Topband: RE: beverages
tom.mcdermott4 at verizon.net
Mon May 24 03:35:12 EDT 2004
Hi Herb, hopefully you'll allow a different Tom to respond..
At VHF and UHF it is possible to maintain polarization on a path. In
that case CP can be advantageous in some circumstances. The loss of 3 dB
comes about when you use a LP antenna to receive a CP signal. If you instead
use a CP receive antenna (of the correct handed-ness) that 3 dB penalty
dissappears. CP is useful on VHF and UHF paths in that if the multipath is
due to a single-hop reflection, that reflection (sometimes, depending on the
reflection angle) can reverse the cirularity of the reflected signal. In
that case, the CP antenna helps reject the multipath component while not
affecting the main component.
On HF, the ionosphere produces a random (and varying) signal
polarization. In this regard, a CP antenna is no better or worse than a
linearly polarized antenna. RHCP is rejected by a LHCP antenna in the same
sense as horizontal polarization is rejected by a vertically polarized
antenna. In fact, there's not so much difference between CP and LP (linear)
as one might think. If two LP antennas are combined the resultant signal can
be LP or CP depending on the phase relationship between the contributions.
Usually the signal is elliptically polarized (somethere in between CP and
LP. Since there's nothing to be gained by using CP (at HF) there is no
incentive to construct the more complicated antenna.
A textbook on optics goes through all the polariation stuff pretty
clearly, my college course (Berkeley Physics Course) contained (really
cheap) linear and circular polarizers, and quarter and half wave plates. A
few experiments with these and a lightbulb help make it all pretty
-- Tom, N5EG
> Would you not consider a circular polarized antenna capable of diversity
reception? Also I recall
> reading some time ago that in theory a RHCP signal into a LHCP antenna has
almost infinite rejection.
> I have never heard of amateurs using CP for anything below VHF and UHF
space communications. However,
> today in the broadcast business CP is almost a given even though you loose
about 3 db in the process.
> You buy it back in signal stability especially with multipath and
> I can't imagine, however, anyone constructing a helix array for 160, but
crossed turnstile dipoles are
> not impossible. I just wonder what CP would buy you on rapidly fading
signals and arrival angles that
> occur on top band.
> Herb Schoenbohm
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