Hello Roger,

 

assuming that the underground antenna behaves like a loop:

For a classic loop area of the loop and the current flow are well defined (the current goes through the wire = circumference of the loop).

This is not the case for the underground antenna, there will be a current flow through an area or even a volume.

But one could define the virtual area of the underground antenna as the area of a classic loop that procudes the same ERP (with the same antenna current) as the underground antenna.

But how to determine this virtual loop area?

Suggestion:

You can measure the relative ERP of your underground loop (at a given antenna current) using daytime WSPR and some local RX station (daytime and local will rule out any propagation changes).

Let's define the virtual area of the underground antenna as A1 (m^2) and the relative ERP as P1 (= dB values from WSPR).

Next you could raise the wire, increasing the loop area by a know value A2). Again measure the relative ERP P2 (as before, using the same WSPR RX stations at daytime).

Now the total loop area is A1+A2, where we know A2 (the above ground part) but not A1 (the virtual underground part).

A1 can be calculated:

At a constant antenna current the ERP will be proportional to the square of the loop area.

So P1 is proportional to A1^2 and P2 is proportional to (A1+A2)^2.

Or: P2/P1 = (A1+A2)^2/A1^2 = (A1^2+2*A1*A2+A2^2)/A1^2

And after some more math:

A1 = ((1+sqrt(P2/P1))/((P2/P1)-1))*A2.

So based on the ERP increase (P2/P1) and the above ground area (A2) of the loop the virtual area (A1) of the "submerged" part can be calculated.

If this is done for some different heights of the wire (= different values of A2) one would have an indication if this theory is valid (the outcome of the virual area should be more or less the same).

 

73, Rik  ON7YD - OR7T 

 

 

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Van: [email protected] [[email protected]] namens Roger Lapthorn [[email protected]]
Verzonden: woensdag 9 januari 2013 15:54
To: [email protected]; [email protected]
Onderwerp: [rsgb_lf_group] Earth electrode antenna with wire at zero height (WSPR 472kHz)

 

Ok, so as the final instalment of this test I have now got the wires from the earth rods coming to the transverter just resting on the wet grass (what was elevated by 1.5m is now zero). It seems to work best just feeding the transverter straight into the rods without matching suggesting low reactance and near 50 ohms.

I'm still getting decent reports! So, it confirms the "loop in the ground" theory, or it is acting as some bizarre E field antenna.

Will again leave it running overnight to get a decent number of spots but already I think we have an answer.

What I can try next - is this ever going to end!? - is elevate the loop part "in the air" to much greater height (about 9m to match the top of the Marconi I just took down) and see how much improvement I get. More in the air implies greater loop area so more signal. Just how much will indicate how much of the loop's effective area is in the soil/rock.

WSPR spots please folks.

73s
Roger G3XBM

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