[amsat-bb] Squint and system loss

[Scott Townley <nx7u@xxxxxxxxxxxxxx>]

Haven't seen this explicitly shown on the -bb but then I haven't been
around for very long...

With as high a gain antenna as is on AO-40 on S-band, it doesn't take
much squint to take you out of the picture.

For me (for some reason), finding spacecraft antenna data is difficult. 
AMSAT-NA (http://www.amsat.org/amsat/phase3d/antennas.html) says the
S-band antenna is a 50cm dish with 18dBic gain.  Another list member
mentioned to me that it was a helical antenna with 16dBic gain.  

In any event, it doesn't take much squint to take your link budget
down.  If the antenna really is the 50cm dish, 9 degrees of squint is
about the -3dB point, and 17 degrees for -10dB.  Realistically, then,
for all but the very best RX setups, you won't hear anything for squint
greater than 20 degrees.  If the helical antenna is correct, it's a bit
looser...-3dB at 15 degrees and -10dB at 27 degrees squint. 

The AMSAT-DL site (http://amsat-dl.org/journal/adlj-p3d.htm) today makes
a useful statement "Experimental transponder operation in mode UL1/S2 is
possible...with squint angles below 20 degrees."  From which I would
infer that the 50cm dish answer is most likely correct.

The spreadsheet that Gene Marcus/W3PM made available on the -bb the
other day is most useful (!!) for calculating out your RX systems.  It
includes squint loss calculations based on a 16dBic TX pattern with zero
sidelobes (a cos(theta)^n pattern) which matches the helical antenna
beamwidths quoted above very closely.  It's at
http://home.hiwaay.net/~mmarcus/download/ao40_s2.xls
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