LF: Earth-mode or not earth-mode

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Thanks for feedback and questions from Rik, Chris, Mal, Stefan and others on and off this list. 

I'm still waiting for someone to explain to me why, if this is (mainly) conduction along metal pipes, which I am sure it is, best results are not obtained with a galvanic connection to the pipe at each end and why best receive results are with an E-field probe. I remain puzzled. Answers please!

May I reply on a few general points raised by some?

What is the definition of earth-mode?

I could not find one, but my understanding is "communication between two points essentially using conduction through the ground". In the modern world it is almost impossible to find anywhere, except remote moors, that is not criss-crossed with buried cables and pipes. My interpretation of earth-mode therefore includes utilising these as a means of helping a signal get from (a) to (b). Hence my term "utilities assisted earth-mode". In mining and military use of earth-mode I think this is also accepted. What I also mean is communicating from (a) to (b) not using a radiated signal or induction as the prime means of propagation.

Why experiment with this form of VLF/ULF communication?

From my tests last year it was clear that local communication "piggy-backing" on these buried utilities seemed to be possible over many kilometres without the need for large and complex antenna systems that are essential to get even a tiny VLF signal actually radiated, as Ossi, Gerhard, Stefan and Chris know only too well.

Questions I'm trying to answer are:

• How best to launch a signal?
• What in the ground (apart from water, soil and rock) helps the signal get from (a) to (b)?
• What works best to pick up the VLF/ULF signal at the far end of the link?
• What frequencies work best?
• What are the noise sources and how can these be minimised?
  
• How close to buried utilities does one really have to be for it to work?
• Can one use rivers, canals or the sea coast for earth-mode?
  
• How little/much power is needed for a given range?
• What modes are most effective?
• What is the practical range limit?
• Is there a practical use for this form of communication?
  

Also, I'd welcome suggestions for further practical tests.  A lot of the experience is directly useful in the reception of DX radiated VLF signals. 

Results have been shared here because others are known to be interested in this aspect of VLF work, but if people think the LF-reflector is not the right place please say so. 


73s
Roger G3XBM

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