Well, this has proved fun! For some unknown reason, the entire circuit decided to stop working regardless of which transistors I installed! I ended up having to completely start again.
Eventually, I managed to get it working with the BC327/BC337 transistors at 3V supply, only to find that the maximum unregulated voltage was barely 300V. Even allowing for a little loading by my 10:1 HV probe, this is too low.
So I played about with the transformers, with interesting results! It would seem that the audio transformers I have are not 1:1 isolating transformers, but output impedance matching units - put them in the wrong way and the current goes up dramatically!
In the end, I found that the unidentified transformers taken from the old emergency lighting switch-mode inverter worked best. With one of those fitted, i'm measuring just over 400V unregulated. How much higher the true figure is, i'll have to wait for the 1GΩ resistor to find out! I also found I had to lower the oscillator capacitor value as the 100uF was causing visible pulsing! 4u7 seems a good value at present.
Friday, 29 November 2019
Generating High Voltage from Very Low Voltage
One of the things I want to do with these Geiger tubes, is to make an ultra-portable 'pocket' device. This is planned for the SI-19BG miniature α tube, which is only about 20mm long!
At present, I'm working on this circuit -
Where possible I've kept the values as stated, but as I don't have any of the specified 2N series transistors, I'm using whatever I have in stock, namely a BC212L and a 2N3904. The 1N914 is replaced by a series string of 47V Zeners and a couple of miniature neon bulbs! The transformer is a miniature audio transformer. The transistors have lower specs than those stated, and this might well affect the results. The closest I have in stock to the specs of the 2N4401/2N4403 are a couple of BC327/BC337 pairs. These have total power dissipation of 625mW same as the specified devices, but slightly lower collector-emitter voltages, however the collector current is higher at 800mA against 600mA. I might try these instead of the quickly-grabbed BC212L and 2N3904.
It is working on the bread-board, but due to loading effects of my HV resistor chain (remember this is only 100MΩ) the voltage reading is poor with the 3V supply (2xAA). With a 9V supply (PP3), the loading is much less of a problem, and I can get the circuit to produce a reading of about 380V with two neons and four Zeners, oh and the neons glow quite nicely! At 3V the neons glow is quite dim, and extinguishes when the voltage is measured.
I quite like the idea of having at least one neon in the feedback circuit - its glow is a good safety check!
I'm not entirely sure what controls the available power of this circuit yet, which may be critical to getting it to work at 1.2V or lower, which is my ultimate goal, so the pocket unit can run on a single AA NiMH cell. This might prove too difficult a voltage to start from, so I may end up using a 3V supply and finding a way to miniaturize the battery! Using higher voltage Zeners will also drastically lower the component count and physical size of the built circuit - as would using a single inductor in place of the transformer.
The 6.3mm fuse clips for the tubes have been delivered. I expect the 1GΩ 2kV resistor to arrive tomorrow. That will massively assist in getting accurate voltage readings!
There is a variation of this circuit that uses the feedback transistor to control the base bias of the oscillator transistor, which is said to give lower current drain, so I might try this out. I'll try the circuit at just 1.5V as well from a single AA cell, and see if it runs!
At present, I'm working on this circuit -
It is working on the bread-board, but due to loading effects of my HV resistor chain (remember this is only 100MΩ) the voltage reading is poor with the 3V supply (2xAA). With a 9V supply (PP3), the loading is much less of a problem, and I can get the circuit to produce a reading of about 380V with two neons and four Zeners, oh and the neons glow quite nicely! At 3V the neons glow is quite dim, and extinguishes when the voltage is measured.
I quite like the idea of having at least one neon in the feedback circuit - its glow is a good safety check!
I'm not entirely sure what controls the available power of this circuit yet, which may be critical to getting it to work at 1.2V or lower, which is my ultimate goal, so the pocket unit can run on a single AA NiMH cell. This might prove too difficult a voltage to start from, so I may end up using a 3V supply and finding a way to miniaturize the battery! Using higher voltage Zeners will also drastically lower the component count and physical size of the built circuit - as would using a single inductor in place of the transformer.
The 6.3mm fuse clips for the tubes have been delivered. I expect the 1GΩ 2kV resistor to arrive tomorrow. That will massively assist in getting accurate voltage readings!
There is a variation of this circuit that uses the feedback transistor to control the base bias of the oscillator transistor, which is said to give lower current drain, so I might try this out. I'll try the circuit at just 1.5V as well from a single AA cell, and see if it runs!
Delaying testing the G-M tubes - with good reason
As any electronics enthusiast could probably appreciate - I'm itching to test these Geiger-Müller tubes! But, I've decided to force myself to wait! Why? Well, although I have my scratch built High Voltage divider chain, it is only 100:1, and built from standard 300V resistors. So I've decided to wait until I can get a very accurate voltage reading - which means waiting for the Next Day delivery of a 1GΩ 2kV resistor, coming from RS Components, for the sake of another couple of quid.
I've also had to enter into a dispute with, yet again, a Chinese ebay seller. The 3W IR LED modules I bought to create extra illumination for the trail camera, turn out to be just 1W. Of course, the seller will now try and give me the run around, but I don't play games with these people!
I've also turned down an appalling counter-offer made by seller "dosimeters_radiometers_counters", who believes the 'Best Offer' option is for wholesale, and sent a counter-offer of exactly the asking price! No, its isn't. I thought only the Far East sellers used that dirty trick!
Amazingly, it's actually stopped raining, and there is sunshine! The ground will still be sodden, but I might make a little foray out later, to play with the Radiofix receiver up on the top of a local hill - away from all the electrical crud!
I've also had to enter into a dispute with, yet again, a Chinese ebay seller. The 3W IR LED modules I bought to create extra illumination for the trail camera, turn out to be just 1W. Of course, the seller will now try and give me the run around, but I don't play games with these people!
I've also turned down an appalling counter-offer made by seller "dosimeters_radiometers_counters", who believes the 'Best Offer' option is for wholesale, and sent a counter-offer of exactly the asking price! No, its isn't. I thought only the Far East sellers used that dirty trick!
Amazingly, it's actually stopped raining, and there is sunshine! The ground will still be sodden, but I might make a little foray out later, to play with the Radiofix receiver up on the top of a local hill - away from all the electrical crud!
Thursday, 28 November 2019
Another 1090MHz Spider Antenna
Its been some time since I last worked on the 360Radar receiver external mount project, due to giving the PVC radome build plenty of time for the cement to cure. So today I finally got around to fabricating the antenna.
The first job here was to drill a bit of PCB stock, to mount the BNC socket on. This was deliberately drilled a little too small, allowing for creating a keying flat during filing it out to size.
With the PCB drilled and roughly cut to size, the BNC panel socket was fitted. The corners were then cut off and the PCB filed until circular.
The circular PCB is single sided, and so is fitted with the copper facing the socket. This is the 'bottom' of the antenna.
Using my 150W iron, the PCB was tinned, and the antenna elements soldered on. Each was cut a little long, to ensure that there was some play in the dimensions.
With all the elements soldered, a marker for 68mm was made on the jig block, and each element measured and trimmed. The BNC to SMA patch-lead was connected, and the ground-plane elements bent to shape. That done, the coax patch-lead was threaded through the antenna mount on the radomes internal equipment board, and the antenna secured in place with hot-melt glue.
A test fit was made to ensure that the driven element of the antenna would fit cleanly into the spire of the radome.The next stage of this project is to size up the equipment board and drill it for mounting pillars to attach the electronics.
I now have the BOI-33 G-M tubes, and most of the necessary parts for the Geiger counter. Ive ordered the correct sized fuse clips to attach to the tubes (6.3mm rather than the common 5mm), these should be with me by the weekend.
Tomorrow, I am going to bread-board a 3V to 400V zener regulated flyback HV generator, for powering these tubes. I've some 100V zeners on order, which will make regulating these circuits a bit easier, as only four would be needed! The prototype tomorrow will use the only zeners I have at the moment in 'high' voltages, so will be a string of eight 47V devices, plus one 24V unit! I could probably get away with fewer if I took the feedback from after the first multiplier, but the regulation will be that bit poorer. I might try both and see how they do.
The first job here was to drill a bit of PCB stock, to mount the BNC socket on. This was deliberately drilled a little too small, allowing for creating a keying flat during filing it out to size.
With the PCB drilled and roughly cut to size, the BNC panel socket was fitted. The corners were then cut off and the PCB filed until circular.
The circular PCB is single sided, and so is fitted with the copper facing the socket. This is the 'bottom' of the antenna.
Using my 150W iron, the PCB was tinned, and the antenna elements soldered on. Each was cut a little long, to ensure that there was some play in the dimensions.
With all the elements soldered, a marker for 68mm was made on the jig block, and each element measured and trimmed. The BNC to SMA patch-lead was connected, and the ground-plane elements bent to shape. That done, the coax patch-lead was threaded through the antenna mount on the radomes internal equipment board, and the antenna secured in place with hot-melt glue.
A test fit was made to ensure that the driven element of the antenna would fit cleanly into the spire of the radome.The next stage of this project is to size up the equipment board and drill it for mounting pillars to attach the electronics.
I now have the BOI-33 G-M tubes, and most of the necessary parts for the Geiger counter. Ive ordered the correct sized fuse clips to attach to the tubes (6.3mm rather than the common 5mm), these should be with me by the weekend.
Tomorrow, I am going to bread-board a 3V to 400V zener regulated flyback HV generator, for powering these tubes. I've some 100V zeners on order, which will make regulating these circuits a bit easier, as only four would be needed! The prototype tomorrow will use the only zeners I have at the moment in 'high' voltages, so will be a string of eight 47V devices, plus one 24V unit! I could probably get away with fewer if I took the feedback from after the first multiplier, but the regulation will be that bit poorer. I might try both and see how they do.
