By Paul Wade NBWT

Parabolic dish antennas can provide extremely high gains at microwave frequencies. A 2-foot dish at 10 GHz can provide more than 30 dB of gain. The gain is only limited by the size of the parabolic reflector; a number of hams have dishes larger than 20 feet, and occasionally a much larger commercial dish is made available for amateur operation, like the 150-foot one at the Algonquin Radio Observatory in Ontario, used by VE30NT for the 1993 EME Contest. But these high gains are only achievable if the antennas are properly implemented, and dishes have more critical dimensions than horns and lenses.

Background

Last September (1993), I finished my 10-GHz transverter at 2 PM on the Saturday of the VHF QSO Party. After a quick checkout, I drove up Mt. Wachusett and worked four grids using a small horn antenna. However, for the 10-GHz Contest the following

161 Center Road Shirley MA 01464

weekend, I wanted to have a better antenna ready.

Several moderate-sized parabolic dish reflectors were available in my garage but lacked feeds and support structures. I had thought this would be no problem, since lots of people, both amateur and commercial, use dish antennas. After reading several articles in the ham literature, I had a fuzzy understanding and was able to put a feed horn on one of the dishes and make a number of contacts of over 200 km from Mt. Washington, in horizontal rain.

But I was not satisfied that I really understood the details of making dishes work, so I got some antenna books from the library and papers from IEEE journals and did some reading. This article is an attempt to explain for others what I've learned. The 10-GHz antenna results from the 1993 Central States VHF Conference suggest that I might not be the only one who is fuzzy on the subject—the dishes measured had efficiencies of from 23% to less than 10%, while all the books say that efficiency should typically be 55%. On the other hand, there are enough hams doing successful EME work to suggest that some have mastered feeding their dishes. One of them, VE4MA, has written two good articles on TVRO dishes and feedhorns for EME.12

There have been some good articles written by antenna experts who are also hams, like KI4VE, K5SXK and particularly W2IMU in The ARRL UHF/Microwave Experimenter's Manual, which is an excellent starting point. However, as I struggled to understand things that are probably simple and obvious to these folks, I did some reading and then used my personal computer to do some of the more difficult calculations and plot them in ways that helped me to understand what is happening. Many of us find a picture easier to comprehend than a complex equation. What I hope to do here is to start at a very basic level and explain the fundamentals, with pictures and graphics, well enough for hams to implement a dish antenna that works well. An accompanying computer program, HDL_ANT, is provided to do the necessary design calculations and to draw templates for small dishes in order to check the accuracy of the parabolic surface.

1 Notes appear on page 22.

HDL_ANT can be downloaded from the ARRL BBS (203-666-0578) or via the Internet from ftp.cs.buffalo.edu in the /pub/ham-radio directory.

Dish Antenna Design

A dish antenna works the same way as a reflecting optical telescope. Electromagnetic waves, either light or radio, arrive on parallel paths from a distant source and are reflected by a mirror to a common point, called the focus. When a ray of light reflects from a mirror or flat surface, the angle of the path it takes leaving the surface (angle of reflection) is the same as the angle at which it arrived (angle of incidence). This optical principle is familiar to anyone who misspent a part of his youth at a pool table! If the mirror is a flat surface, two rays of light that arrive on parallel paths leave on parallel paths; however, if the mirror is curved, two parallel incident rays leave at different angles. If the curve is parabolic (y = ax2), then all the reflected rays meet at one point, as

Parabolic Curve Dish

shown in Fig 1. A dish is a parabola of rotation, a parabolic curve rotated around an axis that passes through the focus and the center of the curve.

A transmitting antenna reverses the path: the light or radio wave originates from a point source at the focus and is reflected into a beam of rays parallel to the axis of the parabola.

Illumination

Some of the difficulties found in real antennas are easier to understand when considering a transmitting antenna but are also present in receiving antennas, since antennas are reciprocal. One difficulty is finding a point source, since any antenna, even a half-wave dipole at 10 GHz, is much bigger than a point. Even if we were able to find a point source, it would radiate equally in all directions, so the energy that was not radiated toward the reflector would be wasted. The energy radiated from the focus toward the reflector illuminates the reflector, just as a light bulb would. So we are looking for a point source that illuminates only the reflector.

