Getting Sectioned

A few people over the years have suggested I should be sectioned! But its actually some coax thats being sectioned here!

Ive been tasked at work with producing and teaching a Basic Broadcasting course, a sort of Radio 101, and I like to have hands on demos of things, rather than just death by powerpoint. My course is to explain in basic terms essentially how radio works and how we broadcast with it! And part of that means a bit about feeders.

So, I decided I needed some coax that I could bare back and use to demonstrate how it works. I could use a bit of RG-214, but its not easy to see at the back of a classroom, so I found a 5m patch-lead (for want of a better term!) of this stuff -

 

This is 3.8 inch RFS Heliflex. Its a semi-rigid cable similar to Cellflex or Heliax, only bigger! Ive done a comparison photo below with a bit of RG-174!

 

I also have this bit of 4" Rigid line. Now this was part of a combiner system some time in the past, and this particular section was modified with an inter-digital filter and two BNC sampling points. It also has an N type adapter on one end!

I am slot sectioning the rigid line, which means essentially cutting a slot down one side so the innards can be seen. Ive removed the sampling points and the filter plungers, and my nibblers made swift work of roughing out the slot, helped by the outer 'braid' of this coax being aluminium tube!
The Heliflex is a bit harder. This needs a staggered sectioning of the open end, plus a slot section down about half its length. Having cut both ends off, I have two pieces with connectors on, each about three feet long. Those connectors are massive, and silver plated!

The picture below shows one section with the end carefully and neatly cut, and the outer sleeve cut back where the slot will go. The next task is to remove the copper outer conductor from the slot area. This won't be easy, as the copper is thick and corrugated.

 Looking into the open end, you can see how this is made up, and also appreciate why we have to use dehydrators to fill them with dry air! The yanks use nitrogen for this. By pressurizing the feeder systems, it prevents moisture getting into the feeders.

I have about four foot of this spare, which I will chop up and donate to local radio clubs as a teaching aid!

The other demo item I want to build is a set of UHF Lecher lines. I already have a set, but need a suitable balun to feed the RF energy into them.

Camels and needles

Im currently attempting the rather tricky task of threading a flexible, stranded wire up the inside of a 30ft fibreglass tube! But, this tube also tapers, and has a constriction in the middle!

So far ive tried various telescopic fishing rods to shove the wire up, but they are either too short, or eventually too fat to get all the way!

Next thing to try then is a thin line and a small but heavy weight!

The idea is to put an antenna element inside this tube, which happens to be Sams flagpole, which was originally put up to eventually become a vertical antenna anyway!

Part of the problem is the presence of an internal bung, which was at the top. I had thought it to be a seal of epoxy, and tried to crack it out with a chisel. Well, it just fell inside! I cant get anything past it, and without cutting more off the end of the tube I cant get it out

CHOTA Antenna Planning

Another post with no photos!

The time has come to start deciding on what antennas will be needed for the CHOTA activation from St Mary's, Lead.September seems a long time away but it will soon be here if im not prepared!

There are several important issues to be considered, of which not a few are essentially mutually exclusive and require compromises. But essentially it boils down to a simple trade off - complexity against frequency agility.

Of course, I could just use a terminated dipole, after all, many Naval vessels do well with them! But, unfortunately, most of my contacts will not be with stations equipped with diversity reception from numerous receive sites around the world fitted with HF log periodics! I could also of course limit my bands of operation, after all this is a one day event and if I cant get enough contacts on 40m then im not trying hard enough!

But I need to be able to demonstrate amateur radio at its best ( I want to be invited back next year!) and a SID (Sudden Ionospheric Disturbance) could knock 40m on the head, and there would I be? Or, perhaps more likely, as occurs with the Doncaster Show, I might find im up against a contest! So frequency agility is a must.

Theres also to be considered the possibility that I might have other operators, so more bands could be utilised. I at least want to have the 20m PSK station running, alongside SSB on 40m, but I'd also like to run a WSPR beacon, perhaps on 10m (IF I ever get it working!)

Two antennas then that Im considering. The first is the ZS6BKW multiband dipole. This variant of the venerable G5RV would give the 'best bang for the buck', and should work fine from the 27ft Larkspur mast. But it would still leave me a single antenna station. The next option, is a Cobweb antenna, covering 20m and up, combined with a 40m dipole (or 40m/80m fan?)

The Cobweb, design variant by G3TXQ, would give me 5 bands, omni-directional HF coverage, and using fibreglass spreaders (which I already have) should be possible to mount atop the Larkspur with a suitably designed bracket. The Clansman antenna kit hung below would give me the 40m inverted V dipole.