Wednesday, 27 November 2019
A more effective 'clicker'
Almost all simple Geiger Counter designs I've seen, use a piezo element or a small loudspeaker, to give the audible 'click'. Ive tested some of these, and to be frank I think they are rubbish! Yes they 'click' but its just the instant of the loudspeaker coil being pulled in. Its not very satisfying.
Since for some of my Geiger builds I'm wanting a simple count indication of this 'click' type, I decided to work out a design that would give a much nicer, cleaner, and louder indication.
The result is a single transistor design, which when triggered by a pulse from the G-M tube, switches a parallel arrangement of an LED, fed by a 1k series resistor, and a low cost miniature 5V buzzer. As this initial circuit is designed for 9V, there is a potential divider made of two 100Ω resistors which provides a roughly 4.5V supply for the buzzer. In order not to draw any current when not needed, the 'ground' connection of the potential divider connects to the buzzer negative, and so to the transistors collector.
To prototype this, I had to rob the buzzer from a cheap Chinese 'pixie' transceiver, that was lying about in the workshop. I also, belatedly, added a series resistor to the transistor base on the breadboard prototype - after destroying one transistor by repeatedly applying raw supply to the base! I also destroyed a few LEDs trying to find a way of using a capacitor to 'extend' the pulse.
Since for some of my Geiger builds I'm wanting a simple count indication of this 'click' type, I decided to work out a design that would give a much nicer, cleaner, and louder indication.
The result is a single transistor design, which when triggered by a pulse from the G-M tube, switches a parallel arrangement of an LED, fed by a 1k series resistor, and a low cost miniature 5V buzzer. As this initial circuit is designed for 9V, there is a potential divider made of two 100Ω resistors which provides a roughly 4.5V supply for the buzzer. In order not to draw any current when not needed, the 'ground' connection of the potential divider connects to the buzzer negative, and so to the transistors collector.
To prototype this, I had to rob the buzzer from a cheap Chinese 'pixie' transceiver, that was lying about in the workshop. I also, belatedly, added a series resistor to the transistor base on the breadboard prototype - after destroying one transistor by repeatedly applying raw supply to the base! I also destroyed a few LEDs trying to find a way of using a capacitor to 'extend' the pulse.
The nightmare of finding parts at low cost
Its really no wonder that so many companies are losing out to internet trade, especially when it comes to the hobbyist. Many small electronics suppliers, who im sure most hobbyists and amateurs would prefer to patronise, either dont have an internet presence, or dont stock enough and varied parts. Major suppliers either charge extortionate small quantity postage rates, or rediculous 'handling fees', or both, and/or have stupidly time consuming and confusing parts selection filters, that make it all but impossible to quickly and easily find and order a simple component. And hence, the likes of ebay thrive, as a small supplier, or private seller, can quickly and easily list their goods, and a buyer can equaly easily find what they want.
Of course its not always as simple as that. The plethora of Far East suppliers using off-hand tricks in listing titles, which make it look like a part is available cheaply, but then you find it isnt, and the number of sellers listing parts they dont actually have, makes it a tedious task. Then of course there are those with stupid prices, and barmy postage.
Take this morning. I wanted to buy some 5MΩ high voltage resistors. Why? Ive just invested quite a substantial sum in G-M tubes, and Im not going to risk a standard 1/4w resistor breaking down short-circuit and destroying my tubes!
Finding these on the big suppliers websites gave me a headache. I found some on ebay that were not extortionately priced, and were in the UK, but in smaller quantities than I wanted, or in the wrong values. I even enlisted the help of forum members to try and locate some, which is what worked in the end!
I now have 50 4M7 3kV rated resistors coming. Its cost me more than I wanted to pay, but it gives me a handy surplus. Now, many of the Russian tubes advise an ideal anode resistance of 5M1, but that can be made up with a lower voltage rated series resistor before the 3kV unit, if desired. It probably wont make a lot of difference. But, the presence of the 3kV rated resistor gives the protection to the tube that is wanted. The drive voltage of 400-500V probably means normal resistors will survive, but the trade of is cost of resistor against cost of tube. These have cost me less than 8p each, against a tube cost of £10-20 or more. I can offer some of the surplus to other builders - who would shun 99p inc. postage for the protection and piece of mind? We'll see!
Of course its not always as simple as that. The plethora of Far East suppliers using off-hand tricks in listing titles, which make it look like a part is available cheaply, but then you find it isnt, and the number of sellers listing parts they dont actually have, makes it a tedious task. Then of course there are those with stupid prices, and barmy postage.
Take this morning. I wanted to buy some 5MΩ high voltage resistors. Why? Ive just invested quite a substantial sum in G-M tubes, and Im not going to risk a standard 1/4w resistor breaking down short-circuit and destroying my tubes!
Finding these on the big suppliers websites gave me a headache. I found some on ebay that were not extortionately priced, and were in the UK, but in smaller quantities than I wanted, or in the wrong values. I even enlisted the help of forum members to try and locate some, which is what worked in the end!
I now have 50 4M7 3kV rated resistors coming. Its cost me more than I wanted to pay, but it gives me a handy surplus. Now, many of the Russian tubes advise an ideal anode resistance of 5M1, but that can be made up with a lower voltage rated series resistor before the 3kV unit, if desired. It probably wont make a lot of difference. But, the presence of the 3kV rated resistor gives the protection to the tube that is wanted. The drive voltage of 400-500V probably means normal resistors will survive, but the trade of is cost of resistor against cost of tube. These have cost me less than 8p each, against a tube cost of £10-20 or more. I can offer some of the surplus to other builders - who would shun 99p inc. postage for the protection and piece of mind? We'll see!
Monday, 25 November 2019
Tubes and Parts, and waiting
OK, so now I have on order the Russian Geiger-Müller tubes - two BOI-33 beta/gamma types (SBM-20 equivalents), the dinky SI-19BG alpha tube, and the whopping STS-6 beta/gamma tube. I believe some less sensitive SI-3BG glass beta/gamma tubes are also coming my way. I also have ordered high voltage transistors, a load of 2N3906 PNP transistors, plus various other parts. These will be used in the counters, but will mostly just bulk my stock of general purpose components.
Ive also ordered some sub-miniature 1:1 audio transformers. These will be used in the HV generator circuits, but I also use them in such things as digital mode transceiver interfaces, and other audio projects, so they are always worth having!
Although the current HV generator works, further reading suggests that using the transformer on it as a transformer rather than a dual inductor package, might well be more efficient. This hopefully will also get me to the 'holy grail' of this project - A 400V G-M tube supply from a 1.2V AA NiMH cell!
I now have to await delivery. The 3W IR illuminators to help light dark areas of the trailcams coverage have arrived today - no instructions and no hint as to which terminal is which (other than making the assumption that the one with the most copper attached will be negative!), as also have the BN73-302 pig-nose ferrites to wind the transformers for the Wellbrook Loop clone amplifier.
Of course, once I build a Geiger counter that works, I'll need to prove that its working! The beta-lights in the Clansman equipment might work for this, as perhaps might the dial illumination on my Silva Mk4M compass, but to be on the safe side, I've also ordered a 2% Thoriated Tungsten TIG welding rod!
Thorium is the most abundant naturally radioactive element on earth. Its long half-life is a touch longer than the age of the known universe, and its slow alpha decay leads to the above decay chain, providing alpha and beta particles to test Geiger counters with! And a Thoriated welding rod is a damn sight safer to handle than old Radium watch dials!
Ive also ordered some sub-miniature 1:1 audio transformers. These will be used in the HV generator circuits, but I also use them in such things as digital mode transceiver interfaces, and other audio projects, so they are always worth having!
Although the current HV generator works, further reading suggests that using the transformer on it as a transformer rather than a dual inductor package, might well be more efficient. This hopefully will also get me to the 'holy grail' of this project - A 400V G-M tube supply from a 1.2V AA NiMH cell!
I now have to await delivery. The 3W IR illuminators to help light dark areas of the trailcams coverage have arrived today - no instructions and no hint as to which terminal is which (other than making the assumption that the one with the most copper attached will be negative!), as also have the BN73-302 pig-nose ferrites to wind the transformers for the Wellbrook Loop clone amplifier.
Of course, once I build a Geiger counter that works, I'll need to prove that its working! The beta-lights in the Clansman equipment might work for this, as perhaps might the dial illumination on my Silva Mk4M compass, but to be on the safe side, I've also ordered a 2% Thoriated Tungsten TIG welding rod!
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| Thorium Series (courtesy Wikipedia) |
Sunday, 24 November 2019
Russian G-M tubes
Any brief search on ebay for Geiger Tubes, will bring up a whole raft of different Russian devices. If you forget to put the word 'tube' you'll get a whole load of shoes...
It seems the Soviets were a bit obsessed with G-M tubes! They can be had in all manner of forms, sizes, and sensitivities! As the photo below (courtesy pocketmagic.net) shows
It would seem that the SMB-20 tube (4th and 5th from top) is considered the standard for low rate homebrew counters, and so this will be one of the tubes I will try. I have on order though a huge STS-6 (2nd from top) and a tiny little Alpha capable device, the SI19BG.
Ive now made the HV generator permanent. Its not a winner in any design elegance awards! I've literally just pulled the components off of the breadboard, and cobbled them together on a bit of Perfboard! Ive made one change - the 10mH choke has been replaced with a 25mH/20mH transformer taken from an old switching supply. Ive wired the windings in parallel, and I'm able to just about get 530V from it now. I don't know how, or even if, using both windings in any way makes much difference. That's something I can investigate later.
With the 500Ω single turn preset, setting an exact voltage is very tricky. Future builds will have proper 100Ω multiturn presets, but for now I've managed to set this one to near enough 430V, which is the ZP1481s voltage. The SBM-20s I believe run at 400V.
One of my main goals with these circuits, is that it should be reproducible using 'junk-box' parts. Now, I have spent quite a bit on parts for this - but generally this is in bulk, and so building up my 'junk-box' further! The electronics for a simple Geiger counter should, ideally, cost nothing to any constructor with a reasonable stock of common or garden parts. The expense should all go on A) the G-M tube, and B) the case to put it all in!
