In the UK OFCOM is not interested in licencing this (and taking your money!), so anyone may make an earth-mode beacon.
For the TX PA a low cost audio IC was used. It produced 4-5W.
Years ago I kept a blog of my exploits. I hope you can get some ideas here.
See https://sites.google.com/site/sub9khz/vlf-using-earth-mode/g3xbm-earth-mode-blog .
I still have my version I built after your article in PW. I could not find anyone local to me to conduct experiments with. I tried an experiment to Rainham marshes in Essex from my QTH in Crayford, in an attempt to see if I could cross the Thames River ( approx 1 mile apart), but it was unsuccessful. I actually got as far as building a 200watt tx using an old hifi unit and a WSPR modulator at 9khz but never got to try it beyond a few 100 yards. my earth system was too short as well.
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