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Roger,
Why not try ros mf-1 into the
ground, that's based round 1 Khz ? no need for a
time locked pc ?
G ..
Sent: Tuesday, June 22, 2010 11:10 PM
Subject: LF: VLF QRP Earth mode: 5.1km DX tonight but no such thing
as a free lunch
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
- 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)..
- 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.
- There is no evidence of induction playing a real (long distance) part in
propagation at 5.1km distance.
- 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.
- 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.
- "Utility coupling" aided VLF earth mode communications could be very
viable over a range of at least 5kms.
FURTHER WORK
- Try an E-field probe at 5km and greater distance. It should reject 50Hz
noise more and may work as well as the loop?
- Try to establish the limit of true induction communications unaided by
ground conduction paths.
- 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
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM GQRP
1678 ISWL G11088
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