Aperture, Gain, and Efficiency

The aperture, gain, and efficiency of an antenna were all defined for antennas in general in part 1 of this series of articles. The aperture of a dish antenna is the area of the reflector as seen by a passing radio wave:

Aperture = nr2, where r is the radius, half the diameter of the dish.

If we replace a dish antenna with a much larger one, the greater aperture of the larger dish captures much more of the passing radio wave, so a larger dish has more gain than the a smaller one. If we do a little geometry, we find that the gain is proportional to the aperture.

The gain of a dish is calculated as described in part 1:

Fig 1—The geometry of a parabolic dish antenna.

with reference to an isotropic radiator, h is the efficiency of the antenna. It might be amusing to calculate the gain of the VE30NT 150-foot dish at various frequencies; use 50% efficiency to make the first calculation simpler, then try different values to see how efficiency affects gain.

How much efficiency should we expect? All the books say that 55% is reasonable, and 70 to 80% is possible with very good feeds. Several ham articles have calculated gain based on 65% efficiency, but I haven't found measured data to support any of these numbers. On the other hand, KI4VE suggests that the amateur is lucky to achieve 45-50% efficiency with a small dish and a typical "coffee-can" feed.3

Practical Dish Antennas

When we first described a parabolic dish antenna, we put a point source at the focus, so that energy would radiate uniformly in all directions both in magnitude and phase. The problem is that the energy that is not radiated toward the reflector will be wasted. What we really want is a feed antenna that radiates only toward the reflector and has a phase pattern that appears to radiate from a single point.

Feed Patterns

We have already seen that efficiency is a measure of how well we use the aperture. If we illuminate the whole reflector, we will be using the whole aperture. Perhaps our feed pattern should be as shown in Fig 2, with uniform feed illumination across the reflector. But when we look more closely at the parabolic surface, we find that the focus is farther from the edge of the reflector than from the center. Since radiated power diminishes with the square of the distance (inverse-square law), less energy is arriving at the edge of the reflector than at the center; this is commonly called space attenuation or space taper. In order to compensate,

Fig 2—A parabolic dish antenna with uniform feed illumination.

14 QEX

Fig 3—The desired dish illumination would provide uniform field intensity at all points on the reflector.

we must provide more power at the edge of the dish than in the center by adjusting the feed pattern to that shown in Fig 3, in order to have constant illumination over the surface of the reflector.

Simple feed antennas, like a circular horn (coffee-can feed) that many amateurs have used, have a cos'O) pattern like the idealized pattern shown in Fig 4. In Fig 5 we superimpose that on our desired pattern; we have too much energy in the center, not enough at the edges, and some misses the reflector entirely. The missing energy at the edges is called illumination loss, and the energy that misses the reflector is called spillover loss. The more energy we have at the edge, the more spillover we have, but if we reduce spillover, the outer part of the dish is not well illuminated and is not contributing to the gain. Therefore, simple horns are not ideal dish feeds (although they are useful). In order to have very efficient dish illumination we need to increase energy near the edge of the dish and have the energy drop off very quickly beyond the edge.

Edge Taper

Almost all feedhorns will provide less energy at the edge of the dish than at the center, like Fig 4. The difference in power at the edge is referred to as

using a simple horn feed.

the edge taper, or illumination taper. With different feedhorns, we can vary the edge taper with which a dish is illuminated. Different edge tapers produce different amounts of illumination loss and spillover loss, as shown in Fig 6: a small edge taper results in larger spillover loss, while a large edge taper reduces the spillover loss at the expense of increased illumination loss.

If we plot these losses versus the energy at the edge of the dish in Fig 7, we find that the total efficiency of a dish antenna peaks with an illumination taper, like Fig 6, so that the energy at the edge is about 10 dB lower than the energy at the center.4'6 This is often referred to as 10-dB edge taper or edge illumination—often recommended but not explained.

