|Click circuit to enlarge|
If you are starting up on the band you may want to look at my TX transverter. With my 20m spaced earth-electrode "antenna" in the ground this was regularly copied by G8HUH some 250km away on WSPR. The ERP with this "antenna" is only about 30uW. Sadly, the FT817 is deaf at 137kHz.
As you will see, it is very similar to my 472kHz design although this is a transmit only design. It is designed to be used with the FT817 at 500mW.
See https://sites.google.com/site/g3xbmqrp3/mflf/137khz_tvrtr .