As said, I have the 'fibreglass' spreaders already. Mine actually say they are carbon composition but I very much doubt it, I will do an absorption test on them at some point. I just need to work out two things - 1, how to mount the whole antenna onto the relatively thin top section of the Larksput mast, and 2, how to mount the poles and wires to the bracket in a way that makes the whole antenna easy to dismantle and put together in the field.

Mobile antenna mount repaired

Since I had the car apart to replace a headlight bulb, I decided to find out just what was wrong with the HF antenna mount. I managed to remove the mount only having to unfasten the very ends of the side trims and the last two fixings of the head lining, which gave just enough room to squeeze my hand through to retrieve the SO239 fixing.

As expected, the center pin connection had separated. Luckily not the pin sheared, but the soldered connection to the coax. It seems under enough torque the pin can turn, and over time the wire has split.

After recutting the coax and inserting it back into the connector, I managed, somehow, to resolder the center. Then after a fair bit f swearing I finally managed to get the mount back through the hole in the roof and have it stay there without falling back out for long enough for me to get hold of it from the top of the car and get the nut on! A few minutes of the good old antenna fitters rooftop pump pliers dance, and its secure again.

Tested with the 40m antenna, found enough activity to verify it was operational again.

Mobile Intermittency

Shortly after obtaining a 40m mobile antenna, it became apparent that there was a problem. Often I would be able to hear nothing on the mobile HF.

It turns out the roof SO239 connector has gone intermittent. I suspect wind loading stresses have fractured the center connection. As I have other vehicle repairs to do tomorrow (yet another blown headlight!) I intend also taking a look at the antenna connection, and seeing if its repairable.

Because of this problem, ive gone much of the winter (what we've had of one!) without a reliable HF mobile, and so made little use of the 40m antenna.

I suspect, that by the time ive fixed the problem, the higher bands will be back in play. One of my tasks will be to build an efficient antenna for the one favorite band I dont have one for - 10m!

ShackaLite

Two things have occupied my technical time today. These were lighting up the shack, and making sure I can deal with it if the workshop catches light!

My workshop also doubles as much needed storage for the rest of the family, so one of my ongoing jobs is to improve this storage (or, as Julie puts it - 'tidy up'). Ive bought six plastic storage boxes today (stackable) to help with this. Part of the problem is I have lots of 'junk', otherwise known as salvaged equipment. These boxes will help me store such things as salvaged circuit boards, RF/Coax patch leads etc.

One of my obsessions is with fire safety. This stems from my earlier, childhood obsession - fire.

When I was younger, fire fascinated me. I would sit watching candles burning. I would also set fire to anything and everything, from dry grass, to cardboard boxes in the woods, to 'volcanos' made from weedkiller and sugar. But I soon came to realise that the real buzz from pyromania isnt destruction - its control. I learnt to respect, but also control fire. From this led to my unending interest in fire extinguishers. From a teenager I knew the classes, colour codes etc. I have already taught Sam how to control a fire, and soon will teach him how to extinguish one!

The workshop is a particular worry, due to the expensive electronic test gear, soldering irons, flammable solvents etc. The two choices are dry powder and CO2. Because of the risk of damage to the equipment, I opt for CO2. Today I finally put my extinguisher into its permanent  fixture on the wall, within very easy reach! (I have another CO2 for domestic use, plus a 1kg powder. I carry a 2kg powder extinguisher as part of my mobile set-up)

I need to go through a lot of the stuff I have and decide if I am ever really going to use it, and if not to dispose of it. Ive already stacked a load of casings/cabinets up to be recycled.

Onto the shack lighting. A strip of LED cool white self adhesive flexible lighting was cut and stuck tot he underside of the top shelf of the shack desk. I just need to solder on the power wires, add an on/off switch, and wire it up to either the shacks solar power system or the main PSU. This three foot strip of LEDs is bright enough to light up the whole desk and most of the rest of the shack.

As I share the shack with the rest of the family (Im in the corner!), and as I had bought the 5m long RGB LED strip just for a bit of a laugh, I also today ran this strip around the ceiling coving, attached the control box and held it in place with a couple of map pins, and ran it to a small 12v SMPSU. These LEDs are enough to light to whole room, and when all the LEDs are on it does in fact result in a general white illumination. But the real fun is with the control box and its little remote! The LEDs can be made to flash in RGB sequence, or to fade in and out all colours in all combinations, etc etc, and the speed or brightness can be adjusted. Ideal for a room the boys play in. I just need to lengthen the power cable and hide it in a bit of trunking.

I still have a fair bit of the cool white LED strip left, I think I will put this in the under stairs cupboard, along with a 7Ahr 12v SLAB and a microswitch on the door! It will make finding my next beer much easier!

USB power monitor

I bought this as my 'frivolous' purchase on ebay. It just sort of took my fancy, and I thought it could be reasonably useful.