When an antenna is receiving a signal from space, such as a satellite or EME signal, there is very little background noise emanating from the sky compared to the noise generated by the warm (300 K) earth during terrestrial communications. Most of the noise received by an antenna pointed at the sky is earth noise arriving through feed spillover. As we saw in Fig 6, the spillover can be reduced by increasing the edge taper, while Fig 7 shows the efficiency, and thus the gain, decreasing slowly as edge taper

Taper Spill Over

Fig 5—A comparison of typical dish illumination with the desired illuminaton.

is increased. The best compromise is reached when GIT, the ratio of gain to antenna noise temperature, is maximum. This typically occurs with an edge taper of about 13 dB, but the optimum edge taper for GIT is a function of receiver noise temperature and sky noise temperature at any given frequency.2

Focal Length and f/D Ratio

All parabolic dishes have a parabolic curvature, but some are shallow dishes, while others are much deeper and more like a bowl. They are just different parts of a parabola that extends to infinity. A convenient way to describe how much of the parabola is used is the f/D ratio, the ratio of the focal length f to the diameter D of the dish. All dishes with the same f/D ratio require the same feed geometry, in proportion to the diameter of the dish. The figures so far have depicted one arbitrary//D; Fig8 shows the relative geometries for commonly used f/D ratios, from 0.25 to 0.65, with the desired and idealized feed patterns for each.

Notice the feedhorn patterns for the various f/D ratios in Fig 8. As f/D becomes smaller, the feed pattern to illuminate it becomes broader, so different feedhorns are needed to properly illuminate dishes with different f/D ratios. The feedhorn pattern must

October 1994 15

be matched to the reflector f/D. Larger f/D dishes need a feedhorn with a moderate beamwidth, while a dish with an f/D of 0.25 has the focus level with the edge of the dish, so the subtended angle that must be illuminated is 180 degrees. Also, the edge of the dish is twice as far from the focus as the center of the dish, so the desired pattern would have to be 6 dB stronger (inverse-square law) at the edge as in the center. This is an extremely difficult feed pattern to generate. Consequently, it is almost impossible to efficiently illuminate a dish this deep.

Phase Center

A well-designed feed for a dish or lens has a single phase center, as described in part 1 of this series of articles, so that the feed radiation appears to emanate from a single point source, at least for the main beam, the part of the pattern that illuminates the dish or lens. Away from the main beam, the phase center may move around and appear as multiple points, as stray reflections and surface currents affect the radiation pattern. Also, the phase center will move with frequency, adding difficulty to broad-

lllumination loss Spillover loss

Ratio Parabolic Reflector
f/D = 0.5 Illumination taper
Edge Taper Spillover

Table 1—Measured Effect of Focal Length Error at 10 GHz f/D = 0.5 Illumination taper = 10 dB

band feed design. Fortunately, we are only considering narrow frequency ranges here.

Symmetry of E-plane and H-Plane

On paper, we can only depict radiation in one plane. For a simple antenna with linear polarization, like a dipole, this is all we really care about. A dish, however, is three-dimensional, so we must feed it uniformly in all planes. The usual plane for linear polarization is the E-plane, while the plane perpendicular to it is the H-plane. Unfortunately, most antennas not only have different radiation patterns in the E- and H-planes, but also have different phase centers in each plane, so both phase centers cannot be at the focus.

Focal Length Error

When I started actually measuring the gain of dish antennas, I discovered the most critical dimension to be the focal length—the axial distance from the feed to the center of the dish. A change of V* inch, or about a quarter-wavelength, changed the gain by a dB or more, shown in Table 1 as measured on a 22-inch dish with f/D = 0.39.

