Hello Dmitrijs, If you use the search engine for "2200m" you will find my posting giving the components and turns for the QDX-M operating at 137.5 kHz, plus a url to a reflector with pics of the finished board and a SOTA summit-to-summit contact. Since then we are using the QDX-M with Android phones as the terminal device (Radiomsg is the app), used them in cave-to-surface ops, and have achieved a range of 6 km on the surface using WSPR. The antennas in use are 1m loops that have an effective radiated power of about 0.4 microWatts!.
I have also published a cave oriented article in the Cave Radio and Electronics Journal, No.124 () that gives more details.
Good luck on the LF and MF bands.
Ian VE6IXD
