Have to agree with Drew even if this reduces signal levels at extreme latitudes. I would assume that the P4 centered on the NA&SA would boresight the equator with a beam chosen to match the earth's apparent angular size = arcsin (8000/23500) = 19.9 deg. which corresponds with an antenna with 19.5 dB. At 2.4 GHz an 18-inch dish has about 19.7 dB gain. Such a footprint would only be 3 dB down at the poles.
I would guess that at 145 and 435 MHz the satellite antennas will not be large enough to produce 20 dB* of gain, so only a percentage of their signal will hit earth with a larger amount radiating uselessly into space. The reverse issue for the satellite Rx antenna; only a small percent of the beam will see earth signals. This means mode UV (old Mode-B) will require larger ground station antennas. Calculating this is straight forward path-link calculations. One can plug in the range and Tx power (say 50w) and receiver sensitivity (-145 dBm) to come up with antenna gain requirements. No I will not do that for you this time - hint I have a calculator on my webpage that a clever person could modify - check out Mar's Reconnaissance Orbiter (MRO) Radio Test at the bottom of:
http://www.kl7uw.com/raseti.htm
How does the range to Geostationary orbit compare with apogee on AO-13 or AO-40? I think you will find that a standard AO-13 mode-B antenna system will be adequate (depending on the power and antennas on P4).
73 Ed - KL7UW *Note: on 144 MHz my eme array produces 21.3 dBi gain (a bit large to install on a satellite)
At 12:00 PM 12/14/2007, Andrew Glasbrenner wrote:
Probably a good pointing spot would be where XM and Sirius Point their antennas - just about on the Canadian Border close to Winnipeg- Manitoba 73 Robin VE3FRH.
Probably not if we care about covering the Southern Hemisphere. We really don't know enough even to speculate, but this likely won't be a spot beam like the Ku and C band transponders use. Range will be less than AO-40, so the earth is a fairly big target still.
73, Drew KO4MA
73, Ed - KL7UW ====================================== BP40IQ 50-MHz - 10-GHz www.kl7uw.com 144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w DUBUS Magazine USA Rep dubususa@hotmail.com ======================================