I was surprised at this sensitivity, since my experience with optics and photography suggested that this is not so critical—it would be extremely difficult to adjust a lens or telescope to an optical quarter wavelength. But lenses become more critical to focus as the /"-stop is decreased—an f 2 lens is considered to have a very small depth of field, while an f 16 lens has a large depth of field, or broad focus. The /-stop of a lens is the same as the f/D ratio of a dish—both are the ratio of the focal length to the aperture diameter. A typical reflector telescope has a parabolic reflector of f 8, but a dish antenna with f/D = 0.4 has an f-stop of 0.4, so focusing is much more critical.

More reading located an article which described how to calculate the

Table 1—Measured Effect of Focal Length Error at 10 GHz

Fig 6—Dish illumination at various values of illumination taper.

16 QEX

Feed

Distance

Relative

Gain

N1BWT 19 94

N1BWT 19 94

100 90 30

70 60

30 20 10 0

Edge Illumination Antenna

Edge Taper - dB

Edge Taper - dB

Fig 7—Dish efficiency versus edge taper. The peak efficiency occurs at a taper of about 10 dB.

100 90 30

70 60

30 20 10 0

loss due to focal length error.7 Fig 9 shows the loss as the feedhorn is moved closer and farther than the focus for various/ZD dishes with uniform illumination; the tapered illumination we use in practice will not have nulls as deep as the curves shown in Fig 9. It is clear that dishes with small fID are much more sensitive to focal length error. Remember that a wavelength at 10 GHz is just over an inch.

The critical focal length suggests that it is crucial to have the phase center of the feed exactly at the focus of the reflector. Since the phase center is rarely specified for a feedhorn, we must determine it empirically, by finding the maximum gain on a reflector with known focal length.

If we are using a feedhorn with different phase centers in the E- and H-planes, we can also estimate the loss suffered in each plane by referring to Fig 9.

Lateral errors in feedhorn position are far less serious; small errors have little effect on gain, but do result in shifting the beam slightly off bore-sight.

Notice that the focal-length error in Fig 9 is in wavelengths, independent of the dish size. A quarter-wavelength error in focal length produces the same loss for a 150-foot dish as for a 2-foot dish, and a quarter-wavelength at 10-GHz is just over 'A inch. Another implication is that multiband feeds, like the WA3RMX triband feed, should be optimized for the highest band, since they will be less critical at lower bands with longer wavelengths.8

Total Efficiency

It has been fairly easy to calculate efficiency for an idealized feed horn pattern due to illumination taper and spillover, but there are several other factors that can significantly reduce efficiency. Because the feed horn and its supporting structures are in the beam of the dish, part of the radiation is blocked or deflected, A real feed horn also has sidelobes, so part of its radiation is in undesired directions and thus wasted. Finally, no reflector is a perfect parabola, so the focusing of the beam is not perfect. We end up with quite a list of contributions to total efficiency:

•illumination taper

• spillover loss •asymmetries in the E- and

H-planes

• focal point error

• feedhorn sidelobes

• blockage by the feed horn

• blockage by supporting structures

• imperfections in parabolic surface

• feedline loss

KI4VE suggests that the amateur is lucky to achieve 45-50% efficiency with a small dish and a typical coffee-can feed.3 I suspect that the only way to find total efficiency, or to optimize it, is to make gain measurements on the complete antenna.

Practical Feed Systems

An optimum feed would approximate the desired feed pattern for the f.IJ of the parabolic reflector in both planes and have the same phase center in both planes. Let's examine some of the available feed horn designs to see how well they do:

1. Dipole

Most hams know what the pattern from a dipole looks like—in free space, it looks like a donut with the dipole through the hole. If it is near ground jr a reflector, the pattern in the plane perpendicular to the dipole (H-plane) is distorted to emphasize radiation away from the reflector. The shape of the radiation in this plane is controlled by the distance from the reflector, while the shape of the radiation in a plane parallel to the dipole (E-plane) does not change significantly. This suggests that the best we could do is to find a dish with an edge angle that approximates the E-plane beamwidth and adjust the reflector spacing so that the H-plane beamwidth matches the E-plane. Round disk reflectors are frequently used, but it turns out that the pattern is the same as a half-wavelength rod reflector.

