Message-ID: <E933969C10DD41D0BAD71917389A8D4E@Black>
From: "Markus Vester" <markusvester@aol.com>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTimUDj5oaFLwDGHhjjjFLFLsNVAhjorRAi+zs1LT@mail.gmail.com> <8CD18D871990A26-1370-24FA@Webmail-d118.sysops.aol.com> <4C7FE4DD.7040005@iup.uni-heidelberg.de>
In-Reply-To: <4C7FE4DD.7040005@iup.uni-heidelberg.de>
Date: Sat, 4 Sep 2010 22:57:43 +0200
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Subject: LF: Loop above ground
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Dear Stefan, LF,

this is a simple question but not easy to answer. I think it depends=
 on=20
whereabouts you want to look.

Unlike electric fields, magnetic fields easily penetrates the ground=
 down to a=20
skin depth, which can be some tens of meters depending on frequency an=
d soil=20
conductivity. So in the immediate vicinity of a small loop antenna, th=
e effect=20
of real ground on the field is hardly noticable. Even if you buried it=
 a couple=20
of meters, the loop would still radiate (albeit with some extra loss=
=20
resistance).

On the other hand, looking at the farfield, the skin layer will indeed=
 act as a=20
reflector. This results from the fact that the surface currents have=
 to cancel=20
the loop field deep in the ground. As long as the skindepth is much le=
ss than a=20
vacuum wavelength, this shielding current contributes just as much to=
 the far=20
field as a metal ground plane would do. Thus the radiation resistance,=
 which=20
describes the power transfered to the far field, has to include an ima=
ge loop,=20
and it is doubled to
 Rrad =3D 62.3 kohm * area^2 / lambda^4.

So much for my five cent's worth... I wonder if there are is experimen=
tal=20
evidence of this factor of two, eg. by direct measurement of radiated=
 field=20
strength around a transmit loop antenna.

Best 73,
MArkus (DF6NM)

----- Original Message -----=20
From: Stefan Sch=C3=A4fer
To: rsgb_lf_group@blacksheep.org
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=20
>>effect of ground is already included in the earth antenna picture.=
 For an=20
>>above-ground loop with a mirror image beneath it, radiation resistan=
ce would=20
>>be doubled.<<

Can the ground/soil be seen as a mirror for LF? It depends on the cond=
uctance=20
and Epsilon R of course, but i mean the "normal" conds ;-)

73, cu, Stefan