Re: VLF: 2km on 8,97 kHz using 6W TX power and a 120m earth antenna

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Mike,

You asked about my earth electrode set-up, so I'll try to describe it. 

At the bottom of the back garden I have a copper earth rod about 1.5m long driven into the soil. Here the soil is chalky "clunch". From this earth rod a 2mm diameter PVC covered wire comes up 2m and then runs 10m along the top of the fence before coming up to the bedroom. In the bedroom another short wire 2m long connects to the central heating radiator. A coax cable about 10m long runs back to the TX in the front bedroom effectively bringing the two electrodes to the audio TX connected via a step-up toroidal transformer.

Ignoring the loop within the ground, there is about  30 sq m of loop area in the air. I want to try just an air loop (no ground electrodes) to compare signal level at maximum range.

Before using the radiator and house pipes as the second electrode I had two 1.5m copper rods 10m  nearer the house than the "far" electrode. This one had a baseline of 10m only and was less effective.

I'll try to do a drawing and put it on here later.

73s
Roger G3XBM

, Mike.WE0H <we0h@gmx.com> wrote:

Hi Roger,

Could you show me a picture or describe your Earth Electrode antenna? I have all heavy trees here so no other antenna would likely work. What feedline? What wire size and what length of the electrodes? Does the wire run straight away from the feedline in opposite directions to the earth electrodes or?

Thanks much,
Mike
WE0H



Roger L wrote:

Stefan,

A few observations on your tests using the earth electrode "antenna" today:

  1. In my experience, most of the signal transmission is by induction,

     so I think it would be worth trying to receive the signal with a
     loop antenna when the received signal may be much stronger than on
     the E-field probe. In my case I use a 30turn, 80cm square, loop
     which can be rotated to minimise 50Hz interference.

  2. On my 10m and 20m baseline earth electrodes I was measuring

     resistance of between 40-60 ohms between 1-9kHz, so your figure
     seems very high. This may be as a result of the soil and rock type
     in your area but this is worth checking. If my theory of the "loop
     in the ground" is correct then a poor conductivity soil would
     result in a larger effective loop area within the ground.

  3. Did you try matching the PA to this high impedance through a

     transformer? I think not in this test, so better results may well
     be possible.

  4. Regarding Mal's comment about "you need a 100m high antenna", well

     you know this works, but the point of this experiment is to see
     how effective, or not, an electrode "antenna" is. The results are
     valuable whether good or not.

Here, I have completed a further "by ear" range test using my TDA2003 4W PA into a 20m base electrode pair "antenna" walking around the village in many different directions using the 80cm loop RX at around 1.5kHz. I improved the 50Hz rejection this week by adding another Sallen-Key HPF. When walking over metal pipework in the roads the signal is strong up to at least 0.25km in almost any direction and limited by 50Hz noise. When walking across fields well away from pipework the loop orientation exactly lines up with the base electrode "loop in the ground" at distances up to 0.5km.  I have not managed to hear the signal at any further distance in any direction. My grandson is back staying with us all next week, so very limited chance for more experiments until about June 14th. I have a long list of things to try still!

73s
Roger G3XBM

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