Mobile 8.977kHz VLF loop tests started

Today I started to do my tests on 8.977kHz using my 5W earth mode transmitter at home but using a mobile loop antenna on the car connected to my PC via a tuned preamp. The idea is to be able to drive around and measure signal levels with Spectran software whilst actually on the move.

30t 80cm loop mounted behind the car

The loop was mounted behind the car in such a way that it would detect any ground propagated signals. The loop is about 10cm off the ground.

A drive test to Swaffham Bulbeck was carried out and signals were detected more or less continuously out to 3.5km from home before they disappeared in the noise. Bandwidth used was 0.18Hz with a continuous carrier. Although coverage was as I anticipated, signal levels were not as great as when the loop was actually laid directly on the ground. At one of my usual test sites 3.5km out in the fens there was no copy with this loop arrangement yet there was a decent signal copied with the loop on the ground a few days earlier. I need to do some direct comparisons between the loop on the ground, the loop mobile mounted 10cm above the ground and with the E-field probe on the car roof. Initial indications are that the difference between the EFP and the mobile loop is probably no more that 5-6dB.

Signal received with Spectran and the mobile loop