[TowerTalk] Re: Horizontal + Vertical Polarization Question
Martin Ewing AA6E
aa6e at arrl.net
Fri Nov 21 09:39:14 EST 2003
K9OM asks:
Has anyone experimented with feeding both a horizontally polarized and
vertically polarized antenna simultaneously on HF? Such as, feeding a
horizontally polarized yagi and a vertical on 20, 15, or 10 meters?
------
Well, in my time, we called this circular polarization. That's what you have if
you feed H and V 90 degrees out of phase, anyway, and if the phase centers of
the two antennas coincide. If the antennas are not symmetric, you'll get
elliptical polarization, i.e., circular plus linear. If the antennas are
physically offset, you'll get "interesting" interference fringes on top of the
normal radiation pattern.
Circular polarized antennas are insensitive to the (linear) polarization angle
of the incoming wave, so that eliminates one source of QSB. (Propagation can
twist the polarization angle, but typical HF propagation favors H polarization.
It's the same reason your Polaroid sunglasses work.) On the other hand, a
right-hand circular antenna rejects left-hand circular waves, so if both sides
are playing this game, you'd better agree which sense to use. The optimum
antenna (on receive, but same argument for transmit) for a linearly polarized
wave is linearly polarized at the same orientation. If you use circular receive
for a linear transmit, you are going to be down 3 db against an optimum linear
antenna. So there's no free lunch.
73, Martin, AA6E
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