It seems the Soviets were a bit obsessed with G-M tubes! They can be had in all manner of forms, sizes, and sensitivities! As the photo below (courtesy pocketmagic.net) shows
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| Soviet G-M tubes |
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| SI19BG Alpha window G-M tube |
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| HV Generator Board, and HV probe board |
One of my main goals with these circuits, is that it should be reproducible using 'junk-box' parts. Now, I have spent quite a bit on parts for this - but generally this is in bulk, and so building up my 'junk-box' further! The electronics for a simple Geiger counter should, ideally, cost nothing to any constructor with a reasonable stock of common or garden parts. The expense should all go on A) the G-M tube, and B) the case to put it all in!
HV Go, G-M No Go, And Inside the Euromarine Radiofix Mk5
Late last night, I worked up the Geiger HV generator on breadboard. The first attempt didnt give me more than the supply voltage! Clearly, I'd got something wrong in building it, so I pulled everything out and did it again - this time it worked!
Using my newly built HV resistor chain, I was able to set the HV generator up to show 43V on my DMM, which equates to 430V. This is in the middle of the Mullard ZP1481 tubes 'Geiger Plateau'. I didnt have a 100Ω preset for the voltage setting, so was using a 500Ω with a 100Ω fixed resistor in parallel. This was very temperamental, and hard to adjust! Ive some 100Ω 10-turn presets on order which will make that much easier!
The 4M7 anode resistor was connected right on the tubes anode pin, and a 2x100k potential divider in the cathode to ground path was provided to allow me to connect the oscilloscope to look for pulses.
Sadly, not one single pulse was detected! After a lot of checking and testing, including with various 'beta-light' sources, I've sadly had to come to the conclusion that the tube is dead. I will of course retain it and test it again once I have another working tube and a working counter circuit. I have Russian tubes on order, but they will take some time to arrive.
With the intention of maybe retuning it for better coverage of the Aero-NDB band, I've also had the Euromarine Radiofix Mk5 open. Internally, the most prominent feature is the ferrite rod antenna. From the wiring I can see that there are two coils wound together on this. The circuit itself looks to be a very simple superhet. There are three IF transformers, which are all Toko 468kHz units, plus an oscillator coil (Red). Five BC171B transistors and a germanium diode detector.
Being a 468kHz IF, this rules out retuning to cover up to 550kHz, as this would require tuning through the IF! But, it seems there are few beacons in the UK above 440kHz anyway (not sure of the rest of Europe), so if I decide to retune, it would only be shifting around 40kHz, and still clear of the IF.
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| Parts ready to prototype, and the HV divider board |
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| HV generator, and Mullard ZP1481 G-M tube |
Sadly, not one single pulse was detected! After a lot of checking and testing, including with various 'beta-light' sources, I've sadly had to come to the conclusion that the tube is dead. I will of course retain it and test it again once I have another working tube and a working counter circuit. I have Russian tubes on order, but they will take some time to arrive.
With the intention of maybe retuning it for better coverage of the Aero-NDB band, I've also had the Euromarine Radiofix Mk5 open. Internally, the most prominent feature is the ferrite rod antenna. From the wiring I can see that there are two coils wound together on this. The circuit itself looks to be a very simple superhet. There are three IF transformers, which are all Toko 468kHz units, plus an oscillator coil (Red). Five BC171B transistors and a germanium diode detector.
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| Ferrite rod directional antenna |
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| Simple Superhet |
Friday, 22 November 2019
Divide And Conquer
With a High Voltage generator to build, I though it might be a reasonably sensible idea to be able to measure that high voltage. Now, apart from my oscilloscope, which I'm not confident of the calibration of, I don't have any test gear for hundreds of volts at a few tens of microamps.
So, a HT potential divider was called for. This is a straightforward device, simply a suitable chain of high value resistors, in series with the multi-meters own internal impedance. But first, I needed to know exactly what that impedance is!
Now, the manual says 11.5MΩ but how far can you trust a Chinese manual? So I decided to measure it. OK, so how do you measure the internal impedance of a voltmeter? Well, by balance of course!
Here's the set-up - a fixed voltage is applied across the meter and a series resistor. I chose 10V as this makes everything really easy. As I expected the impedance to be in the 10 to 12MΩ region, I started with a 10MΩ fixed resistor in series. If the series resistor matches the internal impedance - the meter would read half the voltage, in this case 5V. It actually read a little more. So I added another series resistor, a 1M5, and it read a little below. So, it wasn't 10MΩ and it wasn't 11.5MΩ! But 11MΩ.
So I now knew the exact impedance of the meter, and I want to be able to easily display up to 1kV. So making use of the meters ranges I decided on a 10:1 ratio, so that for instance a 500V HT would read 50V on the meter. This would still give me two decimal places, so 100mV accuracy in the HT reading. The exact resistor chain values were worked out using Ohms law, I wont go into them here but its sufficient to say I needed the chain, including the meters impedance, to give a current of 4.5uA.
As I was using whatever I happened to have in stock, this resulted in a chain consisting of 5x 10MΩ, 5x 8M2, 1x 6M8, and a 4M7 preset potentiometer, set up as a variable resistor (wiper and one end tied together). The use of the preset meant I could swing the final resistor value around the expected couple of Megohms, to calibrate the chain and get exactly 10:1. This was all built 'snake-like' on a bit of Perfboard. I then used my bench PSU, which goes up to 32V, to test and calibrate the chain - adjusting the preset to give a 3.2V reading.
I'll post up a photo of the completed device later.
Ive also had a play with the Euromarine Radiofix unit today. A pair of headphones were found that would work with it. The RDF unit has a mono 3.5mm jack, but although this pair of 'phones has a 4-way 'mobile phone' jack, one earpiece works. Radio 4 LW on 198kHz and RTE-1 on 252kHz are both incredibly strong, but even then the nulls in the antenna pattern are sharp and deep enough to get a bearing! Despite the level of local noise, and the incredibly weak signals, Sherburn (2.3miles), Finningley (21miles) and Leeds-Bradford (21.5miles) can all be received and a bearing taken.
Of course, this unit was designed for maritime navigation, when there were still coastal beacons, and so covers 180 to 400kHz. The aeronautical beacon band extends up to 550kHz, so I will consider retuning it to cover from 250 to 550kHz, if possible.
So, a HT potential divider was called for. This is a straightforward device, simply a suitable chain of high value resistors, in series with the multi-meters own internal impedance. But first, I needed to know exactly what that impedance is!
Now, the manual says 11.5MΩ but how far can you trust a Chinese manual? So I decided to measure it. OK, so how do you measure the internal impedance of a voltmeter? Well, by balance of course!
Here's the set-up - a fixed voltage is applied across the meter and a series resistor. I chose 10V as this makes everything really easy. As I expected the impedance to be in the 10 to 12MΩ region, I started with a 10MΩ fixed resistor in series. If the series resistor matches the internal impedance - the meter would read half the voltage, in this case 5V. It actually read a little more. So I added another series resistor, a 1M5, and it read a little below. So, it wasn't 10MΩ and it wasn't 11.5MΩ! But 11MΩ.
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| Set-up to measure meter inpedance |
So I now knew the exact impedance of the meter, and I want to be able to easily display up to 1kV. So making use of the meters ranges I decided on a 10:1 ratio, so that for instance a 500V HT would read 50V on the meter. This would still give me two decimal places, so 100mV accuracy in the HT reading. The exact resistor chain values were worked out using Ohms law, I wont go into them here but its sufficient to say I needed the chain, including the meters impedance, to give a current of 4.5uA.
As I was using whatever I happened to have in stock, this resulted in a chain consisting of 5x 10MΩ, 5x 8M2, 1x 6M8, and a 4M7 preset potentiometer, set up as a variable resistor (wiper and one end tied together). The use of the preset meant I could swing the final resistor value around the expected couple of Megohms, to calibrate the chain and get exactly 10:1. This was all built 'snake-like' on a bit of Perfboard. I then used my bench PSU, which goes up to 32V, to test and calibrate the chain - adjusting the preset to give a 3.2V reading.
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| High Voltage Divider Chain (and a random SCR!) |
I'll post up a photo of the completed device later.
Ive also had a play with the Euromarine Radiofix unit today. A pair of headphones were found that would work with it. The RDF unit has a mono 3.5mm jack, but although this pair of 'phones has a 4-way 'mobile phone' jack, one earpiece works. Radio 4 LW on 198kHz and RTE-1 on 252kHz are both incredibly strong, but even then the nulls in the antenna pattern are sharp and deep enough to get a bearing! Despite the level of local noise, and the incredibly weak signals, Sherburn (2.3miles), Finningley (21miles) and Leeds-Bradford (21.5miles) can all be received and a bearing taken.
Of course, this unit was designed for maritime navigation, when there were still coastal beacons, and so covers 180 to 400kHz. The aeronautical beacon band extends up to 550kHz, so I will consider retuning it to cover from 250 to 550kHz, if possible.
Thursday, 21 November 2019
Bit of G-M H.T.
A long, long time ago, someone kindly donated me a Mullard ZP1481 Mica End-Window Geiger-Müller tube. Whether this was working or not was not known, and it was my intention to build a circuit to test it. Of course, I got sidetracked and its been sat on the shelf waiting for me to get around to it.
So this weekend, it is slated as my no.1 workshop task! My original design, if I recall, called for a rather special pulse transformer, which I didn't have at the time and still don't! The circuit I intend building this weekend uses a much simpler method of generating the required high voltage by utilising inductor back-EMF (flyback) and capacitor-diode multipliers.
With any luck, the circuit will work, the tube will work, and I can then add on a counter/indicator circuit, turning the circuit into a fully fledged Geiger Counter!