2. Dual Dipole

The H-plane beamwidth can be narrowed by adding a second parallel dipole over a plane reflector, such as the EIA(sometimes erroneously called NBS) reference antenna.9 This is a reasonably good feed with good symmetry for reflectors with f/D around 0.55 and has been used with good success for 432-MHz EME.

3. Penny-Splasher

The penny-splasher feed is equivalent to a dual dipole with reflector— the slots in the waveguide act as dipoles.10

4. Rectangular Horn

The beamwidth of a born antenna is controlled by the horn aperture dimen sion, but a square horn has different E- and H-plane beamwidths. We can make it rectangular with the aperture dimensions adjusted so that the E- and

H-plane patterns and beamwidths are similar. G3RPE described this technique and showed thatat 10 GHz itcan only illuminate dishes with/7D greater than 0.48 if the horn is driven by common WR-90 waveguide.11 However, the smaller WR-75 waveguide is also suitable for 10 GHz and could

Narrow Beam Scalar Horns
Illumination taper = 10 dB a: f/D = 0.75

Illumination loss Spillover loss

Illumination taper = 10 dB d: f/D = 0.45
Magnetic Field Parallel Wires
Illumination taper = 10 dB e: f/D = 0.35

Fig 8—Dish illumination patterns tor dishes of various f/D ratios. 18 QEX

drive horns which would illuminate an f/D as small as 0.43.

With a rectangular horn, it is difficult to achieve both a common phase center for the E- and H-planes and similar patterns in both planes. The horn section of the HDL. ANT computer program calculates the phase centers and allows adjustment of dimensions to change them. Kraus shows a series of patterns for horns with different flare angles, and some of them approximate the desirable feed pattern of Fig 3.12 However, no phase information is given; W2IMU once told me they were terrible, and I accept his authority.

5. Circular Horn

A circular horn antenna, since it is symmetrical, might be expected to provide a fairly symmetrical pattern. Unfortunately, it doesn't, and the phase centers are different for the E- and H-planes. The beamwidth is controlled by the diameter of the horn—for wide beamwidths, the horn may have no flare, like the coffee-can feed, or cylindrical horn, often used at 1296 MHz.13

Some improvement in the pattern may be provided by adding a choke flange to a cylindrical horn.14 Further improvement is possible by adding slots in the flange, though radiation patterns are shown in only one plane.15

All of the above feeds have cos^O) patterns similar to Fig 4. Many of these were developed for radar applications, where feed inefficiency may be compensated by increased power. More recently, satellite communication has prompted research into more efficient feed antennas, particularly for deep dishes (small f/D,) with reduced sidelobes and better G/T. Here are a few of the many variations that have been described, chosen for their potential for construction without elaborate machining:

6. Clavin Feed

The Clavin feed is a cavity antenna fed by a resonant slot, with probes that excite a second waveguide mode to broaden the pattern in the H-plane to match the E-plane.16 Radiation patterns approximate our desired feed pattern, Fig 3, while maintaining a good phase center. Fig 10 is a sketch of one I made from a 1-inch copper plumbing pipe cap. It is best for deep dishes with f/D in the 0.35 to 0.4 range. The resonant slot makes it more narrowband than the others (not a problem for amateur use), and the smaller size would have less feed blockage than the "Chaparral" or Kumar feeds, so it might provide better performance on smaller dishes.

eo-15.0

A scalar feed is one that has no inherent polarization; the word "scalar" means that the electric, field distribution is independent of the axis in which you look at the distribution. The result is that scalar horns have equal

eo-15.0

F'eed Axial Displacement from Focus in Wavelengths

F'eed Axial Displacement from Focus in Wavelengths

Fig 9—The loss due to axial displacement of the feed from the focus point is highly dependent on the f/D ratio.

1 Inch Copper Pipe Cap, Cut Down

WR-90 WAVEGUIDE

#14 Wire

0.185

0.185

SLOT 0.83 x 0.08

Dimensions in Inches

Fig 10—A Clavin feed for 10 GHz, made from a 1-inch copper pipe cap.

