*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<http://sites.google.com/site/g3xbmqrp/Home/earthmode/lordsgrdfarm%28CW%29.wav?attredirects=0&d=1>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<http://sites.google.com/site/g3xbmqrp/Home/earthmode>.
73s
Roger G3XBM
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM GQRP 1678 ISWL G11088
