10 GHz dual-mode feedhorn - testing

Back in January I built a W2IMU 10 GHz dual-mode feedhorn using dimensions calculated from W1GHZ’s HDL_ANT program. I used a 50mm length of 22 mm od. copper pipe for the input section and 98mm length of 54 mm od. copper pipe for the output phasing section. The flare section was cut from 1.2 mm thick copper sheet which was annealed before shaping and soldering. Some simple bench testing indicated that it was working and I also managed to carry out some limited tests with the feedhorn mounted at the dish.

The CST Studio model and simulation indicated that S11 return loss should be around -20dB at 10.4 GHz. I recently put together a 10 GHz - 1 GHz modular down-converter to use with my Viavi analyser and can now measure return loss.

Using a SMA connector positioned 11.4 mm from the end, with a pin diameter of 1.27 mm" (0.050") and probe length 6.35 mm (0.25") I couldn’t get a good return loss at 10 GHz. I tried slightly different probe lengths with little success and ended up adding a M3 tuning screw opposite the SMA connector. With a probe length of 7mm and the tuning screw adjusted I now measure a return loss better than -25dB at 10.4 GHz - close to the simulated results.