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2m FM from Geo (RAgeo)



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.

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 4) The uplinks would generally be unpublished and PL protected.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.

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)

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.

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



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