Re: LF: Re: Re: G3XBM - very long carrier VLF transmission test Dec 15th

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Chris/Mal et al,

Certainly all my tests to date have been with what the US military calls "earth mode". This is propagation through the ground primarily by conduction and induction. What puzzles me, and some others, is that reception at a distance of 1 to 5.6km has been better using a loop antenna at the RX end rather than another pair of earth electrodes, suggesting the induction field is the stronger component. I am not an electromagnetic theory man and my physics is very rusty, so please excuse my ignorance here again.

Mal is partly wrong about the use of earth electrodes. These have been used by the US military for VLF/ULF/ELF radiated communication with submarines worldwide. They have also been rejected as not secure enough for purely through-the-ground communications between buried bunkers because of the radiated component in the air! 

Where a very large "in the air" antenna structure is not practical, I suspect the results from an earth electrode are not that far down: on 136 and 500kHz even my small 20m spaced earth electrode antenna is less than 8dB down on the best "in the air" antenna I have so far managed to erect in my tiny plot of land. 

My unanswered questions are:

1. Exactly what "helps" earth mode signals get as far as they do? I find it amazing that just a few watts into my system is detected so well so far away (5.6km best DX, but further is certainly possible in some directions yet to be checked).

2. How much signal is radiated in an earth mode set-up? With a signal that is probably induced into overhead cables and conducted along miles of pipework and cables in the ground, railway tracks and even natural structures like water courses and rocks, then surely a tiny proportion must be radiated. OK the radiated signal may be minuscule, but we have already seen that with very slow QRSS and the power of Spectrum Lab and similar it is possible to detect these weak signals at a considerable distance. The attenuation of the radiated component is far less than the induction field component (6dB rather than 18dB attenuation for double distance I believe to a first order). Does anyone know how to calculate the amount of RF radiated in an earth electrode antenna of given dimensions?

3. Assuming a very small amount of signal is radiated, what are the limits for amateur communications using earth electrode structures? With 100-200W and 600m long earth electrodes nearly 50km has already been achieved in Germany.

So, OK I'll be very lucky to be heard more than 10km away by anyone on Wednesday, but I have already been surprised many times on VLF/LF and it is not totally beyond the bounds of possibility that, with QRSS6000 or similar, detection at a moderate distance may be possible with a stable signal source (mine is questionable I know) and suitable weak signal detection techniques.

I continue to promote earth electrodes because, for me, they have worked. I do not deny that a large and well engineered vertical antenna would be better, but for many of us this is simply not an option. 

Investigations, and the fun, continue...

73s
Roger G3XBM

On 14 December 2010 13:48, mal hamilton <g3kevmal@talktalk.net> wrote:

Chris

Generally speaking if you want to propagate a LF signal that covers hundreds of Kilometres use an elevated Aerial and a good ground system.

The earth electrode TX and ground wires system is only intended for short distances and received using the same method, howerver there will be RF leakage and signals will also be copied  at short distances using an elevated aerial but maybe copied further afield when the TX station is using several hundred watts or Kilowatts.

 There is a lot of misleading and spectulative information about regarding earth electrode antennas. What some are using for so called antennas I am using as a ground for my elevated antennas.

g3kev

 

 

----- Original Message -----

From: Chris

To: rsgb_lf_group@blacksheep.org

Sent: Tuesday, December 14, 2010 12:18 PM

Subject: LF: Re: G3XBM - very long carrier VLF transmission test Dec 15th

Hi Roger,

I was under the impression that your earlier tests had shown that reception from your earth electrodes at any distance was only possible by also using earth electrodes. This suggests that those of us using an aerial stand little or no chance, correct?

From what I recall of tests done forty or so years ago with friends, using wideband audio into the ground, any form of 'in air' reception attempt was seriously bad. The reception of a signal injected into the earth as the conduction medium also required the receive system to be in the same medium i.e. the earth.

Do prove me wrong...and good luck with the tests.

Vy 73,

Chris, G4AYT, Whitstable, Kent.

----- Original Message -----

From: Roger Lapthorn

To: rsgb_lf_group@blacksheep.org ; Nick Swales ; Chris Osborn ; Gary - G4WGT ; vlf0308@abelian.org ; paula.taylor@metoffice.gov.uk

Sent: Sunday, December 12, 2010 10:51 PM

Subject: LF: G3XBM - very long carrier VLF transmission test Dec 15th

This Wednesday, Dec 15th, I will run tests on 8.7608kHz with very long continuous TX periods of several hours interspersed with gaps of 0.5 to 1 hour. Transmissions will start at 0900GMT and finish at 2300GMT. I will reveal the TX pattern after the test so the test is done "blind".

I'd be most grateful if suitably equipped stations could look for any possible signs of the signal, even though I shall only be using the 20m spaced earth electrodes and 5W as used for my local earthmode tests. Those able to screw the bandwidths right down may just be able to detect something, although I think it is unlikely at any great distance.

15.12.10    
0900-2300z 
8.7608kHz   
Carrier "on" periods of 30mins or more

Good luck to anyone having a go.

73s
Roger G3XBM

--
g3xbm-qrp.blogspot.com/
www.g3xbm.co.uk
www.youtube.com/user/g3xbm
G3XBM   GQRP 1678    ISWL G11088

-- 
g3xbm-qrp.blogspot.com/
www.g3xbm.co.uk
www.youtube.com/user/g3xbm
G3XBM   GQRP 1678    ISWL G11088