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2m FM from Geo (RAgeo)



Hi AMSATs,

An FM transponder in Geostationairy orbit, very interresting idea but it 
will never work.
You will have one channel for hundredthousands of amateurs having their big 
guns fixed to this geo-sat and open fire when it is switched on :-)

>A Two meter FM Channel from Geo. (this should be fun...)

>Besides putting a wideband linear transponder to provide plenty of
>weaksignal bandwidth to satisfy that need, I would also like to put on
>a geo bird a single FM channel transponder that could be heard all over
>the continent by mobiles with just a whip on the roof.

>Crazy?  I dont think so.  By my reckoning, a 10W transmitter could do
>it on 2m and fit well within the power budget of the satellite.

Its not crazy, its technically possible, but the problem is more the small 
number of amateurs who have no dicepline or the ones who uplink without 
knowing they're on a satellite uplink.

>Here is how I would design it:
>
>1) It would be operated as a directed net under a "net control"
>
>2) It would have 2 uplinks, 1 for net control (priority), and the other
>    for checkins or other stations.
>
>3) It would be made available on a "reservation basis" so that people,
>   clubs, events, could request and schedule it in advance.
>
>4) The uplinks would generally be unpublished and PL protected.
>
>5) Emergencies or priority traffic would be accepted at any time under
>    the judgement of the net control
>
>6) Reservations would be via an automated (though managed) WEB page.
>
>7) Reservation requests would be submitted via electronic means and if
>    accepted, would be sent in return randomly selected 70cm FM uplinks
>    and the proper PL for THAT scheduled sesssion..  The "random"
>    algorithm would be driven by rules established established by the ITU
>    frequency coordinator.

Very interresting, but you know it would take a lot of organisation work ??
Besides that, it will only be used as "local chat transponder"...

And who will get the transponder when more reservations at the same time are 
done ????
May I guess ? :-)

>USES: (Remember, it will be available 24 hours a day, nothing at all like
>the "FM birds")...  Here is how we could use it:
>
>* Mobiles during Noon and Midnight times (when sat points at USA)
>* ("Oscar" arrays maybe needed mornings and evenings)
>* Special Events, Demos, AMSAT meetings, COnferences, Hamfests
>* OPEN periods, or DXpeditions...
>* HAM radio RESPONSE TEAMS going to far-away places
>* The HAM radio channel
>* Limited Disaster or emergency support, (but it is usually too public)

Same question, who will get the transponder when more events at the same 
time want to have the acces to the transponder ??

>Link Budget:
>Transmitter power: 10W  40 dBm
>Satellite Ant Gain      10 dB
>Polarization loss       -3 dB   (TX circular, mobile receives linear)
>Path Loss             -166 dB   (40,000 km)
>Mobile Whip gain*        5 dB   (Actually -10 to +8. Please see notes!)
>Received signal level -114 dBm  (.45uV) full quieting on modern radios)

>Why FM?  Because FM (above threshold) yields much better signal to noise
>ratio than SSB.  And the purpose of this channel is not to "dig-out-the-
>weak-one" but to communicate meaningful information reliably among a large
>group of people under the guidance of a directed net.

Yeah above threshold, but with the same amoung of power you would recieve a 
much better SSB signal. Don't forget earth noise and man made earth signals 
(like taxi stations in 2m ham band... who have also wipe antennas)

>MOBILE ROOF WHIPS:
>I have done many experiments with mobile OMNI whips and satellites
>and the NORMAL 5/8ths wave mobile antenna will NOT work well in this case,
>because it directs its gain at the horizon (+5 dB) but has a -5 dB or more
>null looking up at 30 to 50 degrees which is where a GEO bird will always
>be.

>A better antenna is a 19" whip in the center of the roof.  From it, you
>will always get about 5 dBi gain up at the GEO bird.  An even better whip
>is a 4+ foot long 3/4 wavelength whip because it will give almost 8 dBi
>gain upwards toward the GEO arc.  (except within 20 degrees latitude of
>the equator (south of Florida)).

>SO, I think it would be very useful to HAM radio.

>Of course, It would be stupid to operate it the way we currently
>waste our on orbit FM assets... So dont even think about it...

>(Hummh... but think of what we could do with UO-14 or SO-50 if we just did
>(some planning... but that is another subject)...

>de WB4APR, Bob

A geostationary satellite is ideal for broadcasting between 
main-ground-stations. Maybe it would be a better idea to use such a 
transponder as repeater-backbone, or even a 
packet-radio-intercontinal-back-bone. Today we still use this commercial 
internet, where we have the possibility to do it via satellite.

I'm not against the idea, but I would see it in another way.

73 de PE1RAH, William
---

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