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

[Burns Fisher]

FWIW, this page https://www.amsat.org/two-way-satellites/ao-7/ says that
AO-7 has a CP antenna as well.

On Mon, Mar 12, 2018 at 12:58 PM, bruce hunter <bhunter3 at mindspring.com>
wrote:

> 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
>
>
>
>
> ---
> This email has been checked for viruses by AVG.
> http://www.avg.com
>
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions
> expressed
> are solely those of the author, and do not reflect the official views of
> AMSAT-NA.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
>