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?

On Mon, 4 Aug 2008 11:49:12 -0700 (PDT), Chris
wrote:

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.

Hi Chris,

The best way to resolve this terminology problem is to ask the ones
with the objections - it was their term after all.

Barring that, they could be talking about directivity vs. frequency.

Either way, the enumeration of your required characteristics should
point out how useful any system would be as their own may aid or
conflict due to their own abilities to support your goal.

73's
Richard Clark, KB7QHC

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

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.

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...

"Chris" wrote in message
...
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?

Hi Chris

What directivity is required from this antenna?
Whats the limit of the budget available for the development of the design
of the slot array?

Jerry KD6JDJ

On Aug 4, 11:49 am, 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 of the things that occurs to me is that it can make a big
difference just how you feed the array. This is something I've
pondered with respect to linear arrays of dipoles at lower
frequencies. If I feed the array from one end and depend on the
length of transmission medium between the feedpoints, e.g. one
wavelength between each radiator so they're fed in-phase, the phasing
will change linearly with frequency. This is hardly noticeable if the
array is short and therefore has low gain; but if it's long so the
beamwidth is, say, 10 degrees, then a 3% change in frequency (.03*360
degrees) will tilt the pattern by about one beamwidth. In a waveguide-
fed system where the propagation velocity depends significantly on
frequency I suppose things could be even worse.

But an alternate way to feed such an array is with a tree of splitters
so that the propagation from the input port to each radiator is the
same time delay. In that case, the radiators will stay in phase and
yield a high gain over a considerable bandwidth. In such a case, you
do still have to account for the fact that the mutual impedance among
the antennas will cause the elements at different points along the
array to represent different loads to the transmission tree "leaves"
and therefore cause different phase shifts, but that's something you
can account for.

I recall once reading about a radar antenna that used a "geodesic
lens" to give equal time delay from the feedpoint to each radiating
element; it was basically a fan-shaped waveguide with a wrinkle in it,
deepest at the middle and flat on the outer edges, which caused the
central transmission path to be the same electrical length as the
outer paths (and all the others).

Cheers,
Tom

K7ITM wrote:
On Aug 4, 11:49 am, 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 of the things that occurs to me is that it can make a big
difference just how you feed the array. This is something I've
pondered with respect to linear arrays of dipoles at lower
frequencies. If I feed the array from one end and depend on the
length of transmission medium between the feedpoints, e.g. one
wavelength between each radiator so they're fed in-phase, the phasing
will change linearly with frequency. This is hardly noticeable if the
array is short and therefore has low gain; but if it's long so the
beamwidth is, say, 10 degrees, then a 3% change in frequency (.03*360
degrees) will tilt the pattern by about one beamwidth. In a waveguide-
fed system where the propagation velocity depends significantly on
frequency I suppose things could be even worse.

This is actually used deliberately to steer a beam by changing the
frequency. Consider a radar with a magnetron that can be tuned...



But an alternate way to feed such an array is with a tree of splitters
so that the propagation from the input port to each radiator is the
same time delay. In that case, the radiators will stay in phase and
yield a high gain over a considerable bandwidth. In such a case, you
do still have to account for the fact that the mutual impedance among
the antennas will cause the elements at different points along the
array to represent different loads to the transmission tree "leaves"
and therefore cause different phase shifts, but that's something you
can account for.

And for which phased array designers spend significant amount of time.
There are "sweet spots" that are easier than others, and if you have
lots and lots of elements, then the fact that edge elements have
different neighbors than all the ones in the middle might be ignored.
All depends on the sidelobe requirements, mostly.



I recall once reading about a radar antenna that used a "geodesic
lens" to give equal time delay from the feedpoint to each radiating
element; it was basically a fan-shaped waveguide with a wrinkle in it,
deepest at the middle and flat on the outer edges, which caused the
central transmission path to be the same electrical length as the
outer paths (and all the others).

There's about as many schemes for feeding a phased array as grains of
sand at the beach it seems.

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
add path after fast to reply

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