Fig 11—This photo shows the technique of mounting a dish using a frying pan with a rolled edge. Also note the Clavin feed used with this dish.

Dc3qs Feed

beamwidths and sidelobes in both azimuth and elevation. This can't be achieved with a standard flared horn, so scalar horns are usually preferred for dish feeds. The symmetry also makes them suitable for both linear and circular polarization. The W2IMU dual-mode horn and the "Chaparral" and Kumar feeds below are scalar feeds.

7. W2IMU Dual-Mode Horn

Diffraction from the edge of a horn causes sidelobes that reduce efficiency. In the W2IMU dual-mode horn design, there is a flare from a small section, which only supports the lowest waveguide mode, to a larger section that supports two waveguide modes.51718 The size of the flare controls the relative amplitude of the two modes, and the length of the large section is chosen so that the two modes cancel at the edge of the horn because they travel at different phase velocities in the waveguide. The cancellation eliminates the sidelobes and thus puts more energy onto the reflector. The requirement for a larger horn makes this feed optimum for larger

20 QEX

f/D reflectors, in the 0.5 to 0.6 range.

8. Chaparral Feed

The "Chaparral" feed is a type of scalar feedhorn often found on TVRO dishes, with a series of cavity rings surrounding a circular waveguide.195-4 The rings modify the pattern to approximate our desired feed pattern, Fig 3, while maintaining a good phase center. This feed is best for deep dishes, with f/D in the 0.35 to 0.45 range. Fine adjustment of the pattern is possible by changing the protrusion of the central waveguide in relation to the surrounding rings.

Note: I have not seen any mention of the location of the phase center, but my experiments show that it is controlled by the location of the outer rings, not the central waveguide.

9. Kumar Feed

The Kumar feed is a scalar feedhorn similar to the Chaparral feed, but with a single larger outer ring, so construction is somewhat simpler.20 Radiation patterns approximate our desired feed pattern. Fig 3, while maintaining a good phase center.

Ham-band versions of this feed have been described by VE4MA for 1296, 2304 and 3456 MHz.221 Like the Chaparral feed, it is best for deep dishes, with f/D in the 0.35 to 0.45 range, with similar fine adjustment.

Complete Dish Antennas

Many of the papers describing feed horns show great detail of the horn performance, but very few even mention what happens when a reflector is added. The reflector may add too many uncertainties for good research, but our goal is to make a good working antenna. We want high efficiency because a dish has the same size, wind loading, and narrow beamwidth regardless of efficiency—we should get as much performance as possible for these operational difficulties. In other words, if I am going to struggle with a one-meter diameter dish on a windy mountain top, I certainly want one meter worth of performance!

In order to compare the different feeds, I wanted to measure the gain of several of them with the same reflector, to find their performance as complete antennas. I made a mechanism from an old slotted-line carriage and some photographic hardware that allows the feed to be moved in three dimensions with fine control of adjustment, so the feed position can be adjusted for maximum gain.

The emphasis here is on smaller dishes intended for mountaintopping and other portable operation, so maximum gain with minimum size and weight is a definite consideration. For other applications, there would be other considerations; EME, for instance, would mandate maximum performance.

Parabolic Reflector

I have managed to collect a half-dozen parabolic reflectors of various sizes and origins, and I wanted to know if they were useful at 10 GHz. First, for each dish I measured the diameter and depth in the center of the dish in order to calculate the focal length and f/D ratio . This can only be an approximation for some dishes, due to holes or flat areas in the center. The focal length is calculated as:

16 depth

The HDL ANT computer program does the calculation and then generates a Postscript plot of a parabolic curve for the specified diameter and f/D ratio. For each reflector, I made a series of plots on a laser printer for a range of f/D values for antennas in general near the calculated value, cut out templates, then fitted them to the surface to find the closest fit. For 10 GHz, the surface must be within ± 1 mm of a true parabola for optimum performance, although errors up to ±3 mm result in only 1 dB degradation.22 I selected several reflectors with good surfaces and discarded one that wasn't even close.

