I was always under the impression the 2.15 dBi and 3 dBi applied to antennas
in free space, the GP being on one side of an infinite plane at the fed end.
Indeed in real would cases in the vicinity of earth or other large (relative
to the antenna) objects the differances (particularly in FM service) become
negligable. Note too: dBi is relative to a (theoretical) isotropic radiator
(which is a mathamaticily logical standard of comparison), vs. dBd which is
relative to an ideal center fed 1/2 wave dipole in free space.


"Ed" wrote in message
. 192.196...

Every reference and factory specification for
standard grounplane and vertical dipole antennas I've seen indicates
the
standard vertical dipole has a horizontal gain of 3 dBi, and the
groundplane 2.1 dBi.



Those are pretty curious references then. Why would they dwell on the
HORIZONTAL gains of VERTICAL antennas? There must be something left
unsaid in what you are trying to express because cross polarization
would drive down sensitivities by 20 to 30 dB.

For another matter, those values you quote bear very little
resemblence to typical 2M FM operation, unless it is from the Space
Shuttle. Height above ground variations in gain easily washes over
any differences you might perceive. I can see a variation of 2dB in
just raising a groundplane from 40" off the turf to 120".


Richard,

You and I seem to be talking different languages !! :^)

My reference to horizontal gain is gain measured in the horizontal
plane...... that is, measurements are taken broadside to the vertical
antenna elements, in this case, both the vertical dipole and a
groundplane.

Gain measurements taken in any other plane in any other plane than
horizontal tend to be rather useless since most VHF mobile communications
takes place horizontally..... even distant repeaters tend to be close to
the horizon.

Cross polarization is not an issue in VHF operations since all
commercial and amateur FM operations I'm familiar with use vertical
polarization.

As far as height variations having effect on gain.... you are
talking about path gain, or system gain. I am speaking specifically of
antenna gain, which I believed to be the question of the original poster.

And as I have already pointed out, most factory specifications
for vertical dipoles and groundplane antennas are as I already listed.

Ed K7AAT