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.