LF: VLF QRP Earth mode: 5.1km DX tonight but no such thing as a free lunch

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Second try at this post (without picture that may have been too large)......

Further tests today to work out how far a signal on 838Hz will propagate through the ground or air and how this happens.   First thing I tried was to go along the same country lane to the south of the village as last evening at a distance of 4.2km (0.6km further than last night). Nothing, zero, ziltch, and disappointment.

So, I then returned
 to 
the very same spot as last night (3.6km) and sure enough the QRSS3 
signal was 
clearly visible on the Spectran display. This was with the loop pointing
 home across the fields. Then the crunch point: I put the loop on the
 ground flat on the road and the signal went up. So, this was pretty
 conclusive proof that the signal was being aided by pipework in the 
road. To check this, I moved 50m into a field and away from the road and
 tried listening/looking for the QRSS3 signal with the loop both flat on
 
the ground and facing home - nothing at all. This was conclusive proof 
that, at this distance at least, conduction through pipes was helping 
the signal reach this far.

Next test was to go to a different 
roadside location some 4km west of the QTH and try the same test again. 
Nothing copied at all, either with the loop vertical or horizontally on 
the ground. I repeated the same test on the way back towards home about 
0.5km west of G6ALB's QTH, but again nothing seen.

Before coming 
home, slightly disappointed, I ventured up along a fenland road where 
the soil is really dark peat. To my surprise, the signal was copied 
again at good strength on the roadside 4.1km west of the QTH with
 the 
loop facing the QTH. Again, I repeated the earlier test and placed the 
loop horizontally on the ground. The signal was SO strong that the 
Spectran trace turned red and I could clearly hear the QRSS3 signal by 
ear!  

A drive up a road called Lord's Ground Drove resulted in 
the signal still being copied well just beyond Lord's Ground Farm at a 
distance of 5.1km. At this point I phoned my wife at home and 
asked her to switch the TX over to 10wpm CW and the signal was recorded 
with Spectran and clearly audible with the narrow filters in. Using the 
crystal earpiece instead of the PC I was not able to hear the CW in the 
50Hz noise. Listen to this recording at 5.1km from the TX but 
use narrow filters around 838Hz to hear the 10wpm beacon signal clearly.

CONCLUSIONS

1. At local distances (up to 0.5km) the signal propagates by both conduction through the ground (aided by pipes in the ground) and by induction (proved by the orientation of the loop when well away from roads and pipes)..
2. At greater distances (3-5kms) the signal reaches its destination by conduction through the ground aided by pipes in the roads and with inductive coupling into the loop at the RX.
3. There is no evidence of induction playing a real (long distance) part in propagation at 5.1km distance.
4. When the signal is being "utility coupled" the direction with respect to the TX earth electrode pair (and loop formed in the ground) is not important. What matters more is whether there are metal pipes between the TX and RX locations; if these are not present then the signal will not be received, I think, at any great distance. There is some possibility that coupling may also be possible as a result of overhead cables, but this has not been tested.
   
5. Even allowing for the conduction aided by pipes, I am amazed that just a 4W TX into a 20m baseline earth electrode pair can be copied so well at 5.1km (and probably considerably further), and even more surprised that 10wpm CW copy was possible at this range.
6. "Utility coupling" aided VLF earth mode communications could be very viable over a range of at least 5kms.
   

FURTHER
 WORK

1. Try an E-field probe at 5km and greater distance. It should reject 50Hz noise more and may work as well as the loop?
   
2. Try to establish the limit of true induction communications unaided by ground conduction paths.
3. Sea coast earth mode test with one electrode in the water and the other at the TX end, say, 100m inland from the water's edge. Just how far would you be able to receive the signal along the coast using another electrode pair or an induction loop?
   

So, some useful further tests, but some disappointment. "There
 is no such thing as a free lunch", as they say.  For real VLF work 
Stefan's true radiation experiments are where the action REALLY is.

All good fun and I've learnt a lot. There's a screen shot of the received QRSS3 signal (strong!) plus the recording of the 10wpm CW signal at 5.1kms on my earth mode website .

73s
Roger G3XBM


-- 
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G3XBM    GQRP 1678      ISWL G11088