QEX

turability. Figure 22 shows one example, but some others have been published. We will examine a few to see what is possible over a range of f/D.

The NRAO Scalar feed in Figure 22 was recently replaced27 with a large dual-band feed that is 5.2 X in diameter at S-band. Figure 30 is a graph of the S-band efficiency with this feed, which is best at an f/D of about 0.6. However, the 85 foot dish has an f/D = 0.42. The goal in radio astronomy is lowest noise, so the feeds are optimized for best G/T, the ratio of gain to noise temperature, rather than maximum efficiency. Using the text output of the FEEDPATT program, we can see that the edge illumination from this feed at an f/D = 0.42 is about -17 dB, while the edge illumination at the best efficiency is -8.2 dB. The result of the reduced edge illumination is less spillover, since the spillover "looks" at warm earth rather than cold sky. In Figure 30, the spillover at an f/D = 0.42 is much smaller than at best efficiency; the tradeoff is a small decrease in efficiency for a significant reduction in noise. Although the dish is optimized for noise rather than efficiency, measured aperture efficiency was 59%, using a radio star for calibration.

The Parkes Radiotelescope in Australia is 210 feet in diameter, with an f/D of 0.41. From the description, the 5 GHz two-hybrid-mode feed installed in 196828 was slightly over 3 X in diameter, but the reflector is so large that feed blockage is negligible. The graph in Figure 31 shows excellent efficiency at an f/D of about 0.45. Note that the polar plot of feed radiation shows an increase in energy away from the center of the dish, like our desired pattern in Figure 2. The antenna was recently upgraded to allow for rapid feed changes for different frequencies, and a 2.3 GHz feed29 designed for reception of the Galileo mission. The 2.3 GHz feed is also a two-hybrid-mode design, 3.3 A; in diameter, designed with an edge illumination of-16 dB for best G/T. The feed pattern data has good phase performance to the edge of the dish. The efficiency curve for the 2.3 GHz feed shown in Figure 32 has best efficiency at an f/D of 0.55, but much less spillover at the actual f/D = 0.41. This large radiotelescope also has high measured efficiency, 63% at 2.3 GHz, with the feed optimized for noise rather than efficiency.

A "High-efficiency Horn Feed" was described by Shukla.30 The graph in Figure 33 shows excellent efficiency at an f/D of about 0.75.

Spillover and Side Lobes

Perhaps we should take a lesson from the radio astronomers. The radiotelescope feeds above all operate at a point on the efficiency curve to the left of the peak, or lower f/D, for reduced spill-over. W7PUA suggests that since spillover increases side lobes and side lobes are always bad, adding noise and interference potential, we should make any compromise to the left side of the peak.

Computer Program

For the benefit of hams without a computer, I show performance for a wide range of feeds. The FEEDPATT program does all the calculations and plots graphs like the ones above. For those with access to the Internet, the FEEDPATT program and all the data files for feed patterns are available on my 10 GHz Web pages: www.tiac .net/users/wade/feedpatt.zip or www.qsl.net/-nlbwt. You can also get them from the ARRL QEX Web site.33 See the README.TXT file for details of program operation.

The output graphs from the program are in PostScript format, ready for printing on a laser printer or for viewing and printing on a PC using the free Ghostscript4 software. On my PC running Windows95 or WindowsNT, I run the FEEDPATT program in one window and Ghostscript gsview in another to view the output as I work.

With the data files for all the above feeds, it should be possible to graph the potential performance of any of them with your dish for any frequency of interest. For other feeds, if you can find, calculate or measure a radiation pattern, you can calculate a graph of estimated efficiency. Please send me a copy of any new feed data.

Feed Pattern Measurement

Antenna gain measurements on large antennas like dishes are difficult31 and require a large antenna range. Pattern measurements are even more difficult, since side lobes may be more than 20 dB down, so even small reflections can distort the pattern. On the other hand, the small aperture of a feed horn requires much smaller distance for far-field measurements. Most of these feeds are 2 A. or less in diameter, so an antenna range that is 2 feet long at 10 GHz (or 10 feet long at 1296 MHz) would be fine. If all reflecting obstacles can be four times as far away as the range length (8 feet

Continued on page 32.

G4ALN Penny Feed measured at 10.368 GHz Figure 35

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