"Jeff Liebermann" wrote in message
...
On Sat, 5 Sep 2009 20:30:30 -0500, (Richard
Harrison) wrote:
Art wrote:
"Chris, what I believe he is referring to is that computer programs
support a tipped vertical over one at right angles to earth."
Quick! Tell your local broadcaster.
We don`t need theory to show that antennas for transmitters and
receivers work best together when they are exactly parallel in space
unless something in the transmission path is redirecting the
polarization of the signal. In aligning microwave antennas humdreds of
times, I`ll swear this is true every time, regarfless of Corriolis, no
matter which side of the equator I was on. Try it. You`ll be convinced
too.
Best regards, Richard Harrison, KB5WZI
Ummmm.... Faraday Rotation?
http://en.wikipedia.org/wiki/Faraday_effect
It's not too horrible at microwave frequencies, but becomes noticeable
as the frequency decreases. I've done some crude polarization studies
at VHF and UHF frequencies with a rotating dipole, and found some
rotation on line of sight paths. However, we were looking for a
polarization distribution and didn't really spend any time getting
accurate numbers for a line of sight path.
* Fraday rotation is much greater in the ionosphere where there are more
free electrons, apparently. L-band and C-band links with geostationary
satellites use circular polarisation for this reason (as well as others when
a mobile terminal is involved) - the degree of twist is measured in turns.
At Ku-band it's down to a few degrees.
Also many broadcasters use elliptical polarization (a mix of
horizontal and vertical). However, that's not to eliminate any
Faraday Rotation effects. It's to deal with the effects of
reflections and refraction, which create nearly random polarization at
the receiving end. Also, because stationary broadcast antennas are
horizontally polarized, while vehicle mounted broadcast antennas are
usually vertical.
* In the UK, VHF FM radio was launched with horizontal polarisation mainly
because of a belief that vehicle-generated nois was predominantly vertically
polarised. This belief derived from an experiment, but the conclusion may
have contained an error. However, it was clear that VHF reception in
vehicles was impaired by this choice because a simple vehicular antenna is
vertically polarised. As vehicular reception grew, and when commercial
radio started at VHF, a vertical component was added to the transmitted
signal to improve matters. Nowadays, most of the transmitting antennas for
VHF FM radio provide 'mixed polarisation'.
I was once asked why the British drive on the left side of the road.
My instant answer was Coriolis Effect. I claimed that due to the
earth's rotation, it's easier to make left turns on the left side of
the Atlantic Ocean and easier to make right turns on the right side of
the Atlantic. It took an excessively long time for even the sharpest
student to catch the joke/hoax. Sigh.
* Isn't the real answer connected to knights in shining armour on their
horses and the side that they, traditionally, carried their jousting pole?
.... but where did the tradition come from? ... is that to do with the
traditional way a right-handed person mounts a horse?
Drivel: At 2.4GHz, most wi-fi wireless routers use vertically
polarized rubber ducky antennas. Yet, most laptops have their
antennas in the top of the LCD frame which are horizontally polarized.
Same with PCB antennas found on most PCMCIA cards.
* A vertical whip on a device with a horizontal chassis, and horizontal
power/data cables, probably radiates plenty of HP as well as VP. Years ago
(before the widespread use of normal-mode helix antenna) I worked on a
scheme that involved STC/ITT 'Starphone' hand-held transceivers. The
base-station antenna was VP but the handsets each contained a PCB loop
antenna where the axis of the loop was parallel with the long dimension of
the handset, that is, generally wrong! ... but the scheme worked quite well
and there was no chance of whip antennas getting broken.
Anyway, nobody cares much about antenna polarization:
* Well, some still do!
Chris
http://picasaweb.google.com/lh/photo/HC8B4F-AnCQF6I_u0k3MYg
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn03.html
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558