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Re: re: QRM 2m, question for sat builders



I was responding to a question asking why 70 cm downlinks were preferred. If
we assume omnidirectional antennas only, then high speed downlinks are
impractical for microsat size spacecraft and the receiver sensitivity is
limited by the noise temperature of the Earth. The minimums are 18 dB at
10m, 3 dB at 2 m and 0 dB at 70 cm, relative to 290 K. Path loss, relative
to 70 cm, is -23 dB at 10 m, -9 dB at 2 m and 0 dB at 70 cm. 2 m is then 6
dB better than 70 cm in the absense of man-made noise.

73,

John
KD6OZH

----- Original Message -----
From: "Andrew Glasbrenner" <glasbrenner@mindspring.com>
To: "John B. Stephensen" <kd6ozh@verizon.net>; <amsat-bb@amsat.org>
Sent: Tuesday, February 15, 2005 11:05 UTC
Subject: Re: [amsat-bb] re: QRM 2m, question for sat builders


> >The path loss goes up with frequency but antenna gain goes up at the same
> >rate for a constant size antenna. The pointing accuracy must be greater
at
> >higher frequencies but for the size antennas that amateurs use, this
isn't a
> >limiting factor below 2 GHz.
>
> To bring this back to the original point, the required use of directional,
gain antenna systems for receive is one of the factors that makes Mode J
more difficult to the beginner. Theoretical this and that aside, my
experience on the LEOs has shown that you can receive a 2m satellite
downlink from LEO on just about anythng better than a wet noodle, and the
same just can not be said for mode J.
>
> 73, Drew KO4MA
> ----
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----
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