Today I restarted my earth-mode and induction communication tests at VLF using with my 4W transmitter and 10m separated earth electrodes. After some false starts, I ended up by field testing a simple receiver consisting of 30t of wire 80cms square into an active high pass filter followed by 3 stages of audio gain into a crystal earpiece. 2N3904 transistors were used at each stage. With the loop over my shoulder I set the TX sending 1 second "beeps" at 1kHz and went walking....
Even with 20-30dB of 50Hz rejection it was still an issue in many places. Walking across the field behind my house, and well away from cables and pipes, the signal could be heard in the earpiece at RS53 to a distance of 0.35km. Mains hum and LW/MW interference (the RX is not in a screened box) rather than signal level limited further range. This week I'll add more high pass filtering, put the RX in a screened box and make the whole thing more immune from BC breakthrough. As I can hear this signal by ear at 0.35km with essentially no narrow filtering, then I'm confident that twice as far will be possible with QRSS/WSPR and a PC based receiver. Not bad for 4W at 1kHz.
9 May 2010
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