Dual-band Feedhorn for the DSS Offset Dish |
I recently completed a new transverter for 5760 MHz in a fairly smallpackage ¾ it fits on top of my 10 GHz transverter next to the wedgewhich supports the RCA DSS offset dish. I designed a 5760 MHz feedhornfor the dish using my HDLANT21computer program, built one, and modified the transverter slightly to allowfor quick changing of the feedhorns with two wingnuts. Now I had a package,shown in Figure1, for a compact two-band rover station.
I was wondering if it was possible to make a good dual-band feed whenDick, K2RIW, mentioned that WR-112 waveguide covers both 5760 MHz and 10368MHz; even though the handbooks don't list it as usable for 5760, the cutofffrequency is slightly lower so it still works.
The next problem was designing a feedhorn to cover both bands with decentillumination for the dish. A few trial calculations showed that a 10 GHzhorn providing -10 dB edge illumination taper would provide a -3 dB edgeillumination at 5760 MHz -- most of the energy would miss the dish! Onthe other hand, a horn designed for 5760 would have a much narrower beamat 10 GHz, so the outer portions of the dish would receive very littleillumination energy; only the performance of a much smaller dish wouldresult. After some fiddling of the numbers, I found a compromise whichmight have the same loss of efficiency at both frequencies.
The final design, using the HDLANT21 template shown in Figure2, has an illumination taper of roughly -16 dB at 10.368 GHz, so it issomewhat under-illuminated, and roughly -5 dB at 5760 MHz, somewhat over-illuminated.I adjusted the horn length to match the phase centers at 10.368 GHz, sinceit is most critical at the higher frequency. Here is the templatefor the dual-band horn.
The next problem was getting a good VSWR at both frequencies. The surplusWR-112 waveguide-to-coax transitions I had weren't very good at 5760 MHz,so tuning was required. I put a small ball bearing inside the waveguideand moved it around with a magnet on the outside until I located a spotwhich improved the VSWR at 5760 MHz without making the 10368 MHz VSWR toomuch worse. Then I marked the spot, drilled and tapped the waveguide, andput in a tuning screw. Next I adjusted the screw for best VSWR at 5760MHz, then put the BB back in and looked for a spot which improved bothfrequencies. A second screw was added here, then both screws adjusted fora compromise with reasonable VSWR at both frequencies. The final tuninghad a VSWR under 1.6 at both 5760 MHz and 10368 MHz, but it is not a broadbandmatch.
The next day, I set up a sun noise measurement at 5760 MHz, with similarresults: the DSS dish with a single-band horn feed has an efficiency ofabout 60%, while the dual-band feed is around 50%; the gain differenceworks out to about 1 dB on this band.
Q - Is a dual-band horn possiblefor lower bands?
A - Yes, with a larger offset dish.A dish should be at least 10 wavelengths in diameter for good performance,so the 18 inch RCA dish isn't big enough below 5760 MHz.
