[amsat-bb] Re: Antenna Polarization Technical Question
kc6uqh at cox.net
Sun Aug 5 13:44:08 PDT 2012
Just a reminder, a QFH antenna is circularly polarized over the whole
envelope of the antenna. A sharp null exists on the back side. A one
wavelength, one turn has gain at low angle side radiation and a 4 dB loss
overhead, where the distance to the ground station is the smallest.
Certainly this is a good fit for satellites.
Turnstile antennas and patch antennas are linear polarized at the sides and
of course are the easiest to implement on a satellite.
My point is all circular antennas are not equal and having an antenna with
gain on the sides opposed to the center of the antenna is very desirable.
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Bob Bruninga
Sent: Saturday, August 04, 2012 3:25 PM
To: Thomas Doyle
Cc: amsat-bb at amsat.org; andrew abken
Subject: [amsat-bb] Re: Antenna Polarization Technical Question
> Not sure why anyone would want to maintain the
> orientation of the satellite in such a way that
> would cause the direction of circular
> polarization to change during the path.
Lets try this approach... As I said before, By the laws of physics, what
comes out one side of a circular polarized low gain antenna as RHCP comes
out the opposite side as LHCP.
Now given that, and the fact that someone in Maryland is in the center of
the RHCP beam, then by the laws of physics, the guy in California must see
mostly LHCP. No matter how much one of those persons demands that he
deserves the RHCP beam, by definition, someone else somewhere will get the
LHCP one, and the geometry changes at least every 10 minutes or so and every
time the spacecraft rotates a bit.
So one might say, "point it down" then only the person in Kansas will see
the main beam and those in CA or MD will be completely off the sides almost
70 degrees from the main beam. Mot people do not realize how LOW these
satellites are. The only solution is to put satellites so high, that "down"
is about the same to everyone (geostationary altitude). But then that takes
100 times more altitude, and that takes 10,000 times more power.
Better to just live with the laws of physics... I guess.
>On Sat, Aug 4, 2012 at 2:47 PM, Bob Bruninga <bruninga at usna.edu> wrote:
>>> I believe that is true but that does not explain
>>> why the optimum polarity setting on the receive
>>> end would change during a pass.
>> That's easy. The circularity on a pair of crossed dipoles (about all
you can get on a spacecraft) May be designed for Right hand circularity when
viewed from the prime direction. But by definition, that save waveform will
be LHC when viewed from the opposite direction.
>> And since the geometry to any one observer is constantly changing by
almost 180 degrees during an overhead pass, that is why it is very easy to
see, complete change in circularity.
>> Bob, WB4APR
>Sent from my computer.
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