No.2 task in the workshop, will be to change the transistor in the EMG channel receiver of the SARBE-5, and see if that solves the poor receive sensitivity issue.
The 2xAA battery holders arrived today, so one is now fitted, with batteries, into the Euromarine RDF unit. All I need to do now is find some compatible headphones!
So this weekend, it is slated as my no.1 workshop task! My original design, if I recall, called for a rather special pulse transformer, which I didn't have at the time and still don't! The circuit I intend building this weekend uses a much simpler method of generating the required high voltage by utilising inductor back-EMF (flyback) and capacitor-diode multipliers.
With any luck, the circuit will work, the tube will work, and I can then add on a counter/indicator circuit, turning the circuit into a fully fledged Geiger Counter!
No.2 task in the workshop, will be to change the transistor in the EMG channel receiver of the SARBE-5, and see if that solves the poor receive sensitivity issue.
The 2xAA battery holders arrived today, so one is now fitted, with batteries, into the Euromarine RDF unit. All I need to do now is find some compatible headphones!
Wednesday, 20 November 2019
Spot the mistake?
Yes, I made rather a silly mistake on retuning the SARBE-5 AUX channel - I tuned it to 290.05MHz... which is NOT the frequency for N/E ICF! That should be 270.05MHz!
The tuning range as is of these receivers is maybe 20MHz wide, so it maybe that I will have to modify the inductors to get the frequency I want!
I'll start the scanner running though and see if I can find an active frequency close to the original 282MHz, to tune to for testing off-air.
As for the poor sensitivity of the main EMG channel, ive found little fault-finding information, so other than dry joints, my only target at present is the transistor. These of course are obsolete types, but luckily, the Tx side uses a few of them, that I can pinch.
Update - Although im yet to repair the EMG channel, I have found that the AUX channel, designed to center at 282MHz, will tune down to the NE ICF frequency, and in fact lower -Ive successfully tuned it to 262.95MHz - the Zone frequency for RAF Leeming. Whether the 20uV sensitivity is sufficient to hear much, well, that remains to be heard!
The tuning range as is of these receivers is maybe 20MHz wide, so it maybe that I will have to modify the inductors to get the frequency I want!
I'll start the scanner running though and see if I can find an active frequency close to the original 282MHz, to tune to for testing off-air.
As for the poor sensitivity of the main EMG channel, ive found little fault-finding information, so other than dry joints, my only target at present is the transistor. These of course are obsolete types, but luckily, the Tx side uses a few of them, that I can pinch.
Update - Although im yet to repair the EMG channel, I have found that the AUX channel, designed to center at 282MHz, will tune down to the NE ICF frequency, and in fact lower -Ive successfully tuned it to 262.95MHz - the Zone frequency for RAF Leeming. Whether the 20uV sensitivity is sufficient to hear much, well, that remains to be heard!
Tuesday, 19 November 2019
Partial Success!
After working out what I was doing wrong in the test set-up, and also discovering the very small note in the service manual that explained that the AUX channel receives regardless of the state of the 'receive' button (yes it is in the instructions on the back of the radio, but mine are in Swedish or something!), I've managed a partial success in retuning the SARBE-5 - my units AUX channel is now no longer the NATO on-scene SAR frequency 282.8MHz, but instead North-East Initial Contact Frequency (ICF N/E) of 290.05MHz.
As these are single active device receivers, retuning a channel, so long as its within the range of the LC circuit, is just a case of adjusting the appropriate trimmer capacitor. The corresponding trimmer capacitor for the test oscillator also needs adjusting if the self test is to work! If the desired frequency is much further out, then adjustment of the little tapped toroid inductor will also be needed.
The AUX channel is working nicely with the test set. I haven't had it on antenna for long enough to see how well it actually receives aircraft yet - that test will need to be done in comparison with a known good receiver, such as my PRC-344, and will need several hours of listening!
Unfortunately, the main EMG channel is somewhat deaf! Ive adjusted its trimmer capacitor C45 for best reception and its not a patch on the AUX channel. I don't know a lot about self-quenching super-regenerative receivers, so at the moment I don't know what to be looking at to solve this. I suspect since there is so little in the circuit, that its either a dry joint or a poorly transistor.
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| C54 AUX Rx tuning |
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| Waiting for a signal! |
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| C45 EMG Rx Tune |
The SARga continues...
Following on from my last post of the strange behavior of the SARBE-5 with the antenna input connected to the Marconi 2955 test-set, ive now tried it running on battery power, with a spare 12v 7Ah SLA battery (charged to 11.1v). The issue still exists, but its form is slightly different, sufficient for me to be able to tell the 'problem' tone apart from the 1kHz modulation. The 'problem' tone is maybe 800Hz.
I also found that the set worked properly if the BNC was 'just' disconnected from the test-set, such that the center pin was physically unconnected, effectively becoming a small capacitor.
So it certainly seems its something to do with isolation from the test-set. Now, the direct connection ive made does bypass a 10pF ceramic capacitor at the antenna, but the antenna connection is still isolated from either the Tx PA or the Rx circuits by more capacitors - the antenna connects at one end of what is essentially a 5-pole low-pass filter, with the Tx connecting at the other end, and the Rx connecting in the middle (so the Tx 'sees' a 5-pole LPF, while the Rx 'sees' a 3-pole Pi-filter. I can only imaging that without the antenna capacitor, somehow this is allowing a feedback path or loading the circuits in some bizarre way. Maybe im missing something in the circuit! This is a 2-channel unit, and I have been working from the single-channel unit schematics for clarity. Is there something going on in the 2-channel set? Maybe its something to do with the AUX channel working even without pressing the receive button?
I'll tack a 10pF capacitor into my test-rig antenna patch lead and see how I progress from there.
And later...
Well, adding the 10pF capacitor into the test-bed antenna connection has solved the issue. I still have no idea why a direct connection from the test-set to the antenna filters - with no DC present - caused such a problem. I can only surmise that it was somehow loading the filter, and causing problems with the operation of the receiver circuits.
So with the receivers responding properly to the test signal, I have found that the AUX channel, currently 282.8MHz, has an MDS (minimum discernible signal) of around 6uV, and is all but fully quieting at the specified level of 20uV. The EMG channel however, currently 243MHz, is not quite as healthy, with an MDS of around 60uV, fully quieting by 220uV, and a distinct 'squeg' before it starts to receive. With any luck this is just a bit off-tune.
So its back now to the service manual, to brush up on the alignment procedures. I'll also check out whether there is anything froody going on as far as power switching to the AUX receiver, that might account for the constant receive regardless of the receive button!
And, if I can ever work out where my compass set is - i'll mark up and etch some battery terminal plates!
I also found that the set worked properly if the BNC was 'just' disconnected from the test-set, such that the center pin was physically unconnected, effectively becoming a small capacitor.
So it certainly seems its something to do with isolation from the test-set. Now, the direct connection ive made does bypass a 10pF ceramic capacitor at the antenna, but the antenna connection is still isolated from either the Tx PA or the Rx circuits by more capacitors - the antenna connects at one end of what is essentially a 5-pole low-pass filter, with the Tx connecting at the other end, and the Rx connecting in the middle (so the Tx 'sees' a 5-pole LPF, while the Rx 'sees' a 3-pole Pi-filter. I can only imaging that without the antenna capacitor, somehow this is allowing a feedback path or loading the circuits in some bizarre way. Maybe im missing something in the circuit! This is a 2-channel unit, and I have been working from the single-channel unit schematics for clarity. Is there something going on in the 2-channel set? Maybe its something to do with the AUX channel working even without pressing the receive button?
I'll tack a 10pF capacitor into my test-rig antenna patch lead and see how I progress from there.
And later...
Well, adding the 10pF capacitor into the test-bed antenna connection has solved the issue. I still have no idea why a direct connection from the test-set to the antenna filters - with no DC present - caused such a problem. I can only surmise that it was somehow loading the filter, and causing problems with the operation of the receiver circuits.
So with the receivers responding properly to the test signal, I have found that the AUX channel, currently 282.8MHz, has an MDS (minimum discernible signal) of around 6uV, and is all but fully quieting at the specified level of 20uV. The EMG channel however, currently 243MHz, is not quite as healthy, with an MDS of around 60uV, fully quieting by 220uV, and a distinct 'squeg' before it starts to receive. With any luck this is just a bit off-tune.
So its back now to the service manual, to brush up on the alignment procedures. I'll also check out whether there is anything froody going on as far as power switching to the AUX receiver, that might account for the constant receive regardless of the receive button!
And, if I can ever work out where my compass set is - i'll mark up and etch some battery terminal plates!
Strange Test Results with the SARBE-5
I seem to have something a little odd going on with the SARBE-5!
Testing it today, to see if all is well before attempting to retune it, ive found a few curious anomalies. First, the AUX channel receiver gives audio output whether the receive button has been pressed or not! The EMG channel doesnt do this and works just as expected - needing the receive button to be pressed to listen.
But more of a worry is that as soon as I connect the unit up to the Marconi 2955 test-set, both receivers give a roughly 1kHz audio tone. Now, this would be fine, if that tone was the 1kHz 30% AM modulation of the test signal from the 2955, but it isnt! They give that tone regardless of what frequency the test set is set to, or what signal level, or even if the test set has its signal generator turned on! In fact, it only stops if I either disconnect the BNC connection to the test-set, needing only to do so far enough that the center pin un-mates, but the barrel can remain connected, or if I turn the test-sets power off!
Strange...!
I have a few theories. Im not too fussed over the AUX channel's indifference to whether ive switched to receive or not - thats a minor trouble. Its the inability to test with a direct signal from the test-set thats the big concern. Using a very limited test regime, due to the frequencies being used, I tried the receivers by close coupling between antennas - the SARBE-5's own rubber ducky antenna and a telescopic whip on the test-set. On both channels I was able to get normal receive performance, i.e. just background noise until the signal level was sufficiently increased (around -65dBm) and then the modulating tone was heard. So it seems whatever is causing the problem is a result of direct electrical contact between the SARBE-5's antenna connection and the circuits of the Marconi 2955. The ground connection from the coax patch-lead doesnt seem to be an issue. My thoughts are that, either there is a loop of some sort being formed, possibly including the bench PSU that is powering the SARBE-5, or there is some strange feedback occurring. Bear in mind that the receiver stages are self-quenching super-regeneratives, it could be that their own oscillations are somehow being fed back around!
