Art Unwin wrote:
On Mar 17, 11:27 am, Jim Lux wrote:

Richard Harrison wrote:

Art wrote:
"I have an on order a tilting system for my antenna to probe the
polarisation of incoming signals for maximum audio clarity and gain."

That may be interesting but do you ever recall cross polarization of an
incoming ionosphere reflected signal being unreadable because
polarization was wrong?

So many different and quickly changing path variations exist in the
ionosphere that the best antenna to use is based on probability.

Or, use diversity combining. Several researchers in France have done
work with this, and discovered there's very little correlation between
the ordinary and extraordinary rays, so diversity combining is extremely
effective on HF skywave paths. They used physically co-located antennas
that had different polarization sensitivities (a loop and a whip, as I
recall).




E.A. Laporte says on page 215 of "Radio Antenna Engineering":

"To make best use of this effect (randomness of ionospheric waves) it is
desirable to employ complimentary antennas for transmitting and
receiving."

Most commercial HF circuits I`ve experienced and seen use horizontal
polarization. It is because much severe man made interference arriving
at a receiving antenna is vertically polarized.

Interference polarization is not necessarily the case. (I believe
there are measurements that show it is essentially random). More what
it is has to do with the antenna pattern of horizontal and vertical
antennas for sources at ground level and reasonably close. For example,
A horizontal antenna not too high over a ground plane has a null right
at zero elevation.




Best regards, Richard Harrison, KB5WZI


Jim'
My 160M antenna is totaly at 70 feet. Not below or above. With the
tilting and pan mechanism and a couple of relays
it is possible to automate it so that every so often it will cycle
thru all modes using the single antenna. When a louder signal arises
then it is simple to stay on that polarisation
. This combiation thus is a reduction of land space required for two
or more separate antennas.
I was just curious as to what other hams were doing and it appears to
be nothing in this area.

Lots of hams have done things with polarization diversity combining for
HF. Check out Ralph, W0RPK's site at:
http://showcase.netins.net/web/wallio/POLAR.html

He has some actual recorded levels over a 20-30 second interval on a 10m
skywave signal.

The problem with physically moving the antenna is that you need a fairly
fast positioner, since the fades (in any one polarization) are on the
order of 1 second. With a single antenna, you also don't know, a
priori, which way to move it or when to move it (is that an overall
fade, or is the polarization changing). You can use a scanning or
dithering technique (much like conical scan for high gain parabolas), as
long as the scan period is much less than the fading time scale.

With two co-located antennas, you've got lots more possibilities, and
with modern signal processing hardware, it's cheap.


(and, of course, most FM car radios use some form of diversity combining
these days, as do virtually all Wireless LAN access points)

Regards
Art