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

[Paul Andrews]

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



























On Mon, Mar 12, 2018 at 9:24 AM, Mike via AMSAT-BB <amsat-bb at amsat.org> wrote:
> Not sure where the 3db loss when on the opposite polarity came from. It is more on the order of 20db or more.
> I am a TV broadcast engineer and we take video and data feeds via various satellites. I can be locked into a bird in the vertical polarity and looking at the spectrum analyzer you can see all the carriers nice and tall. If I rotate the polarity all the signals go completely away and are replaced by all the carriers on the opposite pole. Broadcast satellites transmit two feeds on the same frequency only separated by polarity. This wouldn’t be possible at only a 3db loss.
> That’s why you must rotate your arrow antennas to follow the spin on the birds else the signal completely disappears.
>
> Michael KC4ZVA
> EL98
>
>
>> On Mar 11, 2018, at 10:51 PM, Todd Deckard <tdeckard at imris.com> wrote:
>>
>> Greetings Jerry,
>>
>> thank you for this (yes I meant AO-92).
>>
>> If I understand, you are saying the the satellite uses a conventional linear antenna.
>>
>> However AO-92 is easy enough to access that a 3db loss from the polarity mismatch b/t linear and circular is not significant.
>>
>> Correct?
>> Todd
>>
>> Sent from my iPhone
>>
>>> On Mar 11, 2018, at 6:46 PM, Jerry Buxton <n0jy at amsat.org> wrote:
>>>
>>> Todd,
>>>
>>> Correct is not really a consideration, the way I see it.  Will a CP
>>> antenna work?  Yes.  Polarization should not matter for AO-92.  (BTW I
>>> presume you mean AO-92, not AO-91 in which case L band just flat won't
>>> work no matter the antenna, period.)     :-)
>>>
>>> I use RHCP because I built my antenna for AO-51 and that is was in use
>>> at the time.  I believe that when CP is used on a satellite, at least in
>>> the AMSAT history, it has been RHCP most of the time so building for
>>> RHCP may have advantages for future satellites.
>>>
>>> Jerry Buxton, NØJY
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> _______________________________________________
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> to all interested persons worldwide without requiring membership. Opinions expressed
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> Not an AMSAT-NA member? Join now to support the amateur satellite program!
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