Given a choice, a reflector with a large f/D (0.5 to 0.6) would be preferable. As described earlier, dishes with small f/D are hard to illuminate efficiently and are more sensitive to focal length errors. On the other hand, a dish that is available for the right price is always a good starting point!

Parabolic reflectors can come from many sources, not just antenna manufacturers. Some aluminum snow coasters (now unfortunately replaced by plastic, but aluminum foil glued to the surface might make them usable) are good, and hams in Great Britain have put dustbin lids into service as effective parabolic reflectors for years.

Homebrewing a parabolic reflector is possible, but great difficulty is implied by the surface accuracy cited above. The surface accuracy requirement scales with wavelength, so the task is easier at lower frequencies. Of course, hams are always resourceful— N1IOL found that the cover from his 100-pound propane tank was an excellent 14-inch parabolic surface and has used it to mold a number of fiberglass reflectors. K1LPS then borrowed a larger cover from a different type of propane tank and found it to be nowhere near a parabola!

Recommended Feed Systems

Since no single feed system is optimum for all dishes, a good feed recommendation depends on the f/D of the particular dish. For shallow dishes (f/D of 0.5 to 0.6), I'd recommend the W2IMU dual-mode horn or a pyramidal horn designed for the exact f/D.5'11 The horn section of the computer program will design the horn and plot a construction template. For deeper dishes (f/D of 0.3 to 0.45), I'd recommend the Chaparral, Kumar or Cla vin feeds.5 20 16 For 10 GHz, a Chaparral horn designed for 11-GHz TVRO use works well; your local satellite TV dealer might be persuaded to order it as an "11 GHz Superfeed."

Mechanical Support

There are two critical mechanical problems: mounting the feedhorn to the dish and mounting the dish to the tripod. Most small dishes have no backing structure, so the thin aluminum surface is easily deformed. K1LPS discovered that some cast-aluminum frying pans have a rolled edge that sits nicely on the back of a dish; Mirro is one suitable brand. This is a good use for that old frying pan with the worn-out Teflon coating, so buy a new one for the kitchen. Tap a few holes in the edge of the old pan, screw the dish to it, and you have a solid backing. A solid piece of angle iron or aluminum attaches the bottom of the frying pan to the top of a tripod. The photograph in Fig 11 shows a dish mounted using a frying pan. WA1MBA uses this technique for a 24-inch dish at his home and reports that it stands up well to New England winters.

The mounting structure for the feedhorn is in the RF field, so we must minimize the blockage it causes. Wc do this by using insulating materials and by mounting the support struts diagonally, so they aren't in the plane of the polarization. Fiberglass is a good material; plant stakes or bicycle flags are good sources, and WA5VJB recommends cheap target arrows. Use of four rather than three struts is recommended—if they are all the same length, then the feed is centered. The base of the struts should be attached to the backing structure or edge of the frying pan; the thin dish surface is not mechanically strong.

Aiming

A quality compass and a way of accurately aligning the antenna to it are essential for successful operation. Narrow beamwidth and frequency uncertainty can make searching for weak signals frustrating and time-consuming. A heavy tripod with setting circles is a good start; hang your battery from the center of the tripod and it won't blow over as often. Calibrate your headings by locating a station with a known beam heading rather than by eyeballing the dish heading; small mechanical tolerances can easily shift the beam a few degrees from the apparent boresight. As W1AIM can testify, having the wind blow a dish over can distort it enough to move the beam to an entirely different heading.

Alternatives

The narrow beamwidth may actually make contacts more difficult, particularly in windy conditions. I have worked six grids from Mt. Wachusett in central Massachusetts using a small Gunnplexer horn. The longest path, 203 km, required a 12-inch lens for additional gain to make the contact on wideband FM; it would have been easy with narrowband SSB or CW.23 For a rover station, a reasonable size horn might be a good compromise, with adequate gain and moderate beamwidth for easy aiming. I often use the 17.5-dBi Gunnplexer horn, with a 12-inch lens ready to place in front of it when signals are marginal.

