31 Mar 2011

Earth mode VLF tests with E-field probe RX on car

As an experiment today I rigged up an E-field probe on the car to try to do something similar to LA3EQ's earth mode tests of the last few days. Instead of WSPR I used QRSS3 on 8.76kHz putting my usual 5W into the earth electrodes (one connection to the copper pipe ground and one at the bottom of the garden 20m away). The E-field probe consisted of my 1.5m long whip into an MPF102 source follower feeding a 2N3904 emitter follower with roughly the same arrangement as the PA0RDT E-field probe. The probe was attached at the top of a 2m long PVC pipe poking out of the rear window vertically. The actual E-field probe was powered by an internal 9V battery and was mounted about 1m above the car roof.

With the laptop PC running Spectran on the front passenger seat I was able to drive around and glance at the screen whilst moving. In the village there was a reasonable copy of the signal but outside of the village I only managed to get a couple of places 1km apart where the signal was strong enough to copy. One was at 2km and another at 2.1km. Compared with a loop on the ground at the same spots last year the signal was at least 10-20dB weaker. Although I am getting reasonable copy of the 11905Hz Alpha beacon the signal level is around 10dB down on the level on the 80m sq loop at home. So, maybe the E-field probe can be improved still.

In summary, with utilities assisted earth mode, for that is what the propagation is, the car mounted E-field probe is, as yet, nothing like as effective as a loop for RX.

I'd like to rig up some way of driving along with a horizontal loop attached to the rear of the car about 0.5m above the ground. This way I could drive around the local area and log where copy is possible. Presently I have to get out of the car with the loop to take a measurement when doing earth mode tests.

So, a useful test even though the results were not outstanding, or even good :-)

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