Date: Thu, 05 Sep 2010 00:59
From: Markus Vester
Subject: LF: Loop above ground

Dear Stefan, LF,
this is a simple question but not easy to answer. I think it depends on whereabouts you want to look.
Unlike electric fields, magnetic fields easily penetrates the ground down to a skin depth, which can be some tens of meters depending on frequency and soil conductivity. So in the immediate vicinity of a small loop antenna, the effect of real ground on the field is hardly noticable. Even if you buried it a couple of meters, the loop would still radiate (albeit with some extra loss resistance).
On the other hand, looking at the farfield, the skin layer will indeed act as a reflector. This results from the fact that the surface currents have to cancel the loop field deep in the ground. As long as the skindepth is much less than a vacuum wavelength, this shielding current contributes just as much to the far field as a metal ground plane would do. Thus the radiation resistance, which describes the power transfered to the far field, has to include an image loop, and it is doubled to
Rrad = 62.3 kohm * area^2 / lambda^4.
So much for my five cent's worth... I wonder if there are is experimental evidence of this factor of two, eg. by direct measurement of radiated field strength around a transmit loop antenna.
Best 73, MArkus (DF6NM)

----- Original Message -----
From: Stefan Schafer
Sent: Thursday, September 02, 2010 7:54 PM
Subject: Re: LF: ERP calculation (revised)
Hello Markus, Roger, LF,
Am 02.09.2010 19:07, schrieb Markus Vester:
>>Note this is using the radiation resistance for a loop in free space, as the
>>effect of ground is already included in the earth antenna picture. For an
>>above-ground loop with a mirror image beneath it, radiation resistance would
>>be doubled.<<
Can the ground/soil be seen as a mirror for LF? It depends on the conductance and Epsilon R of course, but i mean the "normal" conds ;-)
73, cu, Stefan