Message-ID: <189201cb12bd$6cde54a0$7900a8c0@athlon1200>
From: "Dave Brown" <tractorb@ihug.co.nz>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTilgENTZiwZLuNfcfEIiATdr-6mR4dOtJi7OV23p@mail.gmail.com>
Date: Wed, 23 Jun 2010 22:18:28 +1200
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Subject: LF: Re: VLF QRP Earth mode: 5.1km DX tonight but no such thing as a free lunch
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Roger-
As well as pipework, there's a strong possibility for assisted signal 
levels from coupling into buried telephone cables. Longitudinal 
induced audio frequency signals can propagate literally miles around 
copper cable distribution systems, especially where metallic cable 
sheaths are involved. If the cable balance isn't up to scratch then 
audible interference in individual cable pairs can result.
Another possibility is coupling to both underground power cables and 
overhead power lines where similar distances can be covered.

DaveB
ZL3FJ

----- Original Message ----- 
From: "Roger Lapthorn" <rogerlapthorn@gmail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, June 23, 2010 10:10 AM
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<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
>