Message-ID: <5.1.0.14.0.20020712173901.00bc7df8@pb623250.kuleuven.be>
Date: Fri, 12 Jul 2002 17:57:06 +0200
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Ground loss/gain (was: polarization of small loops)
In-reply-to: <Chameleon.1026486509.Girardi@tlvk7v>
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Hello Claudio and LF group,

I think that the "ground gain" and the "ground losses" depends on two 
different
>phenomena. Ground gain should come from reflections from ground at a 
>distant point
>(far field) from the source combining with the direct ray from the same 
>source with
>the proper phase. The type of antenna used shouldn't matter since it will 
>be seen
>as a point source at a large distance; of course the polarization w.r.t. 
>ground
>will be important.
The term "ground gain" was not a good choice for what I meant. I agree that 
ground reflection at a distant point can give up to 6dB additional gain 
("ground gain" ?) and if this reflection occurs in the far field it will 
not affect the antenna (impedance etc...).
But what I was reffering to is the effect caused by the ground creating a 
"mirror image" of the antenna. If the antenna is close to ground (as any 
antenna is at 136kHz) this "mirroring" is in the near field (where any 
losses, reflections etc.. will affect the antenna) and can double the 
radiation resistance. But at the same time any lossy object in the near 
field will increase the antenna losses.

>The ground losses depend on the presence of dissipative materials near the 
>antenna
>(near field); for a small electric dipole the electric field near the antenna
>should be more intense than for a small magnetic loop. On the other hand, the
>magnetic field near a loop antenna should be higher than for an dipole. At a
>distance (far field) there is no difference between the two (for the same 
>radiated
>power, of course). So probably for a monopole it is important to have a 
>dielectric
>with low losses (and ground usually is not), while for a loop a lossy 
>dielectric
>near the antenna has less effect, due to the weaker electric field. But I 
>have no
>idea how lossy is the ground as a magnetic material...
I agree that the groundloss for an "electrical" antenna depend on the 
diëlectric properties of the ground while for a "magnetical" antenna it 
will depend on the permeability properties. So these losses can be 
different for a short vertical monopole (electrical) and a small loop 
(magnetical). But at the same time the "mirroring" of the ground can be 
different for these 2 types of antennas (so ground could be a good mirror 
for a monopole and a poor mirror for a loop - or vice versa).
My intuition would say that groundloss and ground "mirroring" go together, 
so a loop might suffer less from groundloss but at the same time it will 
not benefit from the "mirroring" (based on the principle that "there is no 
such thing as a free meal")

73, Rik  ON7YD