[amsat-bb] AO-91 and circular polarization?

[bruce hunter]

Paul,
thanks for your very nice write-up, and explains why my reception using my RHCP 440 ant(rotor-controlled)
becomes quite unreadable when operating SO-50. And because I don't have a polarity switch for that ant, I can
work SO-50 with better success using my Arrow/HT setup.

According to 2016 Getting Started with Amateur Radio, SO-50 also uses CP as downlink;
"The -Z(bottom') of the satellite has four gold 400 MHz antennas in a turnstile
array with the 436 MHz downlink antenna in the center.
The satellite uses_left-hand circular polarization on the downlink."___
73
Christy KB6LTY

---------------------------------------
Mike KC4ZVA,

All US Domestic Communication satellite in C-Band and Ku-band used
orthogonal linear polarization.  This is also known as "Frequency
Reuse" first deployed by RCA in the 1970s.   In a well designed
commercial satellite system isolation between the Horizontal (0 deg)
and the Vertical (90 deg) polarization should exceed 30 dB.
Isolation is greatest at exactly a 90-degree offset between H-pol and
V-pol.   At a 45 deg offset, polarization isolation is only 3db.
When adjusting a commercial satellite dish, it is better to "null out"
the cross-pol that "peak" the co-pol.  Circular polarization is still
used on some Intelsat standard International satellite links.

I believe FO-29 is the only amateur satellite to use a circularly
polarized antenna on the spacecraft.   All of the CubeSats use small
linear polarized antennas.

The primary reason to use a circularly polarized (CP) ground antenna
with a Cubesat is to avoid the deep (-20 to -30 dB) signal null when
the ground antenna is orthogonal (90 deg orientation offset) with the
satellite antenna.  There will still be times when the CubeSat antenna
is pointed away from the earth and even a circular antenna will not
help.  Since most Cubesate tumble, these LP to LP nulls never last
very long but they are an irritation during a QSO.

When using a CP ground antenna, you are effectively using a signal
(wavefront) that contains both an H-pol and V-pol component that will
always be at 45 deg offset to the linear polarized satellite antenna.
  When using a CP signal you are making a decision to "give up" 3 dB
100% of the time in signal to avoid the occasional -20 to -30 dB
signal nulls possible in a linear pol to linear pol system.

If you watch a satellite operator using a hand help Arrow antenna -
they are constantly peaking the antenna in direction and polarization.
If the Arrow was a CP antenna there would be a lot less "arm twisting"
but the Arrow would be 3 dB less efficient 100% of the time.

If your goal is CubeSat operation a CP antenna could be a good choice.
   It doesn't matter if the CP antenna is RHCP or LHCP because you are
working a linear polarization satellite.    If you are working FO-29
and your launch an LHCP signal and FO-29 is expecting an RHCP signal -
you will experience a -20 to -30 dB loss because RHCP into LHCP will
exhibit high isolation.

CubeSats have relatively strong signals and I highly recommend
experimentation with small helical antennas and cross-pol yagis.  You
will have a lot of fun.   If you want to get some arm and shoulder
exercise use the Arrow antenna but the results will be very good.

73 - Paul - W2HRO




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