Adding a balun

I was not happy with wiring coax directly to the centre of the dipole, so decided to make a variable balun as an experiment. I expected I would be using some twin feeder for future antennas so thought that a set of impedances might be useful.

I decided to provide 50 ohm, 200 ohm, 450 ohm and 600 ohm on the antenna side, with a 50 ohm winding for the unbalanced coax side. This is what I believe is known as a "voltage" balun. I decided on a separate winding for the radio side mainly because all the stuff I've read doesn't, so I thought I'd find out the disadvantages.

I got a ferrite 140-43 toroid off ebay (GBP 2.95) and decided on 8 turns for the 50 ohm "tap". Since I wanted a separate unbalanced winding I actually ended up with 5 windings twisted together to make a 5-filar (quintifilar?) wound transmormer. One of the winding stopped halfway at 4 turns. By wiring the antenna sides in series I got turns ratio of 1:1, 2:1, 3:1 and 3.5:1, giving impedances of 50, 200, 450 and ~600 ohms. These I wired to some 4mm sockets (does anyone call them banana plugs and sockects any more?) so I could easily try different antenna connections. The other winding went to the coax SO239 socket.

I put it in a plastic box to make the assembly easy and to make it vaguely water resistant as I intend it to be outdoors, though semi-under-cover. I decided that I was likely to get RF radiation from the twin feeder connection anyway, so hope the extra radiation from the wiring will not be significant. I am worrying a bit about what might happen if I start putting significant power down it, but since my first experiments will be QRP I am not too worried. I can rebuild it into a metal box if it ever seems necessary.

Anyway, I wired a short length of wire to the dipole (a sort of random impedance feeder), plugged it into the 50 ohm antenna side and checked out the rx. I was amazed to find that reception appeared to be much better. Difficult to quantify but I'd guess about 6dB better (1 S point). I tried different taps and got little difference, and I settled on the 200 ohm setting. I am pretty sure the QRM from the shack PC got better too, but this might just be because that's what I've been told to expect when the coax outer is not feeding interference back up to the antenna.

All in all, it was an interesting first build item. Well worth it. And I am ready for a better antenna, as I can use twin feeder to the balun outside the shack, run it through the balun and into the house on RG58 coax.