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Re: LF: Earth Electrodes

from [Markus Vester] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: Earth Electrodes
From: Markus Vester <[email protected]>
Date: Tue, 14 Dec 2010 13:44:20 -0500
In-reply-to: <[email protected]>
References: <80E0204D3E534137903FE0B8E499597F@IBM7FFA209F07C><[email protected]><C38D87D2AED243D59ED8357663143FF4@IBM7FFA209F07C><[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Dear Roger, Andy,
 
interesting thread - let me state a few thoughts here:
 
- as far as I understand, the purpose of the dual array at the Clam Lake facility was to avoid the nulls in the coverage pattern. In effect, they used two orthoganal loops fed in quadrature.
 
- even at 76 Hz, the skin depth in ocean water is only about 26 meters. Despite that the external noise is being attenuated as well, and that sensitive trailing receive antennas could have been employed, I still think that the signaling was limited to a couple of hundred meters depth.  
- the electromagnetic fields do not really propagate "through the earth", at least not very far. The skin depth in "normal ground" at 9 kHz is only a few tens of meters, and even low conductivity rock has a large attenuation. The farfield radiation happens only by coupling of the subsurface current loop to propagating waves above ground.
 
- in principle a wide spread array of transmitter stations could be used to increase fieldstrangth at a given point. But if you start looking at multiple wavelength arrays you will get unwanted directivity, and would have to steer towards a single target receiver. A much more worthwile effort would be a large receiver array. This can be easily done with small antennas, soundcard recording with GPS injection, data collection via internet, and a posteriori software focussing. One could even focus on multiple transmitters simultaneously.
Best regards,
Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: Roger Lapthorn <[email protected]>
An: [email protected]
Verschickt: Di., 14. Dez. 2010, 18:14
Thema: Re: LF: Earth Electrodes

Andy,

Surely two synchronised TX systems can only increase ERP by 3dB maximum? ...or am I missing something?

Yes, 600m baseline is similar to Sanguine pro rata with frequency and that system got a megawatt TX output signal around the world and to a considerable depth in the ocean with 100% reliability. Luckily we don't need to reach the ocean floors, so our ERP can be somewhat lower (!). A 600m baseline would not be that difficult to arrange for a portable test out in the countryside or across National Trust land or woodlands.

73s
Roger G3XBM

On 14 December 2010 16:54, Andy Talbot <[email protected]> wrote:
Google "Project Sanguine"
 
  http://www.fas.org/nuke/guide/usa/c3i/fs_clam_lake_elf2003.pdf   suggests more than 60km of cable used at 76Hz, so pro-rata not dissimiliar to 600m earth spacing at 9kHz
 
However, they do use two synchronised transmitters, so the effective baseline becomes far greater.  
 
Now, how abot that for 8970Hz.   Two stattion using low cost simple GPSDOs and similar DDS / synthesizers can reasonably guarantee their respective frequencies accuracy to 10^-9 short term  At 10kHz (to keep numbers simple), that is a differnece freq of no more than 10^-5, or 100000 seconds, or one cycle in just over a day.  And over this period, GPS derived sources will average out as spot-on.
 
So, for low cost simple amateur coherent wide spaced VLF signalling frequency is not an issue.    SIgnal element timing can be synchronised to sub microsecnds, again using GPS timing.
 
Maybe worth thinking about.   Reading the posts so far, 'JNT interest could be rekindled, so may yet apply for my NoV - although with a total house plot of only some 25 x  7  metres, there not really any scope for Tx antennas unless I use ground wires looping over the garden wall into and along the road at the back
 
Andy
 


 
On 14 December 2010 16:27, Chris <[email protected]> wrote:
HiStefan,
Yes, but 600m is VERY long! The wire even laying on the ground I would expect to radiate quite well at 137 with that length! And how many watts?? ERP??
I am sure 8970 would do well through the ground at the sort of powers being spoken about. Probably ideal for submarines, as Roger says!
Vy 73, Chris, G4AYT.
----- Original Message -----
Sent: Tuesday, December 14, 2010 2:49 PM
Subject: Re: LF: Earth Electrodes

Hi Chris,

But how do you explain that i have crossed 49.6 km with a 600m spaced earth electrode antenna on 8970 Hz? It was rather summer time than winther (regarding QRN)! With the same antenna, my 137 kHz signal was seen 20 dB above noise near Paris in DFCW-3!?????

73, Stefan


Am 14.12.2010 15:15, schrieb Chris:
Hi Mal,
Yes, that is what I had concluded years ago. No harm in trying though!
Further to your previous e-mail to LF, looks to me like beacons are now becoming the norm on 136/7kHz band and below. I have no problem with that personally, indeed, I think it preferable to a QSO taking forever! I woud like to see full idents though, no matter how slow.
Vy 73,
Chris, G4AYT.




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