Matching the earth-electrode "antenna" at 472kHz

I get asked how I match my earth-electrode "antenna" at 472kHz. Well, it depends on your ground resistance. At my old QTH I just connected the transverter directly to the earth electrodes as my ground looked like somewhere between 40-60 ohms resistive. I used an audio oscillator and looked for half voltage by adjusting the series resistance. At the "new" QTH the earth resistance is much higher so I added a 3C90 step-up 42mm diameter transformer and adjust the tap on the secondary for maximum antenna current. I am sure the purists will have a good laugh, but it works. Here is a photo showing the arrangement. I am sure the purists will ask how did you determine the windings? In good amateur fashion I experimented and found what worked. Incidentally, I tried the earth-electrode "antenna" on 160m and on the first night was spotted in Sweden! I think it works as a loop in the ground and would be bigger at lower frequencies. At 137kHz it should form quite a large loop in the ground, although I have not tried this band at all at this QTH. I got the 3C90 toroid from Farnell a few years ago. I assume these are still available.

3C90 matching transformer and antenna current meter for 472kHz