[amsat-bb] Re: Eagle efficiencies
bruninga at usna.edu
Sun Sep 10 09:39:11 PDT 2006
> Clarification, or more fuel to the fire...
> For a *fixed overall physical size (aperture)*, antenna gain
> proportional to f^2
> Pathloss is proportional to 1/f^2
> So then the total link budget actually *increases* with
> frequency (f^2*f^2/f^2=f^2), since you have an antenna at both
True. But that assumes you can keep the antennas at both ends
pointed at each other. Yes, this DOES give better links at
higher frequencies but requires higher attitude control and
pointing requirements on the spacecraft.
For LEO satelites that need OMNI antennas, then your have
F^2/F^2 and these two effects cancel each other out.
Also, the key term is the "fixed size" comparison. Yes, phasing
together FOUR 12 foot UHF yagi's will give the same overall
"aperture" as a single 12 foot 2 meter yagi, but look how much
easier it is to build a single 2m yagi. And then a 10 meter
Dipole will also give about the same aperture too, and it is
even simpler for the same aperture.
When you get to dishes at both ends, then I agree completely
that all things are equal.
Frequency doesn't matter, though pointing accuracy becomes
harder and harder as one goes up.
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