8 Sept 2013

Earth-electrode antenna for 472kHz in the new garden - not too brilliant

After trying a number of different orientations of earth-electrode pairs in my garden, I have come to the conclusion that this antenna structure does not work very well here in the new QTH. I am a little surprised as the earth-electrode antenna worked well just 350m away at the old QTH. Whatever orientation I try I seem to be always in the -22 to -28dB S/N region with G6AVK 78km away.

Why is it not so good? Well, the most likely answer is that the effective distance between the far and near ground rod electrodes is smaller, even with a similar physical separation. This could be because of buried water pipes and utilities criss-crossing the garden, although I wasn't aware of any, or different soil/rock conditions, although the chalk layer is about as far down here as at the old QTH and the soil is a similar alkaline sort. The fact that the 8.97kHz earth-mode results were so poor last week rather hinted at a problem that was not present at the old QTH.
I haven't quite exhausted every layout possible in  my strange shaped back garden, so there are more things to try with earth-electrodes, but it is looking like I shall have to erect a short Marconi or a fully in the air loop antenna to get better ERP on 472kHz. On my previous tests at the old QTH, a full in the air loop with 1mm diameter wire and an enclosed area of around 80m sq was around 8dB better than the earth-electrode antenna, but far more critical to tune and match because of the hi-Q. If my earth-electrode antenna is several dB down on the performance of the same structure at the old QTH then I could be expecting >10dB improvement or even more.

Half the fun is the testing and finding out what does and does not work.  The coming week we have visitors staying, so not too much time to experiment. I shall have to stick to WSPRing on 20 and 10m.


Dick said...

I'll be honest and admit to my total ignorance of this methodology. However, I would suspect that there is a lot more "going on" below ground than "above". I guess you have to take the extra factors into consideration. However, if you were looking for easy communications, I suppose you would be a big fan of 80M rag-chewing. Enjoy the challenge.

LA4ANA said...

Well, as you noted in an earlier post, you have a lot less copper to connect your earth to, compared with your former QTH. I guess the copper helped quite a bit.
I would strongly recommend to erect an overground loop again. Not only to improve your ERP, but a loop is a quiet rx antenna, less prone to electric field noise from domestic sources. Good luck at your new QTH!

John G4LRR said...

Disappointing but interesting. Seems to prove that the earth currents produce the radiated signal. Seems reasonable as ground waves rely on the currents they produce in the ground. I wondered how much the current in the wire contributed to the signal but it looks like not much. Maybe there is a big metal tank under the garden. Would be interesting to find where that pipe goes. If there is a tank I would have thought there must be a hatch somewhere to fill it.