Construction of a 23cm double bi-quad antenna


I'd often heard John MW1FGQ extolling the virtues of his bi-quad antennas, but when he sent me this picture of his latest "mobile" version, I decided that I really must try one for myself.

MW1FGQ/M with a bi-quad

But I wasn't thinking of a compact "mobile" version, so when I found this information on-line, I decided to start with a double bi-quad design.

I had half a spool of 2mm enamelled copper wire left over from my 12-turn helix, so this was soon bent into shape to make the "element". It's fed with a short length of RG-223 co-ax, fitted with a balun sleeve made from the braiding off some UR67.
Then I found a piece of aluminium plate for the reflector, but it was a bit too small, so I augmented it with some lightweight galvanised fencing wire (doubled over for extra strength, and to reduce the mesh size to 0.5 x 1").

The bi-quad feed The prototype 23cm dual bi-quad

The element is supported above the ground-plane by three plastic tubes, with the feeder threaded down the centre one.
The final construction did end up a little heavier that I'd originally intended (due to the wooden support which I built for the lightweight mesh reflector) but it's still light enough to go up on my 9m wobbly mast.

The completed antenna mounted for horizontal polarisation

The benefits of this antenna are its relatively simple construction and broad beamwidth. Within its first day of testing, I was able to manage a comfortable 3-way QSO with John MW1FGQ in North Wales and Andy G1HBE in Manchester - I'd never managed to find a common beam-heading for the two of them with any of my other antennas!

So the next logical step is to make a vertical stack of two of these, to deliver even more gain with the same beamwidth (but a more compressed vertical pattern). Watch this space...