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5dB 6 dB 7dB 8 dB

Parabolic Dish f/D

example I have seen is a dish fed with an open WR-90 waveguide, which covers X-band (8-12 GHz). This dish is fed by the open end of the waveguide pointing at the dish; an open waveguide is known to act as a moderate gain antenna. I located a published radiation pattern for open WR-90 waveguide7 and graphed it in Figure 8. Clearly, the efficiency this simple feed provides is far lower than the previous ones. The moral of this story is that just because a dish already has a feed does not mean it is a good feed— the original design goal may not have been maximum gain.

Understanding the Graphs

The purpose of these graphs is to help visualize the performance of various dish feeds and compare them so that the best feed available may be chosen for each application. The underlying assumption is that we wish to obtain the maximum efficiency from a given dish, and thus the maximum gain. After all, a dish doesn't get any lighter or have any less wind resistance if we get less than maximum gain from it. On the right side of each graph is a decibel scale, relating the efficiency to a loss in decibels from the theoretical gain for that aperture. The ripple in some of the curves is an artifact of the discrete points used in the numerical integration process; it could be removed by integrating at smaller intervals.

The program only accounts for unavoidable losses: illumination loss, spillover loss and feed blockage loss. These losses are plotted as a percentage loss.There are several other losses found in a real dish:

• Diffraction from the edge of the dish

• Polarization shift due to reflector geometry

• Blockage by feed supports

• Surface error in the parabolic reflector

. Feed SWR

These losses occur in greater or lesser amounts in a given antenna, so that the real efficiency is lower than the maximum possible efficiency shown in the curves. The best antennas I have measured have efficiencies perhaps 15% lower than the curves, while others are significantly worse. A typical efficiency for a moderate-sized dish is about 50%, for a gain 3 dB below the theoretical gain for a given aperture size. An excellent dish has an efficiency of 60% or so, about 1 dB better than a typical dish, while a poorly chosen feed or a poor installation can make the gain several dB worse. A 1 dB difference may not seem like much, but it is huge for an EME station that can't squeeze another 1 dB from the preamp or power amplifier.

Another limitation of these curves is the accuracy of the data available for each feed pattern. Many of the published articles only give data for the major part of the pattern, but not the back lobes. This is sufficient to calculate the shape of the efficiency curve, but the whole pattern is required to calculate the maximum efficiency. Therefore, I have estimated the rest of the pattern based on similar feeds for which data for the whole pattern is available.

Since some of the feeds are physically larger than others and would have more feed blockage loss on a given dish, comparisons on any fixed dish size would make smaller feeds look better. Therefore, most of the following graphs use a reflector diameter about ten times larger than the feed diameter. If you are comparing these feeds for use on your dish, you can run the program for the actual reflector diameter of your dish. More information about the program appears below.

Some Popular Feeds

We have already looked at the W2IMU feed, best for an f/D of about 0.6, and the VE4MA feed, best for an f tD of about 0.4. A feed often used for EME at lower frequencies is the EIA reference antenna,8 a dual-dipole over a ground plane. W7PUA2 calculated the pattern for this feed, which results in the graph in Figure 9 showing best efficiency for an f/D of about 0.5, suitable for some of the large surplus dishes. Another commonly used feed is the "Coffee can" feed,9 an open-ended circular waveguide. The graph for a 0.96 X diameter version is shown in Figure 10, with best efficiency at an f/D of about 0.5. The feed data provided by VE4MA10 is very closely agrees with data in the Antenna Engineering Handbook by Jasik.11 The low efficiency of this feed is due to the high back-lobe level, which I estimated from the open-ended rectangular waveguide pattern in Figure 8 since no data was available. Reduced back lobes, or a better front-to-back ratio, would result in higher efficiency; one way of reducing the back lobes is by adding "choke" rings around the circular waveguide. The

VE4MA feed uses a single ring while a feed that is popular on TVRO dishes, the Chaparral style feed, adds a series of rings around the circular waveguide. The Chaparral version sold as an "11 GHz Superfeed" has excellent performance1 on 10 GHz. The graph for a version12 with a waveguide diameter of 0.846 X is shown in Figure 11, with a peak efficiency at an f/D of about 0.4, which is much better than the coffee can.

Deeper Dishes

Many of the TVRO and surplus dishes have a deeper shape resulting in an f/D less than 0.4, so the previous feeds are not optimal. However, reducing the diameter of a waveguide radiator should make the pattern broader; for instance, we could reduce the diam: eter of a coffee can feed. Figure 12 is the graph for one with a waveguide diameter of 0.77 A.,10 with best efficiency at an f/D of 0.4, compared to 0.5 for the larger coffee can feed in Figure 9.

Similarly, the waveguide diameter could be reduced in a Chaparral style feed. The graph in Figure 13 is for one with a waveguide diameter of 0.744 X, with best efficiency at an f/D of about 0.32. The peak efficiency is much better than the coffee can, but not so good as the larger Chaparral version. Another paper13 suggests cutting small E-plane slots in the waveguide to broaden the pattern. Figure 14 graphs this version, with higher peak efficiency at an f/D of 0.32.

W7PUA found that the pattern of the EIA dual-dipole feed could be broadened by shortening the dipoles. He accomplished this by bending the dipoles so only part of them radiates effectively, and called the result a "Handlebar" feed.2 If the dipole is bent toward the reflector, some reduction of the back lobe results, improving the front-to-back ratio and potentially increasing efficiency.

His calculated data for his type B Handlebar is graphed in Figure 15, with best efficiency at an f/D of 0.3. Another version, called Type A, has best efficiency at an f/D of about 0.4, while the full-length dipoles of the EIA feed provide best efficiency at an f/D of 0.5. This suggests that the Handlebar feed may be tuned over a range of f/D by bending the dipoles to different lengths.

Finally, I found a feed described by Clavin14 (in 1974) that is physically small and can be built with hand tools; I've used this successfully on small

Continued on page 25. Jan/Feb 1998 19

Waveguide Dipole with Cavity Reflector (Clavin 1954) Figure 17

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