[amsat-bb] Re: Ridge Test Results
Laurence 'Mike' Hammer
ldh3 at cornell.edu
Tue Apr 22 06:18:23 PDT 2008
Been watching this thread. The only bits of info I haven't seen go across
Antennas act very differently when the radiator part of the antenna is in
close proximity to the RF ground. If you have modelled the antenna as
being "in empty space", that will account for much of the loss seen when it
is mounted to the sat, which in your design by definition is very close to
the sat frame, and thus both the electrical and RF grounds. Have your
modeller check to see if they have the sat frame, and any conductive
panels, in their model; if not you'll have some (possibly large) matching
adjustments to do.
A 1/4 wave piece of steel measuring tape needs to be at 90 degrees from all
nearby metal to be an actual "monopole" antenna. If you lay it along the
side of the sat, the nearby metal (grounds) will soak up much of the signal.
If I remember right, Prof Campbell's prior-to-CubeSat project (name is
escaping me at the moment) incorporated a proven loop design. I don't
remember the freq though. You might want to check your spec against that one.
If you haven't already, check the entire assembly (antenna on sat) with an
SWR analyzer, or preferably a network analyzer, to make sure the
transmitter is "seeing" a 50 ohm RF load at the input of the antenna line
at your transmit freq. While this doesn't guarantee the antenna is
actually an antenna and not a resistor, it does give a basic check that the
matching network is close for your situation.
GL, Mike N2VR
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