Parabolic Reflector Antenna

Parabolic Reflector Antenna

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 VE3ONT for the 1993 EME Contest.

These high gains are only achievable if the antennas are properly implemented, and dishes have more critical dimensions than horns and lenses. I will try to explain the fundamentals using pictures and graphics as an aid to understanding the critical areas and how to deal with them. In addition, a computer program, HDL_ANT is available for the difficult calculations and details, and to draw templates for small dishes in order to check the accuracy of the parabolic surface.

 
INDEX

1. INTRODUCTION
1.1 Introduction 1
1.2 History 1

2. DISH ANTENNA DESIGN
2.1 Dish Antenna Design 3
2.2 Parabolic reflector 3
2.3 Feed Antenna 4
2.4 Types 5

3. PARAMETERS
3.1 Gain 7
3.2 Beam width 8

4. PRACTICAL DISH ANTENNAS
4.1 Feed Patterns 9
4.2 Edge Taper 9
4.3 G/T 10
4.4 Focal Length and ƒ/D ratio 10
4.5 Phase Center 10
4.6 Symmetry of E-plane and H-Plane 11
4.7 Total Efficiency 11
4.8 Practical Feed Systems 11
4.9 Complete Dish Antennas 11
4.10 Parabolic Reflector 12
4.11 Mechanical support 13
4.12 Aiming 13

5. SUMMARY 14

6. REFRENCE 15





1. Introduction
1.1 Introduction :
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it is highly directive; it functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, that is they can produce the narrowest beam width angles, of any antenna type.[1] In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which wavelengths are small enough that conveniently sized dishes can be used.
Parabolic antennas are used as high-gain antennas for point-to-point communication, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite and spacecraft communication antennas, and radio telescopes. Their other large use is in radar antennas, which need to emit a narrow beam of radio waves to locate objects like ships and airplanes. With the advent of home satellite television dishes, parabolic antennas have become a ubiquitous feature of the modern landscape.

1.2 History :

The idea of using parabolic reflectors for radio antennas was taken from optics, where the power of a parabolic mirror to focus light into a beam has been known since classical antiquity. The designs of some specific types of parabolic antenna, such as the Cassegrain and Gregorian, come from similarly named analogous types of reflecting telescope, which were invented by astronomers during the 15th century.
German physicist Heinrich Hertz constructed the world's first parabolic reflector antenna in 1888. The antenna was a cylindrical parabolic reflector made of zinc sheet metal supported by a wooden frame, and had a spark-gap excited dipole along the focal line. Its aperture was 2 meters high by 1.2 meters wide, with a focal length of 0.12 meters, and was used at an operating frequency of about 450 MHz. With two such antennas, one used for transmitting and the other for receiving, Hertz demonstrated the existence of radio waves which had been predicted by James Clerk Maxwell some 22 years earlier.
Italian radio pioneer Guglielmo Marconi used a parabolic reflector during the 1930s in investigations of UHF transmission from his boat in the Mediterranean. In 1931 a microwave relay link across the English Channel using 10 ft. (3 meter) diameter dishes was demonstrated.
In 1937 Grote Reber built the first radio telescope to use a parabolic antenna and did a sky survey with it, one of the events that founded the field of radio astronomy. The development of radar during World War II provided a great impetus to parabolic antenna research, and saw the evolution of shaped-beam antennas, in which the curve of the reflector is different in the vertical and horizontal directions, tailored to produce a beam with a particular shape. During the 1950s dish antennas became widely used in terrestrial microwave relay communication systems.
The first parabolic antenna used for satellite communications was constructed in 1962 at Goonhilly in Cornwall, England, UK to communicate with the Telstar satellite. The advent in the 1970s of computer programs such as NEC capable of calculating the radiation pattern of parabolic antennas has led to the development of sophisticated asymmetric, multireflector and multifeed designs in recent years.

2. Dish Antenna Design

2.1 Dish Antenna Design :
The operating principle of a parabolic antenna is that a point source of radio waves at the focal point in front of a paraboloidal reflector of conductive material will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point.
A typical parabolic antenna consists of a metal parabolic reflector with a small feed antenna suspended in front of the reflector at its focus, pointed back toward the reflector. The reflector is a metallic surface formed into a paraboloid of revolution and usually truncated in a circular rim that forms the diameter of the antenna. In a transmitting antenna, radio frequency current from a transmitter is supplied through a transmission line cable to the feed antenna, which converts it into radio waves. The radio waves are emitted back toward the dish by the feed antenna and reflect off the dish into a parallel beam. In a receiving antenna the incoming radio waves bounce off the dish and are focussed to a point at the feed antenna, which converts them to electric currents which travel through a transmission line to the receiver.

