Chris wrote:
On Aug 4, 5:19 pm, Jim Lux wrote:
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?
Well.. a parabolic reflector has exceedingly wide bandwidth, and,
furthermore, its beam direction and or focus doesn't change with frequency.

Arrays of dipoles (which is what a slot array is) are (relatively)
narrow band, and the boresight direction/sidelobes may change with
frequency. The mechanical tolerances may also be tighter.

That said, lots of people use flat slot arrays or printed dipole arrays..
See, e.g.,http://descanso.jpl.nasa.gov/DPSumma...mp20051028.pdf

So it appears the statement is true and technically accurate but in my
case it simply isn't relevant.

Yep..

But check the tolerances on your slot array, particularly if you're
operating over a wide temperature range. Aluminum CTE is about 21
ppm/degree C, so over a 100 degree span, that's 0.2%. Probably not a
big deal for the spacing, but the slot size changing will change the
phase, etc. 0.2% isn't much, but you ARE talking about an antenna with
3% BW...