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
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