Its nothing complex, looking through the translucent blue plastic case, it looks to be a simple VOM/Display Driver chip and a 4x 7-seg LED display block. Im guessing that the current monitoring is done after the supply for the LEDs is taken, as with no USB load it does read 0A.

I think its name of 'Charger Doctor' is supposed to indicate its intended use of checking when a USB port is used for charging a device. I have it now connected charging my Kindle

80-90mA seems to be what the Kindle is taking, and with this load the USB port is running at 5.07v.  I did try earlier with one of the RTL SDR dongles, which took about 110mA, and the voltage dropped to about 4.9v

Yes, fairly frivolous. Yes, reasonably useful occasionally. Worth the £1.99 from the far east, free shipping? Absolutely.

DAB RFI Issue Investigations

Attempt to stop the RFI at DAB frequencies by decoupling and filtering the driver PSU have been ineffective because the RFI is being generated by the LED lamps themselves.

This photo shows one of the MR16 LED lamps opened up. The boost regulator boards can clearly be seen siliconed into the connector pins housing.  This is the source of the problem. A direct DC test was set up as below

A very simple test, nothing more than one of the lamps croc clipped up to a 12v 7Ahr SLAB, with a DAB receiver a few inches away. What cant be seen here, other than the telescopic antenna positioned above the lamp, is the spectrum analyser.

With the lamp powered, there was a definite set of spurii between 250 - 300MHz indicated on the analyser. DAB transmissions are just a little lower than this, around 220MHz (this will have significance for US radio hams wishing to use the 1.25m band!) There was no obvious peaks anywhere else from 0 to 1000MHz! That makes it a particularly awkward problem, as its affecting just this spot on the dial!

Although the peaks were not measured, only observed, the addition across the lamps terminals of a 100nF ceramic capacitor dramatically reduced their amplitude. The DAB receiver, which was almost entirely wiped out six inches from the lamp, was now usable somewhat closer. Only by bringing the antenna within a few inches of the lamp was the RFI apparent.

This lamp has now been refitted into the holder in the bathroom, with its 100nF capacitor wire-wrapped to its pins with minimum lead length. With this lamp in place, and the DAB radio back in its usual location on the windowsill, powering the lamp circuit results in badly mushed DAB reception, but it doesnt now drop out entirely.

The position of the receiver is almost equidistant between the modified lamp and the next unmodified one of the three in this circuit. I anticipate that adding the 100nF decoupling capacitors to the other two lamps, plus the addition of a few snap on ferrite beads to the positive supply wire close to the lamps, will resolve the RFI issue.

Really though, the 100nF decoupling capacitor and the ferrite chokes should have been included by the manufacturer within each lamp. Another case of saving a few pennies in production, at the expense of increasing the RF noise floor and causing unacceptable RFI.

DAB interference from MR16 12v LED lamps

Since installing the 12v LED lighting in both the bathroom and the kitchen, i've been aware that the circuits cause interference at around 220MHz, sufficient to render DAB reception impractical. As VHF FM is not affected by this, I havent been particularly quick in getting around to solving it, but Radio DSO is approaching, and I prefer several DAB stations for my normal listening anyway, I thought it time to have a go at it.

My circuits use 12v MR16 lamps fed with one switch mode LED driver per circuit. It was my conviction that the drivers were the most likely source of the interference. Today, I spent some time adding a 10nF bypass capacitor and several ferrite snap-on beads to the wiring close to the driver transformer of the bathroom circuit.

This has made no difference to the level of interference at all.

Further online research leads me to suspect that, contrary to engineering reason that LEDs are passive and cannot cause interference if DC fed, the lamps themselves may be the problem.

It seems that some of these, and quite likely the ones I use, contain a boost regulator to step the 12v supply up a bit, usually to around 18v. These being a class of switch mode supply, will produce pulses in the area of a few tens of kHz, but these being square waves will produce all manner of interfering harmonics.

What I need to do now, is take one of the lamps and feed it with pure DC (easily done from one of my large SLAB backup batteries), to see what the level of interference is from a single lamp. It should then be possible for me to develop a filtering regime per lamp that should cut the level of radiated and conducted interference.

I will also report the problem to the supplier.


The ADS-B collinear is now in its tube, just awaiting some end caps and sealant. Performance at ground level inside seems poor, but of course that isnt its intended operating position! Once sealed up, that will mark the endpoint of my experiments into ADS-B antennas.

Note to self...

A bit of a design change to the CoCo ADS-B antenna

Change the whip section at the top to a 1/4 wave section.

Use thin (2mm) slices of spare dielectric to from spacers between element section, to prevent the braid of each section from touching.

Cut the end of the feedline sheath back by 1/4 wavelength, remove the braid and fold back the foil shield. This coax (CTF100) uses a copper foil rather than a metalised film. Form the folded back foil into a
1/4 wave sleeve. Remove the excess dielectric and inner.