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R: R: Pointing large dishes


Pointing at +7 degrees at apogee ( 16 hours) with a 35 dBi gain dish in
polar mount it make to receive a very strong noise generated and translated
by the transponder itself in the order of 10 dB or more above the noise
floor of a 1 dB overall NF receiving system.

As soon the transponder noise is received with a 3 dB (S+N)/N ratio on the
ground the S/N ratio do not improve any more because increasing the dish
diameter both signal and noise increases by the same level .

Having a 35 dBi gain dish with polar mount i would prefere to chouse a
pointing compromise less than  +7 degrees in order to receive the AO40
beacon and its TLM  all the orbit available time because TLM is also
important at perigee.

Pointing at +7 degrees at apogee with a 3 degrees  pencil at -3dB points it
make to receive the same S/N at apogee and nothing about at perigee when the
SSP latitude is -7 degrees because the dish off point at perigee will be 14

73" de i8CVS Domenico

----- Original Message -----
From: Bob Bruninga <bruninga@usna.edu>
To: i8cvs <domenico.i8cvs@tin.it>
Cc: Roger <rogermj01@attbi.com>; AMSAT-BB <amsat-bb@AMSAT.Org>
Sent: Thursday, August 01, 2002 2:52 PM
Subject: Re: R: [amsat-bb] Pointing large dishes

On Thu, 1 Aug 2002, i8cvs wrote:

> A 10' ( 3 meters) dish at 2400 MHz has a lobe like a pencil of  3 degrees
> at - 3 dB points.
> Since the AO40 inclination is 7.53 degrees the satellite subpoint latitude
> will change from -7.53° South to + 7.53 North during one orbit.

But ALL the time at Apogee (16 hours) is +7 degrees and it is only at -7
during Perogee.  Not worth worrying about... So just set for APOGEE..

See the three links  in the 3rd line down on my WEB page for photos:


No, I have NOT gone back and actually finished the project.  Winter snow
and then work got in the way...

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