[Lf] [Fwd: LF: Ground loss/gain (was: polarization of small loops)]

[Andre Kesteloot]

Rik Strobbe wrote:

> 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