archive: Re: SETI: Polarization Modulation

Re: SETI: Polarization Modulation

Edward R. Cole ( (no email) )
Sat, 05 Sep 1998 10:56:59 -0800

>>At 09:38 AM 9/4/98 EDT, rtyndall@juno.com wrote:
>>>I think it would be feasible to transmit two different channels on
>>the >same freq, if they were on opposite polarities. On the receive end,
>there would
>>be some cross-talk, but the levels would be low.. It might even be
>practical
>>on FM voice.
>>
>Paul Shuch, Ph.D. Wrote:
>>Or FM video! That's exactly what the C-band DOMSATs (Vert and Horiz)
>>and Intelsats (RHCP and LHCP) do. The technique is called "frequency
>>reuse."
>>About 30 dB of isolation is achieved. It squeezes 24 transponder
>>channels, each 40 MHz wide, into 500 MHz of spectrum (3.7 to 4.2 GHz).
>
>Rich comments once again:
>I noticed how tightly packed the H & V channels were, when I first looked
>at
>C-band TVRO. Move the probe servo a little and press 'Fine-tune' and
>Presto.
>your'e on the next channel..

Oh, you guys are taking back to the days of yore when I was a satellite
dealer installing those early systems; more than once would I get messed up
because the polorotator was in the opposite polarity sense. Not only would
you get the adjacent channel but sometimes the adjacent satellite. This
was before preprogrammed receivers!

>The app I was thinking about for FM voice, would be to double up on the
>2 meter FM repeaters in areas where they want more repeater channels.
>Or, It might be just the thing for 2 meter packet.. (or is packet dead)?

We do just that where I work. I'm the sole comm tech for an extensive comm
network which covers an area of 250x50 miles using six remote sites plus
ten portable repeaters. We have VHF, UHF, Marine VHF, and Aircraft band
networks with 63 separate FCC licenses and maintain 120 handheld and 40
mobile radios. Recently, to add a remote base station link, we reused one
of our 99 frequencies, rather than go through the frequency coordination
hassle. What we did is set up the link using horizontal polarization (all
the rest of our antennas are vertical), placed the equipment on the same
frequency and used a different PL tone, to be safe. The antennas are
separated by 60 feet vertically and point at diferent directions, which
also adds some isolation.
73, Ed