RE: [amsat-bb] Re: HEO naïveté
Robert Bruninga
bruninga at usna.edu
Sat Feb 7 10:15:15 PST 2009
>> The microwave thing always gets me though.
>> If the antennas are too big how come they
>> can get them on cube sats?
>
> CubeSats buzz around 180 miles over your
> head. At apogee, AO-13 was 23,000 miles
> from the ground.
And every radio amateur knows that it takes 4 times the power to
double the distance. So lets do the math. The difference in
*power* (antenna gain or whatever) to go from 200 miles LEO to
20,000 miles HEO is 2x2x2x2x2x2x1.6 in distnace or 4x4x4x4x4x4x3
or 10,000 times more power. In dB that is 6+6+6+6+6+6+4 dB or
about 40 dB. That means to get the same performance from HEO as
you get from a 1 Watt cubesat, you would need a 10,000 watt HEO
satellite.
But everyone also knows that satelites in 200 mile orbits spend
95% of their time from 500 to 2000 miles away lower on the
horizon. So the real difference is about say 33 dB maybe.
Usually we make HEO's work by getting 13 dB more power on the
satelilte (20 watts) 13 dB more power in the ground station
antenna, and maybe 7 dB more power in the sateliite downlink
antenna to make up for the 33dB or so greater distance.
Hence, HEO's need more power and big antennas on the satellite
and on the ground to work.
Bob, Wb4APR
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