Conclusions

A parabolic dish antenna can provide very high gain at microwave frequencies, but only with very sharp beamwidths. To achieve optimum gain, careful attention to detail is required: checking the parabolic surface accuracy with a template, matching the feedhorn to the f/D of the dish, and, most importantly, accurately locating the phase center of the feedhorn at the focus.

Notes

1Malowanchuk, B. W., VE4MA, "Use of Small TVRO Dishes for EME," Proceedings of the 21st Conference of the Central States VHF Society, ARRL, 1987, pp 6877.

2Malowanchuk, 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. 3Ralston, M.. KI4VE, "Design Considerations for Amateur Microwave Antennas," Proceedings of Microwave Update '88, ARRL, 1988, pp 57-59. 4Reasoner, H., K5SXK, "Microwave Feeds for Parabolic Dishes," Proceedings of Microwave Update '89, ARRL, 1989, pp 7584.

5Turrin, D., W2IMU, "Parabolic Reflector Antennas and Feeds," The ARRL UHF/Microwave Experimenter's Manual, ARRL, 1990.

6Rahmat-Samii, Y., "Reflector Antennas," Antenna Handbook: theory, applications, and design, Y.T. Lo and S.W. Lee, editors, Van Nostrand Reinhold, 1988, p 15-42. 7ingerson, P. G. and Rusch, W. V. T., "Radiation from a paraboloid with an axi ally defocused feed," IEEE Transactions on Antennas and Propagation, Vol AP-21 No. 1. Jan 1973, pp 104-106. 8Hill, T,, WA3RMX, "A Triband Microwave

Dish Feed," QST, Aug 1990, pp 23-27. 9Turrin, D., W2IMU, "Antenna Performance Measurements," QST, Nov 1974, pp 3541.

10Heidemann, R., DC3QS, "A Simple Radiator for 3 cm Parabolic Dishes," VHF Communications, 3/1979, pp 151-153. 11Evans, D., G3RPE, "Pyramidal horn feeds for paraboloidal dishes," Radio Communication, March 1975. 12Kraus, John, (W8JK), Antennas, McGraw Hill, 1956.

,3Vilardi, D., WA2VTR, "Simple and Efficient Feed for Parabolic Antennas," QST, March 1973, pp 43-44.

14Foot, N. J., WA9HUV, "Second-generation cylindrical feedhorns," Ham Radio, May 1982, pp 31-35.

15Foot, N. J,, WA9HUV, "Cylindrical feedhorns revisited," Ham Radio. Feb 1986, pp 20-22.

16Clavin, A., "A Multimode Antenna Having Equal E- and H-Planes," IEEE Transactions on Antennas and Propagation, AP-23, Sep 1975, pp 735-737.

17Turrin, R. H., (W2IMU), "Dual Mode Small-Aperture Antennas," IEEE Transactions on Antennas and Propagation, AP15, Mar 1967, pp 307-308.

18Turrin, D., W2IMU, "A Simple Dual-Mode (IMU) Feed Antenna for 10368 MHz," Proceedings of Microwave Update '91, ARRL, 1991, p 309.

19Wohlleben, R., Mattes, H. and Lochner, O., "Simple small primary feed for large opening angles and high aperture efficiency," Electronics Letters, 21 Sep 1972, pp 181-183.

20Kumar, A., "Reduce Cross-Polarization in

Reflector-Type Antennas," Microwaves, Mar 1978, pp 48-51.

21Malowanchuk, B W„ VE4MA, "VE4MA 3456 MHz circular polarization feed horn," North Texas Microwave Society Feedpoint, Nov/Dec 1991.

22Knadle, R. T., K2RIW, "A Twelve-Foot Stressed Parabolic Dish," QST, Aug 1972, pp 16-22.

23Wade, P., N1BWT, and Reilly, M„ KB1VC, "Metal Lens Antennas for 10 GHz," Proceedings of the 18th Eastern VHF/UHF Conference, ARRL, May 1992, pp 71-78.

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