Absolutely right. Omni for vhf antennas usually means circular in the
horizontal plane, with minimum radiation above and below the horizon. The
more elements get stacked vertically, thre more the beam compresses to point
just at the horizon and the more gain.
If an antenna truely radiates in _all_directions, it's call isotropic. A
dipole has 2.2 dB gain compared to isotropic, or 2.2 dBi. A vertical dipole
would have 2.2 dBi towards the horizon.
Then the question becomes polarization. horizontal loops for VHF usually
mean the antenna is supposed to radiate omni but remain horizontally
polarized, because most SSB uses horizontal polarization. That makes for a
more complex antenna, consisting of several loops around the mast, and only
an approximation of omni coverage...but it works better than the wrong
polarization.
Virtually all FM repeaters use vertical polariztion. Vertically oriented
whips, colinears, 5/8 wave, etc. provide omni and vertical polarization with
very simple construction.
So unless you want to talk on vhf/uhf SSB from the car, stay away from
horizontal loops.
jeff, wa1hco
----- Original Message -----
From: "John Geiger" <johngeig@yahoo.com>
To: <vhfcontesting@contesting.com>; <vhf@w6yx.stanford.edu>
Sent: Wednesday, April 16, 2003 11:46 PM
Subject: Gain from an omnidirectional antenna
> I have an antenna question for the reflector. On the
> local repeater here, the general consensus is "Get a
> horizontal loop for VHF, they will outperform yagis".
> (I know better, but hey, I just report what I hear).
> So here is my question-I have heard gain figures
> attributed to loops, or to stacked loops, but I don't
> understand how. If an antenna is truely
> omnidirectional, where does the gain come from?
>
> As I understand it, a beam gets its gain from focusing
> its RF in the forward direction. It takes the
> radiation that would go to the front and sides, and
> compresses it into the forward direction instead-or
> something like that. The gain comes because you lose
> radiation in other directions. But if an antenna is
> onmidirectional, it is not compressing the signal into
> one direction more than the others. So how does it
> get gain? The only idea that I can come up with is
> that it must cut down on the vertical lobes, taking
> the energy from several of the vertical lobes and
> focusing it into one or two vertical lobes. It can't
> do it in the horizontal plane, or then it wouldn't be
> omnidirectional.
>
> So am I even on the right track with this question or
> thinking? As you can tell, the local club really
> needs a good course in antenna fundamentals!
>
> 73s John NE0P
>
>
>
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