2200m Wspr From A Small Garden - Danger, Fire Hazzard. Do Not Copy!

Of course after a while even 630m became a little too 'easy'. Perhaps radio amateurs - those who really stick at it - are actually masochists. I looked at the final 630m coil I'd made with a former of a small pond and decided, you know what that coil is titchy small (to quote Roald dahl). What I really needed was a coil with hundreds more turns and which stood two metres from the ground. This was constructed from several formers (the aformentioned pond, and several huge garden pots fixed together with cable ties and 10mm bolts. It took about 200m of PVC covered flex weave to make the coil. Given that the wire costs about £1 a metre I have only now realised how expensive it all was. But...

Prototype 2200m coil

Usig the Rigexpert AA-30 which (thankfully) will go down as low as 100khz I managed to fit a coil to the end of the same wire I used for 630m and have it resonant at 0.137500mhz give or take a hz here or there. This is the Wspr TX frequency for 2200m.

Part of a later coil

2200m Variometer

But what to use for a radio?

Well this was more difficult, but also much simpler. More difficult as I did not have anything I could use to transceive on this band. I also did not want to build another G11, too expensive, or a transverter until I was sure there was any chance of success. Fortunately I did have one of the most useful devices ever put on the earth for radio hams. I am talking about Hans Summers' U3s. I had in fact built several of these over the years and consider Hans a friend such has been our communications over the years. Who said data modes are not sociable?  I knew Hans could provide an LPF kit for the U3s so I ordered one, made it and fitted it to the U3s. Unfortunately the output from the U3s was just too small at these frequencies. The losses in the huge coil must be very high. I needed some more power to enable me to tune the coil for efficiency. Again I plumped for the impedance transformer approach. My SWR meters were useless down here. What to do?

An Ultimate 3S

I found a circuit online for a simple single FET amplifier and soon had it built. I also built a larger LPF as I suspected more power would be required in time. Driven by the U3s I suspected I was seeing about 20w into a dummy load. I used the power meter on my Welz SWR and tuned for maximum forward power. Transferring this to the tapped coil of the antenna it was possible to adjust the tapping point to see something similar. I had a 'tuned' antenna, a Wspr transmitter, an amp with LPF all I now needed was darkness and all would be revealed...

Well not quite. It was Autumn. I transmitted for days/nights and nobody spotted me. I knew there was a signal as I could spot myself, I just did not know how much of a signal. Eventually autumn headed for winter, the clocks fell back, and, with the aid of a LF converter I was able to use the antenna to receive my first LF Wspr spots. G8HUH being among the very first as I recall. Then one day, it was a Sunday, and it was morning about 11am I was spotted for the very first time by a station about 50 miles away in Wales. This made the MF spots seem like a piece of cake, now I was hooked.

The next thing I found was a WW2 NOS RF Ammeter in USA. I remember the theory from my RAE but had never used one. I ordered it. It is a lovely thing and arrived a couple of weeks later. I was disappointed to see such a low amount of current (under 0.7 amps) from my homebrew amp (using IR squared from the RAE it is less than 25w). Given the antenna was probably <1000th efficient it is surprising my signal was spotted at all. It was obvious more power was required if I was ever getting beyond the UK.

Lovely RF Ammeter by General Electric

Doing some more 'research' I came across an old thread which talked of using FET audio amplifiers for LF RF. It seemed there was a UK amplifier with specifications beyond 136khz. I managed to track one down on eBay. When it arrived I was disappointed to find it was dead. It turned out the connections from the autotransformer inside had been cut and the fuse was blown.

First Audio amp - BK CA110 (mono)

 I repaired the connections and replaced the fuse then stood well back. Fortunately all was well. I now had a possible 100w at 136khz if i could figure out how to match it to the antenna.

Bigger Audio Amp = more current

The answer was large ferrite rings and transformation of impedance. It was easy to match the input to the amplifier the U3s did not seem to mind. The output of the amplifier was something else. Many hours of trial and error and eventually I got to see about 1.25 amps into a (big) dummy load on the RF Ammeter. With a few adjustments to the homebrew variometer I was able to replicate this current into the loading coil of the antenna. And VOILA! my first spot into Holland, my U3s had reached as far as Europe. There were far fewer receive stations back then (18 months ago!) and so it took a while, but gradually I was spotted as far as France.

[I learned a lot during these (expensive) experiments. Impedance transformation is all about efficiency. Mismatched impedance simply would not allow the amplifiers to transfer power. Of course I did not just start making these devices I used the information supplied by others for free on the internet. As I find the links (if they are still active) I will add them to this page at the bottom.]

Then I saw a larger amplifier on eBay at a very reasonable price. It was cheap because one channel was down. However it was a 600w unit so I reasoned, even if I could not fix the 2nd channel that still left 3x the power. So of course i bought it. And yes it did deliver 3x the power. In fact as I managed to fix the other channel I was able to get both channels via a centre tapped transformer to feed the antenna. I was now seeing >3amps (3 squared x 50 = 450w?) into the coil. I managed my first (and last) 1000km spots into the heart of Europe. Brilliant!

One channel of the (stereo) BK Electronics MXF 600

Not quite.

This was the cause of my first and last antenna fire. I know I am not the first and in my case I was lucky, it was only the tree which went on fire. And it soon extinguished itself as it rained. But I have a wife and kids, a night time fire was not a risk worth taking. I had seen what other people had reported and decided enough was enough. Also we were due building work which entailed the cutting back of the tree (it survived) and so now I cannot put enough wire out even for a decent 630m signal. So all was packed away. Which leaves me only with my memory of those fun adventures on 630/2200m bands. Now the building work is complete. The amplifiers, coils, variometers are stored away... so who knows, one day perhaps? Actually I do know. Been there, done that. As it says at the top, do not copy. You can put out a 2200m Wspr signal from a small garden. BUT as the wire radiating element has to be shoe horned in fire is an ever present risk at this frequency and at the input power required.

Quote from ON7YD:
Another effect of the high antenna voltage can be the corona effect. This is an electric discharge resulting from the ionization of the air and occurs mainly at sharp ends and edges of the antenna. The corona effect should be avoided as it is a loss of power and, even worse, the arcing eventualy can start a fire. The corona effect is rather seldom reported by amateurs on LF, from the information I received the antenna voltage was in excess of 30kV in all cases. The corona effect can be surpressed by avoiding any sharp ends or edges and mounting corona rings at the ends of the antenna
Of course if I lived on a bigger plot, I'd be back on there in a flash - metaphorically speaking!