As it was superb weather today i decided to put some work into the tx system here.

It consists of two tubes 1" round and 1m long in teh ground, the spacing
was a convenient length in the paddock here at 17m.
75 ohm satellite coax (normally used for other uWave tests!) feeds
across the two earth tubes.
I connected my vna across the end of the coax in the shack and measured
impedance of 94ohm at 9KHz.. not too bad.

A spare TDA2002 was put into service for the simple audio amp (seen this
before?!) and an audio transformer ( found in an old motorolla PMR
speaker) was used to step up the output of the amplifier to the earth
'antenna'.
It was not ideal as the output impedance of this transformer was
measured at about 280ohm with a 4ohm resistor on the low impedance end.

My pc was used to drive the audio amp at 8900Hz with on/off keying to
~1W of power into the 94ohm load (about 11v rms)

The transmitter was not connected to any internal house metalwork, only
the two earth rods outside.
If i do connect to the house earth end i get trouble with my switch-mode
psu shutting down!

I received the beeper at 1.1km away in 50hz bandwidth using
spectrum-lab's down converter to 600Hz, it was quite audible, tests at
2km showed no signal but i did not try lowering the bandwidth of the
waterfall to try and identify it.

The receiving end was the 1m multi turn loop with M0BMU preamp, this was
tried facing the TX location and nothing heard, as i had read somewhere
before the loop had to be flat on the ground to hear anything so it
seems that there was no radiated signal.
Getting the loop closer to the ground through the undergrowth made a
large improvement.

More tests to follow soon!

Very interesting this low frequency stuff...

Rob
M0DTS