This was designed some years ago, but has appeared in QST and the RSGB book "LF Today".
I wanted a simple way of getting on this MF band and this was the result. Several have since been successfully built.
It produces about 10W into 50 ohms, but the ERP will be considerably lower depending on the antenna used. In my case, my "antenna" was just an earth-electrode "antenna" in the ground with the electrodes about 12m apart with the wire back to the shack along the fence not very high. I tried the wire on the ground and it made little difference. My theory is this forms a loop in the ground. The actual loop diameter will get larger as frequency is reduced. Such an antenna has been used for far field amateur TX tests (successfully) at well below 10 kHz.
Most nights my measured 10mW ERP was spotted in Norway on WSPR. It used the FT817ND in split mode TXing on 80m but receiving directly on 630m.
See https://sites.google.com/view/g3xbm4/home/hf-mf-and-lf/homebrew/630m-472khz-transverter .
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