1 Feb 2011

VLF earth-mode success: G6ALB copied at 3km on 8.76kHz

Spectran trace of G6ALB on 8.76kHz at 3km
This evening G6ALB fired up his 8.760kHz earth-mode TX at his home QTH running QRSS3 and 40W into earth electrodes spaced 25m apart. We started listening outside his QTH with a colossal signal, moving to the next village (Reach - about 2km) where the signal was still good copy. Finally we parked up outside my QTH in Burwell exactly 3km from Andrew's TX. Signals picked up in the road with my 80cm portable loop into the tuned preamp were still strong - at least 20dB S/N on QRSS3 (see attached). For some reason I was unable to copy Andrew's signals on my own earth electrode pair: I may have had a fault and need to check why, as I was expecting this to give the best result. Andrew's signal was so strong on the 80cm loop that 12 wpm CW would have been copied. His keyer and 8.760kHz signal were generated using "Direct Digital Synthesis" from a 25 MHz clock using my Digilent Spartan S3 FPGA board.

This is the first time I've received another amateur by earth-mode (conduction). The result suggests much further is possible. Experiments continue!


G6AIG Hugh Gibson said...

I have thought about similar Tx into earth probes. The Earth impedance seems to be very capacitive: What match do you see, and what transformers/matching network seems best? Have you also improved the earth around the spikes, with soot/salt etc? Interested..

Gary - G4WGT. said...

If you do find that your earth electrode system is working OK it would suggest that Andrew's signal was stronger radiated than conducted.

Roger G3XBM said...

Best receive results were with the 80cm loop lying on the pavement. Moving the loop about 3-4m into the front garden from the road resulted in a drop of about 20dB in signal level. Why there was zero copy on my earth electrode "antenna" (used to achieve my own best DX of 5.6km) is totally puzzling as the DX from G6ALB is too far for the signal to be radiated. I had assumed the signal followed the pipes in the road, but in that case the earth electrodes, one of which is directly connected to the mains pipework, should be by far the best.

Gary - G4WGT. said...

OK, you didn't clarify the loop was placed on the ground. I can now understand your reasoning that the ground electrodes should have been better.

Andrew G6ALB said...

> and what transformers/matching
> network seems best?

Currently the TX set-up at G6ALB is a nominal 100 watt AF amp (using the Maplin Mosfet design) driving my ground system approx 25m from the house, via two parallel 3.9R power resistors. There’s no transformer.

I’m feeding against the house mains earth and the heating system.

At the power level set the amp output is 24v RMS and the voltage in the load is 20v RMS. There appears to be no obvious phase shift.

I did notice the drop across the load was different at 1 kHz. I need to quantify that.

Current is therefore ~2A which means the load + feeder is 10R. The feeder consists of two x 1.5mm [insulated] copper conductors and the screen of the companion RG213 coax. Normally this set-up is part of my HF station :)

The ground system is five x 1.2m 10mm dia earth rods, a couple of other rods and about a square meter of galvanised steel mesh buried 30-50 cm under the ground.

This arrangement is all gridded with more of the 1.5mm wire.

Roger G3XBM said...

This evening during further tests looking for G6ALB's signal on 8.76kHz I discovered that best results receiving at my QTH 3km from Andrew were with a small E-field probe rather than a loop or earth electrode pair. With the EFP signals could be detected in my shack with just a 19inch whip! This suggests that the electrostatic component is the dominant one rather than magnetic or simple potential difference measurable between the 2 earth electrodes.