So, apart from the AUX channels switching, it looks like the receivers work, but the test set-up is causing trouble! Probably the easiest next step will be to eliminate the PSU as a suspect, by rigging the unit to battery power. Proper SARBE-5 batteries are virtually unobtainable in a working condition, but luckily the radios work on a nominal 12v, so one of my spare 12v SLA batteries will do.
If need be, I will have to mock-up some sort of 1:1 RF transformer to isolate the radio from the test-set.
Testing it today, to see if all is well before attempting to retune it, ive found a few curious anomalies. First, the AUX channel receiver gives audio output whether the receive button has been pressed or not! The EMG channel doesnt do this and works just as expected - needing the receive button to be pressed to listen.
But more of a worry is that as soon as I connect the unit up to the Marconi 2955 test-set, both receivers give a roughly 1kHz audio tone. Now, this would be fine, if that tone was the 1kHz 30% AM modulation of the test signal from the 2955, but it isnt! They give that tone regardless of what frequency the test set is set to, or what signal level, or even if the test set has its signal generator turned on! In fact, it only stops if I either disconnect the BNC connection to the test-set, needing only to do so far enough that the center pin un-mates, but the barrel can remain connected, or if I turn the test-sets power off!
Strange...!
I have a few theories. Im not too fussed over the AUX channel's indifference to whether ive switched to receive or not - thats a minor trouble. Its the inability to test with a direct signal from the test-set thats the big concern. Using a very limited test regime, due to the frequencies being used, I tried the receivers by close coupling between antennas - the SARBE-5's own rubber ducky antenna and a telescopic whip on the test-set. On both channels I was able to get normal receive performance, i.e. just background noise until the signal level was sufficiently increased (around -65dBm) and then the modulating tone was heard. So it seems whatever is causing the problem is a result of direct electrical contact between the SARBE-5's antenna connection and the circuits of the Marconi 2955. The ground connection from the coax patch-lead doesnt seem to be an issue. My thoughts are that, either there is a loop of some sort being formed, possibly including the bench PSU that is powering the SARBE-5, or there is some strange feedback occurring. Bear in mind that the receiver stages are self-quenching super-regeneratives, it could be that their own oscillations are somehow being fed back around!
So, apart from the AUX channels switching, it looks like the receivers work, but the test set-up is causing trouble! Probably the easiest next step will be to eliminate the PSU as a suspect, by rigging the unit to battery power. Proper SARBE-5 batteries are virtually unobtainable in a working condition, but luckily the radios work on a nominal 12v, so one of my spare 12v SLA batteries will do.
If need be, I will have to mock-up some sort of 1:1 RF transformer to isolate the radio from the test-set.
Saturday, 16 November 2019
Rochdale Rally - DF and SAR parts
Visited the Rochdale radio rally today, quite a small rally but a few interesting bits and bobs. Tom insisted on a brew and bacon butty, but it was worth taking him as he was a good bargain spotter!
No one had the BN73-302 ferrites I wanted unfortunately, so the Wellbrook loop clone is still on hold. However very quickly spotted another SARBE antenna! In very good condition, and the seller chucked me in a SARBE-5 battery. Now, the battery terminals had corroded away (although ive just found the cells inside still read 11.55V) but the important thing is that I now can measure the terminals and fabricate replacements, so I can put together a modern replacement battery for the mk5 unit.
Picked up a couple of dogbone insulators, that will complete the receive random-wire antenna for the FRG-100. Tom acquired a few oddball bits free just for interest.
Tom also spotted a Euromarine Radiofix Navigator - one of the DF units i'd been trying to get on ebay recently! Bagged that for less than the ebay ones would have cost with the postage. The battery holder inside has corroded away, and ive got a replacement on order - if only i'd known at the time I could have got one from the rally and had it working already!
So thats on the bench awaiting the new battery holder, 2xAA, but ive replaced the PP3 clip already.
Interestingly, the center terminal of the SARBE-5 battery is the negative. Once I have measured the connection plate, which ive now removed from the battery, I will make up a replacement using PCB material. The three studs on the edge of the plate fit slots in the end of the battery housing - slots which also exist on the dummy (red) battery! This means I could turn the dummy battery into a battery eliminator, to allow connecting up an external supply.
No one had the BN73-302 ferrites I wanted unfortunately, so the Wellbrook loop clone is still on hold. However very quickly spotted another SARBE antenna! In very good condition, and the seller chucked me in a SARBE-5 battery. Now, the battery terminals had corroded away (although ive just found the cells inside still read 11.55V) but the important thing is that I now can measure the terminals and fabricate replacements, so I can put together a modern replacement battery for the mk5 unit.
Picked up a couple of dogbone insulators, that will complete the receive random-wire antenna for the FRG-100. Tom acquired a few oddball bits free just for interest.
Tom also spotted a Euromarine Radiofix Navigator - one of the DF units i'd been trying to get on ebay recently! Bagged that for less than the ebay ones would have cost with the postage. The battery holder inside has corroded away, and ive got a replacement on order - if only i'd known at the time I could have got one from the rally and had it working already!
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| LF RDF Receiver |
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| The state of the workshop bench |
Thursday, 14 November 2019
SARBE-5 Modifications - Working?
Well, its not easy to say!
After making the initial modifications, that is, removing the crystals and the switching board, and disconnecting the Tx power paths via the Tx/Rx reed switches, I found I wasnt getting any response from the unit! I had a bit of initial success but that soon tapered off! I also noted that I didnt seem to be drawing any current.
It does seem to be though that these problems were due to the somewhat too large for the available space crocodile clips I was using to connect power. After swapping the croc leads for soldered on flying leads, ive had repeatable success, although sometimes seemingly different results for the same tests! Good, measurable currents, and plenty of audio.
Before I can go much further though I have to sleeve the newly created soldered connections for the transducer and power wires, so these cannot short to anything. I also want to remove the battery housing (which means breaching the seal where the power wires go through - not a worry as im not going to be immersing the radio in salt-water!) in order to get to and identify the battery terminals.
The next thing then is to connect a temporary antenna feed from the Marconi 2955 and test the receiver stages with real AM signals! If all is well, then its time to look at retuning.
After making the initial modifications, that is, removing the crystals and the switching board, and disconnecting the Tx power paths via the Tx/Rx reed switches, I found I wasnt getting any response from the unit! I had a bit of initial success but that soon tapered off! I also noted that I didnt seem to be drawing any current.
It does seem to be though that these problems were due to the somewhat too large for the available space crocodile clips I was using to connect power. After swapping the croc leads for soldered on flying leads, ive had repeatable success, although sometimes seemingly different results for the same tests! Good, measurable currents, and plenty of audio.
Before I can go much further though I have to sleeve the newly created soldered connections for the transducer and power wires, so these cannot short to anything. I also want to remove the battery housing (which means breaching the seal where the power wires go through - not a worry as im not going to be immersing the radio in salt-water!) in order to get to and identify the battery terminals.
The next thing then is to connect a temporary antenna feed from the Marconi 2955 and test the receiver stages with real AM signals! If all is well, then its time to look at retuning.
SARBE-5 BITE Results
Ive had chance to put the SARBE-5 onto the bench PSU, and power it up in its BITE mode. It looks like the BEACON mode fails self test, but receive and manual transmit seem to pass. As its only the receiver im interested in, thats quite fine by me.
The next stage then is to get the antenna socket repaired. This time, unlike the mk6, im going to cut the transducer and power wiring, rather than unthread it from the loom, as it is much smaller gauge wire than the mk6.
The antenna socket has a small piece of PCB material, plus a small ceramic capacitor. The PCB is slotted and serves as a keying point for the main board. With the PCB and capacitor, and the transducer, removed, the antenna socket could be reseated and the nut tightened. The transducer was then reinstalled, and the PCB and capacitor soldered back on.
The above views of the electronics package show the complexity and compactness of this unit. The two Tx crystals are easy to see, and being socketed were very easy to remove! Next step before refitting the electronics is to totally disable the Tx by disconnecting the reed switch contacts feeding the Tx and Beacon sections with power.
The above is the beacon switching board, and its specialist IC. Ive removed this as it is only used in the Tx function, and the IC itself is of interest. It shouldnt have any effect on the receivers.
The next stage then is to get the antenna socket repaired. This time, unlike the mk6, im going to cut the transducer and power wiring, rather than unthread it from the loom, as it is much smaller gauge wire than the mk6.
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| Empty case showing antenna socket |
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| Top side of electronics |
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| Under side of electronics |
The above is the beacon switching board, and its specialist IC. Ive removed this as it is only used in the Tx function, and the IC itself is of interest. It shouldnt have any effect on the receivers.
SARBE-5 and 80cm Loop Antenna
I have now received the SARBE-5 (BE375). It is complete, with safety pin and antenna, but it has some internal damage that requires repair, this being a stuck fixing screw (however the stud had broken off so it isnt hampering anything), and a loose, and hence broken connections, antenna socket.
This is a twin UHF unit, with 243MHz primary and 282.8MHz auxiliary channels. In the case of this radio then, unlike the V/UHF SARBE-6, the two channels are entirely independent of each other - there are two Tx crystals, and two Rx sections. This gives me the option of re-tuning the receivers for two different monitoring frequencies.
My first task then, will be to repair the antenna socket. My second task is to ascertain what the battery terminal connections are! That said, I can temporarily connect the bench PSU to the internal battery connection posts. This should allow me to test the unit using the BITE functions, even without a working antenna connection.
