28 Nov 2010

New front-end for 8.76kHz VLF earth mode tests

Today I completed the design of a new "front-end" for my tests on 8.76kHz when using earth electrodes as the RX antenna. Up to now I've used an 80cm loop/preamp at the far end of the test, usually with the loop on the ground for maximum signal pick-up. Now I want to compare results with an electrode pair antenna at the RX, laid across the road to form a pickup loop within the ground around any buried pipework and cables. The electrode pair is about 30-100 ohms, so I needed a low input impedance. I decided on a grounded gate FET amplifier with a simple LC tank circuit in the drain tuned to 8.76kHz fed via an emitter follower into the PC sound card. It has sufficient selectivity to reject 50Hz and lower harmonics and reject the stronger VLF and LF stations which could cause intermod and overload. I've also added a small LC lowpass filter on the input to the FET. This will be field tested at a site 5.3kms from the home QTH as soon as the weather improves: it is currently below freezing day and night!


Hugh Gibson said...

Roger.. I am also playing with earth electrodes. This is not new, and there is lots of information about this mode of communication on the web.. especially cave radio. This works extremely well. It was used in WW1 trenches too. There is a cave radio magazine (CREG) with some interesting articles. Clearly, matching the Tx is not easy as the impedance can be surprisingly capacitive. Increasing the effectiveness of the electrodes with salt/soot etc.. also helps.
for extreme VLF (<2KHz) it is not good (for example you cannot see Schumann resonances easily) but for 2-20KHz+ it also works well. Filtering the 50/60Hz is the main problem. G6AIG

Roger G3XBM said...

Hi Hugh, Yes nothing new but fun to try earth mode with very narrowband software receivers as this opens up distances that would not be so easy, especially with QRP TX powers as I am using.