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

[Mike Thompson]

The 3dB mentioned is a result of the polarity mismatch between CP and
linear polarization. Mike is correct that there is 20dB or more when one CP
is mismatched with another CP, or one linear is mismatched with another
linear, but from linear to CP it's only ever about 3dB. IIRC, AO-92 uses a
1/4w whip for 70cm as the receive antenna for the L-band converter, so
using a CP antenna on the ground will work just fine, and you will always
have a 3dB loss, but it will be consistent. If I was building an antenna
for that purpose, that I was going to operate beyond reach of the antenna,
I would use CP as well.

On Mon, Mar 12, 2018 at 8:24 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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