One thing I am interested in, is how the safety pin actually works? It was my thought that withdrawing the pin, activated the beacon by 'pulling' the on/off switch around 90° so that its magnets aligned with the reed switches, but the pin is a flat bar, and slots into the control switch with it in the 'ON - MAIN' position. At this stage I can only think that the metal of the safety pin somehow modifies the magnetic fields.
Another slight issue with it is that at some point it has had a horrible sticky blue paste added to the seals. The original seal is cork gasket sheet, which I can still get from my local auto spares supplier. Since my car is due its service and MoT, I will be visiting them anyway for filters etc, so will pick up some gasket sheet at the same time.
I now have the 80cm loop antenna working, after a fashion. It is hung up on a curtain rail near the radios, and feeding my FRG-100. Im finding however a great deal of impulse noise across HF. Now, im not convinced this is an issue with the antenna itself, as im also getting this when working mobile HF, even with the engine off, but it is making testing rather trying. I can however notice the directionality of the antenna. Some investigation is needed into the source of this horrendous impulse noise.
This is a twin UHF unit, with 243MHz primary and 282.8MHz auxiliary channels. In the case of this radio then, unlike the V/UHF SARBE-6, the two channels are entirely independent of each other - there are two Tx crystals, and two Rx sections. This gives me the option of re-tuning the receivers for two different monitoring frequencies.
My first task then, will be to repair the antenna socket. My second task is to ascertain what the battery terminal connections are! That said, I can temporarily connect the bench PSU to the internal battery connection posts. This should allow me to test the unit using the BITE functions, even without a working antenna connection.
One thing I am interested in, is how the safety pin actually works? It was my thought that withdrawing the pin, activated the beacon by 'pulling' the on/off switch around 90° so that its magnets aligned with the reed switches, but the pin is a flat bar, and slots into the control switch with it in the 'ON - MAIN' position. At this stage I can only think that the metal of the safety pin somehow modifies the magnetic fields.
Another slight issue with it is that at some point it has had a horrible sticky blue paste added to the seals. The original seal is cork gasket sheet, which I can still get from my local auto spares supplier. Since my car is due its service and MoT, I will be visiting them anyway for filters etc, so will pick up some gasket sheet at the same time.
I now have the 80cm loop antenna working, after a fashion. It is hung up on a curtain rail near the radios, and feeding my FRG-100. Im finding however a great deal of impulse noise across HF. Now, im not convinced this is an issue with the antenna itself, as im also getting this when working mobile HF, even with the engine off, but it is making testing rather trying. I can however notice the directionality of the antenna. Some investigation is needed into the source of this horrendous impulse noise.
Sunday, 10 November 2019
SARBE-6 Trouble?
Hmmm, maybe theres a fault, or maybe im doing something wrong!
A couple of days ago, I carried out the modification to the SARBE-6 as detailed in my previous posts. Now im not sure if its worked as expected, or if my handling the electronics has caused some damage, but it seems to have gone somewhat intermittent!
So, back to studying the schematics! Its possible that ive just misunderstood the effect of the modification, as I did both the Tx and Rx reed mods together. The receiver passes self test, but the Tx doesnt. More worrying though is that with a direct connection from the antenna port to my Marconi 2955, the receiver itself doesnt seem to want to play as it should.
I really need it working properly with the test set before I even think of retuning it!
A couple of days ago, I carried out the modification to the SARBE-6 as detailed in my previous posts. Now im not sure if its worked as expected, or if my handling the electronics has caused some damage, but it seems to have gone somewhat intermittent!
So, back to studying the schematics! Its possible that ive just misunderstood the effect of the modification, as I did both the Tx and Rx reed mods together. The receiver passes self test, but the Tx doesnt. More worrying though is that with a direct connection from the antenna port to my Marconi 2955, the receiver itself doesnt seem to want to play as it should.
I really need it working properly with the test set before I even think of retuning it!
Wednesday, 6 November 2019
Burndept BE515 Switching Scheme
It looks like the switching scheme used in the SARBE-6 is very similar to that used in the SARBE-5, for which I have the circuit diagrams. Switching is controlled by six glass reed switches.
The two SPST reeds for volume and power im not concerned with - their operation remains the same. It is the four SPCO reeds that im concerned with.
The plan is, if at all possible, to make the unit work almost exactly as a live unit, but with no ability to actually transmit. If I want it to be able to still pass all the self tests, then the Tx has to remain physically intact, and electrically operational, but with absolutely no way to access the antenna.
There is some concern over this proposal. As Ive already found, even in test mode, with the case fully closed and with NO antenna, the signal has enough leakage to be detectable on a close by receiver. I can pick it up with my discone antenna from downstairs in the sitting room, as distance of some 25ft! I will need to do some tests to ascertain just how much RF leakage there is in test mode!
Of the four SPCO reeds, two control switching for test mode. The first is an antenna changeover switch, which in test mode disconnects the antenna and connected the LPF (low pass filter) to the BITE circuits. The LPF is part of the antenna matching, and there is no further switching between the Tx and Rx circuits - just a shared common point. If I want test mode to remain workable, then I have to leave this as it is.
The second test mode reed controls power, and is essentially in parallel with the on/off switch reed. This again has to be left alone for test mode to work.
So that leaves the Tx and Rx switching reeds. These control the power supply to the receiver(s) and transmitter, and are wired such that the normally closed contacts are in series, with the Tx reed first in line. This means that, in the absence of a magnetic field from either the Transmit or Receive buttons being pressed, power is routed to the beacon circuitry. With the Receive button pressed, the beacon supply is cut and diverted to the receiver circuits. With the Transmit button pressed, power to the second reed is cut, so removing both the beacon and receiver power supply lines, and diverted to the transmit circuits and AM modulator.
So a simple, and I hope workable, modification scheme develops. The 1st Tx/Rx reeds NO (normally open) contact is disconnected from the rest of the circuitry, this preventing the transmitter being activated by the Transmit button. The 2nd reeds NC (normally closed) contact is disconnected, thus isolating the beacon circuitry.
In this state, test mode should still work as normal, but the beacon and the voice transmitter should never activate. The Receive button would still need to be pressed to listen though. However the radio could be set to permanent receive, without needing the button pressed, by adding a short wire to connect the 2nd reeds NO contact to its common contact, therefore permanently powering the receiver and audio amp when the unit is switched on.
I think though that a permanent receive modification would mean that in test mode the Transmit and Beacon functions would not be testable.
It is my intention to modify my unit to require the Receive button to be pressed, at least at first. When later, I work out how to retune the unit for a different frequency, at that point it will probably make good sense to go to a permanent receive function.
The two SPST reeds for volume and power im not concerned with - their operation remains the same. It is the four SPCO reeds that im concerned with.
The plan is, if at all possible, to make the unit work almost exactly as a live unit, but with no ability to actually transmit. If I want it to be able to still pass all the self tests, then the Tx has to remain physically intact, and electrically operational, but with absolutely no way to access the antenna.
There is some concern over this proposal. As Ive already found, even in test mode, with the case fully closed and with NO antenna, the signal has enough leakage to be detectable on a close by receiver. I can pick it up with my discone antenna from downstairs in the sitting room, as distance of some 25ft! I will need to do some tests to ascertain just how much RF leakage there is in test mode!
Of the four SPCO reeds, two control switching for test mode. The first is an antenna changeover switch, which in test mode disconnects the antenna and connected the LPF (low pass filter) to the BITE circuits. The LPF is part of the antenna matching, and there is no further switching between the Tx and Rx circuits - just a shared common point. If I want test mode to remain workable, then I have to leave this as it is.
The second test mode reed controls power, and is essentially in parallel with the on/off switch reed. This again has to be left alone for test mode to work.
So that leaves the Tx and Rx switching reeds. These control the power supply to the receiver(s) and transmitter, and are wired such that the normally closed contacts are in series, with the Tx reed first in line. This means that, in the absence of a magnetic field from either the Transmit or Receive buttons being pressed, power is routed to the beacon circuitry. With the Receive button pressed, the beacon supply is cut and diverted to the receiver circuits. With the Transmit button pressed, power to the second reed is cut, so removing both the beacon and receiver power supply lines, and diverted to the transmit circuits and AM modulator.
So a simple, and I hope workable, modification scheme develops. The 1st Tx/Rx reeds NO (normally open) contact is disconnected from the rest of the circuitry, this preventing the transmitter being activated by the Transmit button. The 2nd reeds NC (normally closed) contact is disconnected, thus isolating the beacon circuitry.
In this state, test mode should still work as normal, but the beacon and the voice transmitter should never activate. The Receive button would still need to be pressed to listen though. However the radio could be set to permanent receive, without needing the button pressed, by adding a short wire to connect the 2nd reeds NO contact to its common contact, therefore permanently powering the receiver and audio amp when the unit is switched on.
I think though that a permanent receive modification would mean that in test mode the Transmit and Beacon functions would not be testable.
It is my intention to modify my unit to require the Receive button to be pressed, at least at first. When later, I work out how to retune the unit for a different frequency, at that point it will probably make good sense to go to a permanent receive function.
Dismantling the BE515
Without a service manual, or circuit diagrams, im pretty much working blind with the SARBE-6. However, years of servicing public safety radios, plus the service manual for the older, but in many ways similar SARBE-5, means I do have a reasonable idea how to go about it.
The electronics package of the SARBE-6 is made up of a main PCB, plus a couple of smaller plug in modules. The main board fills the whole of the internal space, and is held secure onto a pair of threaded studs. Because the only externally mounted parts are the antenna socket and the battery connection, it 'should' be a simple case of lifting the electronics gently out, after unsoldering the antenna.
A great feature of modern smart phones is the ability to photograph a circuit just before carrying out any work, and immediately annotate it, so you have a record of what was done. This helps when rebuilding, but is also brilliant for spotting where you went wrong!
