sizes of waveguide, and I found coax transitions for all three in my collection acquired at hamfests. We were surprised to find how well these worked: a WR-137 transition with a small rectangular flange provided 46% efficiency, close to our expectation, but transitions with large circular flanges showed much higher efficiencies: 56 and 58%. Finally, the highest efficiency was a WR-159 transition with rounded, rather than square, corners on the inside of the waveguide. While large circular flanges may provide the same effect as the choke flange for cylindrical waveguide feed horns described by WA9HUV,15 we can offer no explanation for the performance of the rounded corner waveguide transition.
All the dishes we had available have an f/D ratio of 0.45. At this f/D, the Kumar, Clavin and rectangular feedhorn all offer very good performance. For dishes with other f/D ratios, the recommendations are the same as those offered in "Practical Microwave Antennas, Part 2"; the Kumar and Clavin feeds are better for the f/D range of 0.35 to 0.45, while rectangular feedhorns may be optimized for any f/D greater than 0.45. If you find a surplus waveguide-to-coax transition^ it may provide performance as good as the ones we measured, but be sure to adjust it carefully; as Table 1 shows, the focal distances, and thus the apparent phase centers, vary widely.
At 5760 MHz, the focal length of the dish is not quite as critical as at 10 GHz since a wavelength is nearly twice as long. However, two of the feeds in Table 1 were measured with varying focal distance to show the loss associated with small focal-length errors. Getting the feed exactly at the focal point is still the most important aspect of dish efficiency and gain.
Finally, match the impedance of the antenna to the transmission line—a low VSWR is important at all frequencies, and even more so at microwaves where transmission line losses are high. In "Practical Microwave Antennas" I made the assumption that this would be obvious, but our measurements here are a reminder that the obvious sometimes needs restatement.
It should be apparent that signifi
Fig 3—Full-size template for a 7.2-dBi horn for 5760 MHz, suitable for illuminating a dish with f/D= 0.47.
cant improvements in dish efficiency are available, compared to previously published feed designs for 5760 MHz. How significant? The measurements show a gain increase of 3 to 4 dB with no increase in dish size, weight or wind loading—enough to double the range of a pair of stations making this improvement.
Don Cook, K1DPP, provided both enthusiasm and machining skills for this project. Unfortunately, he became a Silent Key before it was completed. He was a true friend and helped all the hams who knew him; we all miss him.
1Wade, P., N1BWT, "Practical Microwave
Antennas, Part 1," QEX, September 1994, pp 3-11.
2Wade, P., N1BWT, "Practical Microwave Antennas, Part 2," QEX, October 1994, pp 13-22.
3Wade, P.. N1BWT, "Practical Microwave Antennas, Part 3," QEX, November 1994, pp 16-24.
4Lau, Z„ KH6CP/1, "A 5616-MHz Local Oscillator," QEX, May 1993, pp 15-19.
5Cook, R., N2SB, "5760 MHz from the Junkbox," QEX, May 1994, pp 20-24.
6Lau, Z., KH6CP/1, "A Low-Noise PHEMPT Amplifier for 5760 MHz," QEX, September 1994, pp 28-31.
7Wade, P., N1BWT, "Mixers, Etc., for 5760 MHz," Proceedings of Microwave Update '92, ARRL, 1992,"pp 71-79.
8Hill, T., WA3RMX, "A Triband Microwave Dish Feed," QST, August 1990, pp 23-27.
Milliard, D.L., W0PW, "Antenna Ideas for 3.5, 5.8, and 10.3 GHz," QEX, January 1988, pp 3-5.
10Kumar, A,, "Reduce Cross-Poiarization in Reflector-Type Antennas," Microwaves, March 1978, pp 48-51.
1lMalowanchuk, B. W., VE4MA, "Selection of an Optimum Dish Feed," Proceedings of the 23rd Conference of the Central States VHF Society, ARRL, 1989, pp 35-43.
12Malowanchuk, B. W„ VE4MA, "VE4MA 3456-MHz circular polarization feed horn," Feedpoint, November/December 1991, North Texas Microwave Society.
13C!avin, A., "A Multimode Antenna Having Equal E- and H-Planes," IEEE Transactions on Antennas and Propagation, AP-23, September 1975, pp 735-737.
14Evans, D., G3RPE, "Pyramidal horn feeds for paraboloidal dishes," Radio Communication, March 1975.
15Foot, N.J., WA9HUV, "Second-Generation Cylindrical Feedhorns," Ham Radio, May 1982, pp 31-35.
Effects of the Ionosphere
Was this article helpful?