6m Loop Fed Yagi Project

A few years ago, I bought a 6m yagi from Moonraker and James and I used it on one of our private field days. We found it difficult to get a great match but I thought I’d resurrect the project earlier this year to enter some random Saturday contest that was taking place on 6m.

Unfortunately, for love nor money, although I managed to assemble most of the bits, I couldn’t find the gamma match arrangement in all my Tesco boxes. I needed this because unfortunately the impedance of a three element yagi is well under 50 ohms so unless I went for just a two element beam, I would have to re-engineer things. I recall that a loop had a higher impedance, about 100 ohms. Using a closed loop system with a reflector and a director would bring the loop impedance down, probably by about half (according to the modelling) to achieve 50 ohms. I modelled it and things looked very promising. Wide bandwidth and pretty good gain.

In the end, this became an expensive experiment in that I used up lots of spare parts including a couple of meters of copper tube, an old half wave CB vertical *and* most of the old three element yagi in the process. I managed to slot the copper tube inside one of the sections of the CB antenna and using some 90 degree copper bends (B&Q), I soldered the copper up and jubilee clipped the rest together.

The whole thing ended up 4% too big. Most infuriating. No amount of shuffling the modelling around could replicate this issue as to why but it was resonant at around 48 Mhz not 50.15Mhz as planned. I’ve still not bottomed this out and have recently taken to factor in a 4% difference between computer models and the real McCoy these days. Maybe I should learn how to use NEC V5 (although the last I heard you’re not allowed this outside of USA).

The good news is that it worked. I fired up 100 watts via my TS2000 and I made a number of interesting contacts on the Saturday. Conditions were pretty flat but I still worked several contacts at greater than 100 miles or so and I also recall some EU contacts too. Nothing spectacular – but good fun at only 10m AGL.

The bad news is that the solder joints I made snap and crack under small moments of bending pressure so it’s now scrap. Also beware that for a long term project, copper is relatively heavy for an antenna and in any case, putting copper and aluminium together isn’t good practise. Dissimilar metals tend not to get along side-by-side.