dishes at 10 Ghz1 and 5760 Mhz.15 The original version is graphed in Figure 16, with best efficiency at an f/D around 0.35 to 0.4. Peak efficiency is not as high as that of some other feeds, but on small dishes, the efficiency of the better feeds would be reduced by feed blockage.

Phase Errors

All of these graphs are based on amplitude patterns only for the feeds, because phase data is much more difficult to measure and is rarely available. If the phase of the radiated energy is not uniform over all areas of the reflector, different parts may reflect out-of-phase energy into the main beam and reduce the total main-beam energy, reducing the gain.

Another common problem is feeds that do not have the same phase center in the E-plane and the H-plane, which has the same effect as not having the phase center at the focal point: reduced gain and pattern distortion.

Phase errors are probably the largest cause for low efficiency, so you should not expect to get efficiencies near the calculated values unless the feed has good phase performance. A feed with small phase error still suffers from all of the other losses listed above, so the expected performance of a real dish might be only 15% lower than the calculated efficiency curve.

The only feeds with published phase data are the Kumar (VE4MA), Chaparral, Chaparral with slots and the Koch Multi-ring feed discussed below. All of these have excellent phase performance over a wide illumination angle, so the efficiency curves for these feeds are good for any(f D greater than 0.3. None of these feeds can adequately illuminate an f/D of 0.25, but the bent-dipole Handlebar feeds of W7PUA2 show promise at the lower frequencies.

Other Feeds

Clavin described another feed in 1954.16 It appears to be a dipole projecting through the broad sides of a waveguide with a cavity reflector at the waveguide end, so that the radiation is backwards along the incoming waveguide. The graph in Figure 17 shows lower efficiency than the 1974 version in Figure 16. The 1954 version is described as a significant improvement on those WW2-vintage feed designs found in the old books (unfortunately, some newer books keep copying them).

A dipole with a disc reflector (splasher) has long been a popular simple feed, but I have not located a good pattern for one. The pattern I located was for a version17 with a very large (3 X in diameter) reflector, so it is only suitable for large dishes. Figure 18 is the graph for this feed, with best efficiency at an f /D of about 0.35. The large reflector improves the pattern front-to-back ratio significantly, compared to a dipole with a classic wire reflector or a Yagi with 2 elements. The graph of data from W7PUA2 is shown in Figure 19, with lower efficiency peaking at an f/D around 0.25 to 0.3. I would expect a dipole with a small splasher to be somewhat better than Figure 19, but simple dipole feeds are known to have phase and polarity errors over wide illumination angles.

The VE4MA feed designs were based on a paper by Kumar.18 The original data plotted in Figure 20 is not much different from the VE4MA version in Figure 6.

Another published feed design is a "Multi-ring Coaxial Feed" by Koch 19. The graph for this feed, in Figure 21, shows best efficiency for an f/ D around 0.35 to 0.4. The drawing in the article does not appear to be something that amateurs could fabricate easily, but I have included it because few such feeds have phase data available.

Offset fed Dishes

None of the feeds so far are optimal for an f/D greater than 0.6. A large f/D places the feed far from the reflector, making the feed support structure difficult, so dishes with a large f/D are rare. However, offset fed dishes often require a narrower illumination angle equivalent to a large f/D for a conventional center fed dish. For instance, the ubiquitous RCA DSS dishes need an illumination angle equivalent to an f/D of 0.71. At 10 GHz, we have successfully fed these dishes with small rectangular horns20 and obtained high efficiency.21 The main advantage of the offset fed dish is that the feed and support can be completely out of the main beam, so there is no feed blockage loss.

An additional advantage of offset fed dishes used as satellite receiving antennas is that spillover is "looking" at cold sky, so its G/T performance is better than a conventional dish, whose spillover "sees" warm earth. Thus, an offset fed dish would be a good choice for EME operation if a large enough dish were available.

Many commercial feeds for offset dishes appear to be conical horns with corrugated walls; the corrugations reduce phase error found in smooth-walled corrugated horns. One example of this feed is called a Scalar feed.22 The graph for this feed is shown in Figure 22, with f/D for best efficiency around 0.5 to 0.55. The article states that the 10 dB beamwidth is about 75% of the flare angle, so this feed could be tailored to a different f/D by changing the flare angle of the horn. This physically large feed, more than 5 A. in diameter, was used at 5 GHz on the National Radio Astronomy Observatory (NRAO) 85 foot radiotelescope in West Virginia. The estimated aperture efficiency was 60%. Although the article suggests that this feed is only useful on dishes greater than 30 A. in diameter, offset dishes should not have any limitation as long as the feed does not block the beam.

Another possibility for offset dishes is the W2IMU dual-mode feed. All the amateur-band versions that I've seen have a diameter of about 1.3 X, and G3PH023 is using one to illuminate an offset fed dish at 10 GHz. From Figure 5, the f/D for best efficiency is about 0.6, so it is not optimal for many offset dishes. However, the original article24 also described a larger version with a diameter of 1.86 X. The graph for the larger version in Figure 23 shows that the f/D for best efficiency is about 0.8.

A W2IMU dual-mode feed optimized for the DSS dishes could be built, with an intermediate feed diameter suitable for an f/D of 0.71. This dual-mode feed works by generating and propagating two modes, TEU and TMU, in the output section. Since the modes propagate at different guide velocities, there is a length of output section for which the two modes cancel and eliminate side lobes and back lobes generated by currents in the rim of the horn. The polar plots in Figures 5 and 23 show no discernible side lobes. The trick in changing the diameter would be to get the length of the output section right so that the two modes cancel. The length of the output section would have to be adjusted until side lobes were eliminated from the feed pattern, but once the right length was found, everyone could copy the dimensions just as we do for other feed designs.

Helical Feed Antennas

Helix antennas are broadband and provide circular polarization, but are not often used as feeds. I was referred by I5TDJ to one design,25 a 1.59 turn helix with a very large reflector (30 inch diameter at 2 GHz), which presumably improves the front-to-back ratio. For a very large dish, this feed is graphed in Continued on page 30.

Helix 7 turn with Ground Plane dia. = 0.35 A. Figure 27

Circular Polarization

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