Bruce in alaska wrote:
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.


1/2 degree kinds of beamwidth aren't that tough to get. (a meter or two
aperture at 10GHz gets you in the ballpark, a third that at 30GHz Ka-band).

beamwidth is almost entirely a function of the physical extent of the
antenna (that is, a 20 meter antenna will have comparable beamwidth,
regardless of whether it's a reflector or phased array)

Sidelobes are a function of the control over amplitude and phase across
the aperture. You can use a smooth surface (as in a reflector) or good
element and feed design (as in a low sidelobe phased array). There are
phased arrays (electrically steerable no less) with sidelobe levels more
than 50 dB down. That's an impressive number for ANY antenna (for a
reflector, you're going to be paying a lot of attention to surface
finish and even more to the feed design)

You might want more aperture on an uplink for the gain, as opposed to
the beam properties. You could spend more on the fixed antenna and less
on the power amplifier (e.g. use a 12m antenna instead of a 6m antenna
and a PA that's 1/4 the size.. either one will have a beam that is
substantially bigger than the satellite you're pointing at). There *IS*
a limit.. cost starts to rise pretty quickly above a particular size,
because the structure to hold mechanical tolerances is big and
expensive. (e.g. the Deep Space Network 72 meter antennas cost a good
chunk of a billion dollars back in the 60s)

It's more a cost thing. If you only need to point in one direction at
one frequency, and it needs to have large physical aperture, then a
fixed reflector is probably the cheapest solution (particularly over life).



Here's a relatively old design (1969) at 16 GHz with 3 degree beamwidth,
pretty decent sidelobes, and steerable over 60 degrees

http://ntrs.nasa.gov/archive/nasa/ca...1970002178.pdf