LF: Sub-9kHz walkabout tests

[Roger Lapthorn <rogerlapthorn@gmail.com>]

This week I've restarted tests using 3-4W into my 10m spaced grounded electrodes at 1-9kHz. 

On receive I'm using an 80cm loop into a Sallen-Key active HPF followed by 3 stages of audio gain into a crystal earpiece. With the TX sending 1 second 1kHz beeps, I went out into the fields behind my house on the edge of the village to see how far I could receive the signal. The signal was still audible at 0.35kms in several different directions, but the limit was set by MW/LW broadcast breakthrough and mains hum, so with a bit of attention I'm sure further is possible. The next test today is using a VLF up-converter (see my website) feeding into the FT817. Selectivity, mains hum and BC rejection should be much better. An attempt at a MOSFET based up-converter design yesterday failed, I think as the LO signal was causing blocking or other desensitisation.

I'm intrigued by what mode the signal is propagating: the loop orientation for best reception clearly indicates induction, so I guess the TX end grounded electrodes and the feed wire (about 1-2m in the air) form a loop in the ground. The signals at this range are stronger than when I tried some years ago using grounded electrode pairs at the RX end too. Putting the loop on the ground, for example, made signals weaker. The tests were carried out in a mix of directions and there is no evidence that cables and pipework were aiding range. I should try a proper loop at the TX end I guess.

It will be interesting to see if range is greater today with the FT817 and upconverter. Also I'll try at 1kHz, 5kHz and 8.7kHz to see if there is any noticeable difference. If successful, I want to try WSPR and QRSS to see how these perform. The range achieved so far was "by ear" listening for a clearly audible beep in the earphone.

This is hardly the great DX achieved by Stefan, but fun nonetheless to experiment with ...and no big kite antennas needed!

73s
Roger G3XBM
 



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