LF: More earth mode DXing today (5.6kms)

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Earth Mode DX up to 5.6km (utilities assisted, 4W TX)

Today I resumed my QRSS3 VLF earth mode tests using 4W at 838Hz into 20m spaced earth 
electrodes. The aim of the test today was to determine the absolute 
maximum range possible by utilities assisted earth mode. Best result was
 5.6km along Commissioner's Fen to the west of Burwell, the home 
QTH. Signals 
we just copied at this distance using the 80cm, 30 turn, loop and preamp
 
into the PC running Spectran (see picture from the Spectran screen). A test at 6km resulted in no detectable 
signal.
At one point about 5km from the QTH the signal was so strong that I could 
copy it when some 20m away from the road (presumably with buried pipes) 
or when holding the loop high in the air vertically orientated above my head! Another possible
 clue to the mode of propagation was fenland drainage water channels. 
Signals always seemed to be somewhat stronger when close to these. 

So, 
conclusions today are:

(1) The propagation is definitely "utilities assisted" conduction apart from the last few metres of induction into the loop. Even so, I'm amazed that the signal can be seen so clearly on Spectran when running QRSS3 with such low power (4W) to such a small earth electrode antenna with just 20m spacing. 5.6km is a very long way from my QTH.
(2) Even at 2.4kms from the home QTH, the signal induced in the ground via buried pipes was stronger than any detected by pure induction. I tried various loop orientations to see if there was any "pure induction" signal, but I was unable to see any.
(3) There is weak evidence that at the more distant points some assistance may be being given from water channels criss-crossing the wet fenland. This needs further testing, but I suspect there is something in this.
(4) There is no evidence that overhead electricity cables are helping: when orienting the loop for best coupling into these the signal actually decreased.

Further work:

(1) Try using a larger loop at the RX end. I have in mind a 3 or 4 turn loop some 5 x 5 metres that I can lay flat on the ground. This will allow stronger coupling into the loop from any buried utilities. Also, with a small loop (80cm) it is a bit "hit or miss" where this is placed to get decent coupling when signals are weak. Also, a simple wire loop can be left in the road when cars go by, whereas my 80cm loop on a wooden frame always has to be moved...or be crushed.
(2) Try using an electrode pair antenna at the RX end. I'm thinking of straddling the roadways or drainage channels with this to see if the received signal is stronger and can be detected further from the utilities.
(3) Increasing the TX power. 100W should allow more chance of communication over moderate distances with less dependence on buried utilities or by pure induction (or even some radiation). I need to get hold of a 100W amplifier.
(4) Try an E-field probe antenna at the RX end using an earth electrode RX antenna.

All good fun again after a short break. 

Early next week I'm hoping to run some QRSS3 tests on 137kHz using 1W into the same earth electrode antenna. I am only expecting reception in the next village but I will let people know when I'm testing just in case. Work on a real 136kHz transverter is about to start as is work on a credible 136kHz antenna in my very small garden. With this I hope to do some QRSS and WSPR tests starting in the autumn on 136/137kHz.

73s
Roger G3XBM
-- 
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http://www.g3xbm.co.uk
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G3XBM    GQRP 1678      ISWL G11088

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