Tom, I didn't read all of the mail on this subject until after I posted my
comment. And my comment was just a simplistic observation in support
of the advantages of stacks. Just glad that we agree on the
general observations. Didn't want to get into issues about diversity
(which isn't what is happening, contrary to one of the posted notes) since it
would only generate more email--for which I have no time at
the moment.
I have the capability to do spatial and polarization diversity here--by
linking
my two 781s together so they track in freq. And I have the start of a pretty
big station with lots of antennas for options. I have wanted to run my
soundblaster with the DSP software SpectraPlus to actually make quantitative
measurements of the advantages of diversity. (BTW Spectra Plus works
GREAT to compare RX filters. Just insert noise from a noise bridge and look
at the audio output spectrum. Won't do the full range of the filters but
it can get you down to --60dB or so. Shows that the 781 filters
are better than the 1000D filters for shape factor. But also shows that
the 781 has more out-of-band white noise above the filter passband that can
contribute to operator fatigue!)
But I can't agree with the dual polarity issue where you state that
it will always increase fading. The deepest fading is when linear
polarization
is rotating and there is NOTHING to get signal that is crossed polarized.
This has been shown by using circular polarization for receiving linearly
polarized signals that are rotating as a way of reducing fading.
I think this may help to explain the issue that keeps coming up about
quads having less fading than Yagis. While both are linearly polarized,
the Yagi has a number of additional polarization "filters" called directors
that
may increase the rejection of orthogonal signals. Whereas the quad has
directors that are not polarized (closed loops). When added to the fact that
the quad has vertical wires in the driven element and the currents may
not be perfectly balanced to cancel any vertical signal (due to no balun
or a poor balun) then the quad may reject vertical signals less than the
Yagi resulting in less fading for vertically polarized signals.
Just some food for thought about quads--might start a whole new thread!
John W0UN
>This is all explained in my most recent reply to Eric written yesterday.
>
>Thanks for the input. It validates an effect I have long noticed on 160 and
>80 meters, and something commercial texts have mentioned for years.
>
>It isn't the fact the antenna has "spatial diversity", it is the fact the
>antenna is better focused that reduces fading. Diversity ONLY works with a
>intelligent voting system, and that requires dual receiving systems from
>antenna to signal output.
>
>Transmitting or receiving sky wave signals with "dual polarity"
>antennas...contrary to some antenna advertisements...always increases
>multipath and fading.
>
>73 Tom
>
>
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