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!

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.

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!

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.

Brown transducer wire to right hand pin

Battery connections

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.

casing showing transducer

Electronics top side

Electronics under side, Tx crystal top left

Tx/Rx switching

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.

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!

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.

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).

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!

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.

80cm and 110cm shielded loops

 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!

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.

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.

Complete loop, with radio and switched attenuators, ready for testing

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.

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!

The bare ends of the loop

 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.

Removing a section of jacket and shield from an offcut

 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.

Shield and jacket twisted off

 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 laterally cut section of shield

 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.

Bridging section ready to be soldered on

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!

Loop connection wires fitted and bridge piece soldered on one side

Feed-point wiring and bridge piece completed

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.

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?

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!

120cm and 80cm sections as cut

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.

VHF DF 1/2λ loop with integral 1/4λ sense antenna

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.