Twisted Gamma Match

Attention has turned again at this station to the wonderful and intriguing magnetic loop.

Having been doing some more technical reading, I thought I'd try out a twisted gamma match, brought to our attention by Prof. Mike Underhill probably more than anyone else.

Now, most people who make a magloop use a primary 'Faraday' loop, which couples to the secondary, larger loop.  This works well, especially if the Faraday loop is squashed into an oval, which tends to yield easier matching.

But I was curious whether the gamma match would work as well as - or better than - the two-loop system, although the overall efficiency of the antenna was already superb.

Solder shield to one side of the loop, and the gamma wire to the centre..

So, out came the blowtorch and solder, and off came the Faraday loop!

I connected the shield of the coax directly to the bottom of the large loop, and then soldered about 2 metres of relatively heavy duty (30A) insulated equipment wire to the centre conductor.  I twisted this, with no idea what might work and what might not, until the wire wrapped up half one of the bottom pipes, up one whole side, and a few inches along one half of the top pipe.  Prof. Underhill seemed generally to do much the same, loosely wrapping one whole half of a circular loop with the gamma match wire.  I used a crocodile clip to connect the end of the twisted gamma match to the top rail of the magloop; you eventually solder this in place.

I tuned-up by ear with the magloop's 0-200pF air-spaced capacitor, to pleasantly find the SWR a perfect 1:1.  Running on WSPR at 5W in poor daytime conditons on 20m showed the system was working as expected.  Comparison of my received signals by US and Canadian stations showed I was level-pegging with other transmitting stations known to have very efficient beacon systems.

More tests needed, but the twisted gamma is very easy to do, seems non-critical as to design, and is less prone to being disturbed when knocked or moved than a Faraday loop.  You can find lots of information about loops and matching systems here. 

Update: I've rebuilt the magloop in wider-bore, 28mm copper.  This has slightly changed capacitor requirements, but works perfectly well.  Using the twisted gamma match, I've been particularly pleased with its performance on 40m, where nightly PSK on 10W and WSPRing on 5W reveals an efficient antenna, more especially considering it is working indoors! On 60m, it's more than once been the only antenna getting across to the US, which is remarkable when up against outdoor antennas across the EU!