dichroic subreflector 2

[kurtulmehtap]

We would like to build a dichroic subreflector which will be used in a S+X Band Parabolic Antenna.
Dichroic surfaces are transparent to one frequency and reflect another.
In our case the frequencies are in the S and X bands.
How difficult is to design and build such a dichroic subreflector?

 

[biff44]

I ran a program that used a 20/30 GHz antenna with two feed horns, and a lowpass plastic reflector surface. The benefit of the two feed horns was that you could optimally illuminate the one main reflector at the two frequencies. Normally with coaxial feeds, you can optimize only one frequency, and the other frequency spills over the edge. At 30 Ghz, we were only bouncing 60% of the power off the dichroic reflector, but that was ok for our applicaiton.

As I recall, the reflector was just a piece of microwave boards with a pattern of X's photolithed onto the surface.

I think in your case, the hard part is to pass one frequency while totally reflecting the other. Since the two freqs are far apart, you might have to get the shape of the metal reflector pattern just right.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[Higgler]

which are you reflecting, hopefully X band.
Is this linear polarization? I hope so, it's easier.

It's pretty easy, just print some 1/2 wavelength lines, space them 3/4 wavelengths apart in all directions and you'll reflect X band with negligible results at your lower band.
Make the line width about 0.050".

If you want a very optimized result, that'll take some comupter work.

kch

 

[kurtulmehtap]

Yes we are reflecting the X Band and the Subreflector has to be transparent to the S Band.
The other Specs are:
RHCP polarization for X-Band receiver (8.025-8.375 GHz)
LHCP polarization for S-Band receiver. (2.2-2.3 GHz)
RHCP polarization for S-Band transmitter (2.04-2.12 GHz)

Also there is a need for supporting autotrack. So the feed has to support this.
Higgler mentioned CAD for the subreflector design.
Is there any software available for designing Dichroic Reflectors and autotrack feeds?

 

[Higgler]

good separation on the frequencies.

Does autotrack require the antenna beam and sidelobes to be tightly controlled? Sidelobes typically come up with a dichroic reflector and closely spaced reflecting elements help minimize the degradation for the high band, though it can hurt for the lower band. At the low bands, the surface looks like a capacitor or lens and the energy propagating through it gets delayed.

A reflecting surface for X band will change it's polarization. i.e. if you want RHCP, you have to use LHCP on your feed horn to bounce RF energy off this reflecting surface.

software is $50K with a long and painful learning curve, it'll be cheaper to hire someone (hmmn, we do that this stuff coincidentally, it'll cost less than the software). Ansoft HFSS and CST Microwave Studio are the two main ones.

If you want to experiment in the lab making your own, get some quarters (25 cents) and array them in X and Y, it's a low pass filter that'll reflect your X band. We did this to validate codes for frequency selective surface software, it was cheap and quick. It works pretty well and since they're round, it's good for circular polarization. A quarter is 0.78 Lambda at 8 ghz and should work ok for you. Space them around 1.2 inch centers.

kch