The problem is getting it up over the threaded studs! There is very little clearance around them. Now, im sure in the Burndept factory they will have had a jig for this! But I have no such, and must rely on carefully prising the board up using dental picks and jewelers screwdrivers.
The battery and transducer (loudspeaker/microphone) connections also need desoldering. It would be a shame to cut the intricate cable loom lacing, plus, there may be reasons that the wires follow the paths they do (RF stability etc), so I will try and extract them from the loom, with the hope of being able to carefully reinsert them later.
With the electronics removed, I should be able to work out the switching sequences, and from that devolve a plan to put the set into permanent receive mode.
As it turned out, the transducer and battery wires were held to the loom in only two places, and were very simple to remove. I now have the electronics package out of the case, and can start working on the modification plan.
Of course the fastest way to kill the Tx would be to simply pull the crystal, which happens to be socketed! But that would leave a powered oscillator and PA, which could cause problems.
The electronics package of the SARBE-6 is made up of a main PCB, plus a couple of smaller plug in modules. The main board fills the whole of the internal space, and is held secure onto a pair of threaded studs. Because the only externally mounted parts are the antenna socket and the battery connection, it 'should' be a simple case of lifting the electronics gently out, after unsoldering the antenna.
A great feature of modern smart phones is the ability to photograph a circuit just before carrying out any work, and immediately annotate it, so you have a record of what was done. This helps when rebuilding, but is also brilliant for spotting where you went wrong!
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| Antenna connects to straight reed leg |
The problem is getting it up over the threaded studs! There is very little clearance around them. Now, im sure in the Burndept factory they will have had a jig for this! But I have no such, and must rely on carefully prising the board up using dental picks and jewelers screwdrivers.
The battery and transducer (loudspeaker/microphone) connections also need desoldering. It would be a shame to cut the intricate cable loom lacing, plus, there may be reasons that the wires follow the paths they do (RF stability etc), so I will try and extract them from the loom, with the hope of being able to carefully reinsert them later.
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| Brown transducer wire to right hand pin |
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| Battery connections |
As it turned out, the transducer and battery wires were held to the loom in only two places, and were very simple to remove. I now have the electronics package out of the case, and can start working on the modification plan.
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| casing showing transducer |
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| Electronics top side |
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| Electronics under side, Tx crystal top left |
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| Tx/Rx switching |
Hedgehogs
I have numerous hedgehogs visit my garden, and ive built a cat proof feeding station to help fatten them up ready to hibernate.
What the heck has this to do with radio and electronics? Well, I know how many are visiting due to my trail camera of course!
This was a great investment! Along with a set of eight high capacity NiMH AA's from 7Dayshop, this has been revealing the comings and goings of these wonderfully cute nocturnal critters for a couple of weeks now. However, one small problem is that its active IR illuminator doesnt reach very far, and there is often activity out in the shadows.
To try and improve the far field of view, tonight I will be experimenting with the contents of a bargain crate of 8h military IR cyalume sticks! Im hoping that these will help reveal what is going on in the dark corners of the video frames!
What the heck has this to do with radio and electronics? Well, I know how many are visiting due to my trail camera of course!
This was a great investment! Along with a set of eight high capacity NiMH AA's from 7Dayshop, this has been revealing the comings and goings of these wonderfully cute nocturnal critters for a couple of weeks now. However, one small problem is that its active IR illuminator doesnt reach very far, and there is often activity out in the shadows.
To try and improve the far field of view, tonight I will be experimenting with the contents of a bargain crate of 8h military IR cyalume sticks! Im hoping that these will help reveal what is going on in the dark corners of the video frames!
Android BOINC dried up
Well, it looks as though the source projects for BOINC running on Android devices has thoroughly dried up! No new units since the start of the week and none on the horizon, so ive shut down my dedicated data crunching devices, for the time being.
I will leave the client active on my mobile, as that will allow me to see when new work becomes available.
This is a real shame, as there must be millions of CPU hours going to waste among the vast numbers of Android users.
I will leave the client active on my mobile, as that will allow me to see when new work becomes available.
This is a real shame, as there must be millions of CPU hours going to waste among the vast numbers of Android users.
Tuesday, 5 November 2019
Burndept BE-515 SARBE-6
One of my newly acquired collection items is a very good condition, fully working Burndept BE-515 transceiver. Commonly known by its designation SARBE-6, this is the 6th incarnation of the Search And Rescue Beacon Equipment (SARBE).
For very obvious reasons, before I do any general listening to this, in fact, before I will even leave it with the battery connected, I need to disable the transmitter! If your reading this, be aware that im an ex-public safety radio engineer - i have the necessary equipment to work on these safely! So what follows is just a few technical notes. Eventually, I hope to be able to not only provide instructions to safely disable the beacon and transmit features, but also conversion details to allow them to be repurposed as novel monitoring receivers.
The SARBE-6 provides automatic or manual distress beacon transmission, AM voice transmission, and short range reception, on the two international distress frequencies of 121.5 and 243MHz. You may spot that these are harmonically related. This is a dual channel unit, but the two channels are not user independent - both channels operate together. So transmission and reception occurs on both VHF and UHF at the same time.
The above is a view inside the unit. Its very complex in there! The transmitter is crystal controlled, however the receiver(s), if it follows the architecture of the mk5 model before it, uses regenerative receiver techniques. This gives the possibility of easily retuning the receive channels. One point of note, at least from the mk5, and ive no reason to think the mk6 is any different, is that the regenerative receiver stage does not have an isolating RF amp ahead of it. This means that in theory it could radiate its oscillation back out the antenna and be detected - this is why it is so important to modify these correctly!
The units were rated to be impervious to saltwater to 10m depth. How the accomplish this for the controls is quite ingenious. The casing is aluminium, and so transparent to a magnetic field. All the switches are internal reed switches, operated by external magnets! It will be partly by changing the connections to the on/off and Transmit reed switches that I accomplish the disablement of the transmitter.
My unit passes all self tests, so is fully operational - hence why I am keeping the battery off! I have tested the receiver using an external source as well.
So I have two tasks with this - disable the transmitter, and work out how to retune the receiver. If only I could find a service manual! I have the manual for the mk5, but I can see that they are not quite the same internally. If possible, I will modify it so that the transmitter cannot operate in 'live' mode, but will operate when routed internally to the BITE (Built-In Test Equipment) circuits in 'test' mode. If this cannot be done satisfactorily then I will remove the Tx crystal, disconnect the Tx supply lines, and also remove the PA transistor!
IMPORTANT - IF YOU ACQUIRE ONE OF THESE WITH A BATTERY - DO NOT UNDER ANY CIRCUMSTANCES OPERATE IT!!! THE SIGNAL FROM THESE IS MONITORED BY AUTOMATIC DIRECTION FINDING ACROSS THE WORLD 24/7
For very obvious reasons, before I do any general listening to this, in fact, before I will even leave it with the battery connected, I need to disable the transmitter! If your reading this, be aware that im an ex-public safety radio engineer - i have the necessary equipment to work on these safely! So what follows is just a few technical notes. Eventually, I hope to be able to not only provide instructions to safely disable the beacon and transmit features, but also conversion details to allow them to be repurposed as novel monitoring receivers.
The SARBE-6 provides automatic or manual distress beacon transmission, AM voice transmission, and short range reception, on the two international distress frequencies of 121.5 and 243MHz. You may spot that these are harmonically related. This is a dual channel unit, but the two channels are not user independent - both channels operate together. So transmission and reception occurs on both VHF and UHF at the same time.
The above is a view inside the unit. Its very complex in there! The transmitter is crystal controlled, however the receiver(s), if it follows the architecture of the mk5 model before it, uses regenerative receiver techniques. This gives the possibility of easily retuning the receive channels. One point of note, at least from the mk5, and ive no reason to think the mk6 is any different, is that the regenerative receiver stage does not have an isolating RF amp ahead of it. This means that in theory it could radiate its oscillation back out the antenna and be detected - this is why it is so important to modify these correctly!
The units were rated to be impervious to saltwater to 10m depth. How the accomplish this for the controls is quite ingenious. The casing is aluminium, and so transparent to a magnetic field. All the switches are internal reed switches, operated by external magnets! It will be partly by changing the connections to the on/off and Transmit reed switches that I accomplish the disablement of the transmitter.
My unit passes all self tests, so is fully operational - hence why I am keeping the battery off! I have tested the receiver using an external source as well.
So I have two tasks with this - disable the transmitter, and work out how to retune the receiver. If only I could find a service manual! I have the manual for the mk5, but I can see that they are not quite the same internally. If possible, I will modify it so that the transmitter cannot operate in 'live' mode, but will operate when routed internally to the BITE (Built-In Test Equipment) circuits in 'test' mode. If this cannot be done satisfactorily then I will remove the Tx crystal, disconnect the Tx supply lines, and also remove the PA transistor!
Friday, 1 November 2019
110cm and 80cm Shielded Loop Antennas Built
Both the 110cm and 80cm loops are now complete physically. At 110cm, the loop is huge! So much so in fact that ive had to reinforce the site of the gap in the shield, in order to help prevent it bending at this point under its own weight! This was done by stretch wrapping PVC tape around the dielectric in the gap, up to the same thickness as the now removed copper. The original section of jacket was then refitted, taped in place, and then the section of jacket removed from the 'bridge piece' added on top for more stiffness, and again stretch taped in place. The extra thickness, and of course the red PVC tape, shows where the gap in the shield is.
The next stage is to rig up a support stand to hold them for testing, and to add the amplifier. The loops will then be tested on my FRG-100 receiver.
Of course, all this may be put to one side, while I play with more surplus radio! This all depends how long the shipping takes on my newly acquired SARBE-5 and SARBE-6 survival radios!
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| 80cm and 110cm shielded loops |
Of course, all this may be put to one side, while I play with more surplus radio! This all depends how long the shipping takes on my newly acquired SARBE-5 and SARBE-6 survival radios!
