Good points about polarization. If the signals and/or noise are polarized
predominantly in one state, then RDF may not be a good predictor of SNR
performance, particularly if the antenna receives predominantly in an
orthogonal polarization. On the other hand, if the polarization state of
the signals and noise evolve randomly with no preference for any one state,
which is often assumed for skywave signals, then RDF will be--on average--a
good receiving metric, subject to the previous stated qualifications about
the spatial distribution of the received noise. However, some of the past
discussions on this reflector about preferential polarization of skywave
signals on 160 may call into question the assumption of randomly polarized
signals.
73, John W1FV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Richard
(Rick) Karlquist
Sent: Wednesday, December 17, 2014 3:19 PM
To: Lee K7TJR; 'Terry Posey'; 'John Kaufmann'; topband@contesting.com
Subject: Re: Topband: 8 circle: DXE vs Hi-Z
All this discussion about RDF overlooks the issue of polarization. If you
make an array of verticals with a certain RDF (assuming noise comes from all
directions uniformly), the array will be better than an individual vertical
by the RDF factor. However, what I have found is that a horizontally
polarized antenna, such as a low dipole frequently receives considerably
better than a vertical. In that case, you would be better off using an
array of low dipoles. The reason why horizontal polarization can be better
is that the horizontal component of terrestrial based noise is highly
attenuated over distance as a ground wave.
Rick N6RK
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