In article
,
Chris wrote:

I've been looking at a few satcom antennas for a project I'm working
on. We were originally looking at parabolic dishes but have recently
taken a look at slot arrays. A low profile solution is desirable
which would lead to a small dish or a low (but very wide) slot array.
Some of the people we've talked to have warned us that slot arrays are
narrow bandwidth. I thought I understood this issue, but I'm
beginning to doubt myself.

Suppose a slot array could transmit at Ka frequencies from 30 to 32
GHz at a bandwidth of 3%. If the center frequency of transmission is
31GHz a 3% bandwidth would be equal to 930MHz or 465MHz on each side
of the center freq. This may be narrow to an antenna designer but
this is more spectrum than we'd ever hope to get on the satellite
anyway. I'm a little confused over why this is an "issue", which
makes me think I have a misunderstanding of the terminology.

Can anyone clear this up for me?

One thing that has not been mentioned IS that if your dealing with
GeoSync SATs, they are spaced VERY Close together, and sometimes
actually just a few miles apart, across the Clark Belt. This means
if you have a Transmitting Antenna that can't be directed at just
one SAT Position, (better than 2 Degrees) you will be illuminating
more than one SAT, AND consequently, if those multiple SATs have Inputs
in the same Band as your up-linking, you will be using other folk's
spectrum and transponder space, which is a GIANT NO NO. You are
REQUIRED to use an Uplink Antenna that will only illuminate just the SAT
your licensed to work, and that includes any Side-Lobes off your antenna
as well. This is why Parabolic Antennas are almost ALWAYS used in
Uplinking to GeoSync SATs. They tend to have very tight Beamwidths,
and few sidelobes that get in the way. Panel and Slot Arrays just don't
have the Beamwidth Control and Minimal Sidelobe specs for this type
of use, and that's why they aren't used in this application.

--
Bruce in alaska
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