VHF RDF Loop with Integral Sense Antenna - 2nd Try
Ages ago I tried this loop, and abandoned it due to construction difficulties. I mentioned a few posts back that I have had a second try at it, and this is the write up!
The antenna design is this one here http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/radcom/1991/06/page29a/ and the details of it are rather sketchy.
Now, one thing I do know, is that my dimensions are not exact! It is intended to have been cut for 145.500MHz, but im pretty sure im at least a couple of cm out in places! The handle is a section of LCF12-50 1/2inch Cellflex, and the loop itself if formed from a length of LDF2-50 3/8th inch Cellflex. The outer jacket, shield copper, and dielectric foam were removed from roughly one half of the loop, with the dielectric foam and outer jacket refitted later. So although it looks like its all one piece of coax, only one half (roughly 1/4λ) still has the outer copper shield, and this forms the 'sense antenna' element.
At the top of the 'handle' the outer shield and the inner conductor of the loop are both soldered to the outer shield of the 'handle'. The other end of the loop solders to the inner conductor. At the bottom of the 'handle' I have soldered on a single hole chassis mount BNC socket. Short 'tags' of the outer shield were left after cutting the parts, in order to solder to.
Where possible, spaces were then filled with off-cuts of dielectric and/or jacket, and taped over. The red tape on the side of the antenna indicates the side with the 'sense' element.
I have yet to perform any tests with the loop. Today has been too wet to work outside, and the indoor VHF QRM rules out testing inside. I also need to take some accurate measurements to calculate exactly what frequency it is resonant at.
The antenna design is this one here http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/radcom/1991/06/page29a/ and the details of it are rather sketchy.
Now, one thing I do know, is that my dimensions are not exact! It is intended to have been cut for 145.500MHz, but im pretty sure im at least a couple of cm out in places! The handle is a section of LCF12-50 1/2inch Cellflex, and the loop itself if formed from a length of LDF2-50 3/8th inch Cellflex. The outer jacket, shield copper, and dielectric foam were removed from roughly one half of the loop, with the dielectric foam and outer jacket refitted later. So although it looks like its all one piece of coax, only one half (roughly 1/4λ) still has the outer copper shield, and this forms the 'sense antenna' element.
At the top of the 'handle' the outer shield and the inner conductor of the loop are both soldered to the outer shield of the 'handle'. The other end of the loop solders to the inner conductor. At the bottom of the 'handle' I have soldered on a single hole chassis mount BNC socket. Short 'tags' of the outer shield were left after cutting the parts, in order to solder to.
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| Detail of the connections to the 'handle' |
Where possible, spaces were then filled with off-cuts of dielectric and/or jacket, and taped over. The red tape on the side of the antenna indicates the side with the 'sense' element.
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| Complete loop, with radio and switched attenuators, ready for testing |
Building the 80cm Shielded Loop Antenna
Building with Cellflex feeder is a rather tricky operation! Although it is flexible, it is also rather springy! The solid outer copper shield and the thick copper plated inner mean this is a job for the hacksaw and pipe-cutter, rather than the side cutters!
The 80cm diameter loop requires a Cellflex length of around 251cm. Ive tried to keep the cuts for the connections as small as possible, but it will still be approximate. This doesn't really matter though, so long as im within a couple of cm.
The pipe-cutter makes a good job of removing the outer sheath, but is not so good on the shield when near the ends of the cable. Here the hacksaw is preferred. I found that due to the springy nature of the cable, clamping it in the workbench was also a requirement!
With the ends of the loop prepared, the next task was to create a 'bridging piece'. This section will bridge the gap in the shield at the feedpoint, making the shield continuous at this point. This was measured and cut, using a pipe-cutter, from an off-cut.
Once the pipe-cutter was through, the section was carefully twisted off, over the dielectric foam, by hand so as not to risk crushing it.
The hardest part of the build was cutting the section of shield lengthways. To do this, it was carefully clamped by its ends in the vice, and both jacket and shield carefully cut through with the hacksaw. Any swarf was then removed with needle-nose pliers and a fine needle file.
The photo below shows the loop ends and the 'bridging piece' ready to be soldered together. Before this is done, the connecting wires to the inner loop will be added, as access to the inner loop ends will be very restricted once the loop is joined.
Because of the huge thermal mass that this cable presents, even adding a few wires was an industrial job, requiring the use of my 150W soldering iron. A short length of red/black speaker cable was split and used for the loop connections, with a length of green hook-up wire used for the ground connection.
Fitting the 'bridging piece' was more akin to fitting domestic central heating pipe! The section was 'sweated' onto the loop, the close fit allowing the solder to 'wick' into the joint. This was a trade-off between using enough heat to properly flow the solder, and not melting the dielectric or the jacket too much!
The green ground connection was soldered into a trough in the corrugations of the shield, in order that a section of surplus jacket can later be added over the copper.
Physically, the loop is now almost complete! What remains is to make the break in the shield at a point 180° from the feed-point. To find this point i will use a fabric tape.
Once physically completed, the next stage will be to add an LNA.
The 80cm diameter loop requires a Cellflex length of around 251cm. Ive tried to keep the cuts for the connections as small as possible, but it will still be approximate. This doesn't really matter though, so long as im within a couple of cm.
The pipe-cutter makes a good job of removing the outer sheath, but is not so good on the shield when near the ends of the cable. Here the hacksaw is preferred. I found that due to the springy nature of the cable, clamping it in the workbench was also a requirement!
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| The bare ends of the loop |
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| Removing a section of jacket and shield from an offcut |
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| Shield and jacket twisted off |
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| The laterally cut section of shield |
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| Bridging section ready to be soldered on |
Fitting the 'bridging piece' was more akin to fitting domestic central heating pipe! The section was 'sweated' onto the loop, the close fit allowing the solder to 'wick' into the joint. This was a trade-off between using enough heat to properly flow the solder, and not melting the dielectric or the jacket too much!
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| Loop connection wires fitted and bridge piece soldered on one side |
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| Feed-point wiring and bridge piece completed |
Physically, the loop is now almost complete! What remains is to make the break in the shield at a point 180° from the feed-point. To find this point i will use a fabric tape.
Once physically completed, the next stage will be to add an LNA.
Short Shielded Receiving Loops
Been thinking of this for some time. I have a length of surplus LCF12-50 feeder that simply is never going to be used for feedline again, as I cant afford the connectors. So why not use it for something else?
One obvious use is a loop antenna. The solid outer shield lends itself well to being used as the E-field shield of a 'magnetic loop', if of course we discount the practical issues of cutting the stuff! I have been contemplating making a clone of the Wellbrook loop antenna with it, but, the Wellbrook is a balanced loop, using just the 'pipe' its formed from as the antenna, in this case that would be the shield of the feeder, meaning the inner conductor would be redundant. At present this seems a bit of a waste, as there are other designs that would make use of all of the parts of this rather expensive cable!
So, Im looking at the Short Shielded One Turn Loops, as per PA0FRI/PA2GZKs designs.
So, after removing the manky PVC tape and the old cable markers, I roughly rolled the LDF12-50 out and measured off an approximate loop with a 120cm diameter. On cutting this section off from the remainder, I found that the left over section itself is near bang on for an 80cm loop!
Now, I have already used about 6 inches of the LCF12-50 to form the handle of a VHF DF loop, which is in the testing phase. This is another version of the loop I attempted way back in 2015.
The plan is to form the loops, and then use an 'off the shelf' LNA with them for initial tests. 120cm is a pretty big beast, so a comparison of performance between the 120cm and 80cm loops will be interesting. My main interest for these is in MF and LF work.
With the pieces cut, the next job is to prepare the ends for connection, and to remove the central section of the shield to provide the break in the shield loop. As the shield loop is continuous at the feed point, this means I will have to sacrifice a small section to be used to rebuild the shield at the feedpoint, maybe a couple of inches. As this is fairly flexible stuff, there is no need just yet to form it into a perfect circle, which will require a bending jig to be built.
Unfortunately, on measuring the actual length for the 120cm loop, I found I was a little short! I could have made a 114cm loop, but didnt like the idea of the 'odd' dimension. So, instead, I will cut the length to 345.5cm, for a 110cm diameter loop. This also gives me plenty of offcut for bridging the gaps of the feedpoints.
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| Surplus hunk of LDF12-50 1/2inch feeder |
One obvious use is a loop antenna. The solid outer shield lends itself well to being used as the E-field shield of a 'magnetic loop', if of course we discount the practical issues of cutting the stuff! I have been contemplating making a clone of the Wellbrook loop antenna with it, but, the Wellbrook is a balanced loop, using just the 'pipe' its formed from as the antenna, in this case that would be the shield of the feeder, meaning the inner conductor would be redundant. At present this seems a bit of a waste, as there are other designs that would make use of all of the parts of this rather expensive cable!
So, Im looking at the Short Shielded One Turn Loops, as per PA0FRI/PA2GZKs designs.
So, after removing the manky PVC tape and the old cable markers, I roughly rolled the LDF12-50 out and measured off an approximate loop with a 120cm diameter. On cutting this section off from the remainder, I found that the left over section itself is near bang on for an 80cm loop!
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| 120cm and 80cm sections as cut |
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| VHF DF 1/2λ loop with integral 1/4λ sense antenna |
With the pieces cut, the next job is to prepare the ends for connection, and to remove the central section of the shield to provide the break in the shield loop. As the shield loop is continuous at the feed point, this means I will have to sacrifice a small section to be used to rebuild the shield at the feedpoint, maybe a couple of inches. As this is fairly flexible stuff, there is no need just yet to form it into a perfect circle, which will require a bending jig to be built.
Unfortunately, on measuring the actual length for the 120cm loop, I found I was a little short! I could have made a 114cm loop, but didnt like the idea of the 'odd' dimension. So, instead, I will cut the length to 345.5cm, for a 110cm diameter loop. This also gives me plenty of offcut for bridging the gaps of the feedpoints.
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