2.2 Parabolic reflector :

The reflector can be of sheet metal, metal screen, or wire grill construction, and it can be either a circular "dish" or various other shapes to create different beam shapes. A metal screen reflects radio waves as well as a solid metal surface as long as the holes are smaller than 1/10 of a wavelength, so screen reflectors are often used to reduce weight and wind loads on the dish. To achieve the maximum gain, it is necessary that the shape of the dish be accurate within a small fraction of a wavelength, to ensure the waves from different parts of the antenna arrive at the focus in phase. Large dishes often require a supporting truss structure behind them to provide the required stiffness.
A reflector made of a grill of parallel wires or bars oriented in one direction acts as a polarizing filter as well as a reflector. It only reflects linearly polarized radio waves, with the electric field parallel to the grill elements. This type is often used in radar antennas. Combined with a linearly polarized feed horn, it helps filter out noise in the receiver and reduces false returns.

Focus


Fig 2.1 - Geometry of Parabolic Dish Antenna

2.3 Feed antenna :

The feed antenna at the reflector's focus is typically a low-gain type such as a half-wave dipole or more often a small horn antenna called a feed horn. In more complex designs, such as the Cassegrain and Gregorian, a secondary reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point. The feed antenna is connected to the associated radio-frequency (RF) transmitting or receiving equipment by means of a coaxial cable transmission line or waveguide.
An advantage of parabolic antennas is that most of the structure of the antenna (all of it except the feed antenna) is nonresonant, so it can function over a wide range of frequencies, that is a wide bandwidth. All that is necessary to change the frequency of operation is to replace the feed antenna with one that works at the new frequency. Some parabolic antennas transmit or receive at multiple frequencies by having several feed antennas mounted at the focal point, close together.

2.4 Types :

Parabolic antennas are distinguished by their shapes:

• Paraboloidal or dish - The reflector is shaped like a paraboloid. This is the most common type. It radiates a narrow pencil-shaped beam along the axis of the dish.
o Shrouded dish - Sometimes a cylindrical metal shield is attached to the rim of the dish. The shroud shields the antenna from radiation from angles outside the main beam axis, reducing the sidelobes. It is sometimes used to prevent interference in terrestrial microwave links, where several antennas using the same frequency are located close together. The shroud is coated inside with microwave absorbent material. Shrouds can reduce back lobe radiation by 10 dB.[2]

• Cylindrical - The reflector is curved in only one direction and flat in the other. The radio waves come to a focus not at a point but along a line. The feed is sometimes a dipole antenna located along the focal line. Cylindrical parabolic antennas radiate a fan-shaped beam, narrow in the curved dimension, and wide in the uncurved dimension. The curved ends of the reflector are sometimes capped by flat plates, to prevent radiation out the ends, and this is called a pillbox antenna.

• Shaped-beam antennas - Modern parabolic antennas can be designed to produce a beam or beams of a particular shape, rather than just the narrow "pencil" or "fan" beams of the simple dish and cylindrical antennas above. Two techniques are used, often in combination, to control the shape of the beam:

o Shaped reflectors - With a single feed antenna, the only option is to alter the shape of the reflector(s). The parabolic reflector can be given a noncircular shape, and/or different curvatures in the horizontal and vertical directions, to alter the shape of the beam. This is often used in radar antennas.

 "Orange peel" antenna - Used in search radars, this is a long narrow antenna shaped like the letter "C". It radiates a narrow vertical fan shaped beam.
o Arrays of feeds - In order to produce an arbitrary shaped beam, instead of one feed horn an array of feed horns clustered around the focal point can be used. Array-fed antennas are often used on communication satellites, particularly direct broadcast satellites, to create a downlink radiation pattern to cover a particular continent or coverage area. They are often used with secondary reflector antennas such as the Cassegrain.


Parabolic antennas are also classified by the type of feed, that is, how the radio waves are supplied to the antenna:

• Axial or front feed - This is the most common type of feed, with the feed antenna located in front of the dish at the focus, on the beam axis. A disadvantage of this type is that the feed and its supports block some of the beam, which limits the aperture efficiency to only 55 - 60%.[2]

• Off-axis or offset feed - The reflector is an asymmetrical segment of a paraboloid, so the focus, and the feed antenna, are located to one side of the dish. The purpose of this design is to move the feed structure out of the beam path, so it doesn't block the beam. It is widely used in home satellite television dishes, which are small enough that the feed structure would otherwise block a significant percentage of the signal. Offset feed is also used in multiple reflector designs such as the Cassegrain and Gregorian, below.

• Cassegrain - In a Cassegrain antenna the feed is located on or behind the dish, and radiates forward, illuminating a convex hyperboloidal secondary reflector at the focus of the dish. The radio waves from the feed reflect back off the secondary reflector to the dish, which forms the outgoing beam. An advantage of this configuration is that the feed, with its waveguides and "front end" electronics does not have to be suspended in front of the dish, so it is used for antennas with complicated or bulky feeds, such as large satellite communication antennas and radio telescopes. Aperture efficiency is on the order of 65 - 70%[2]

• Gregorian - Similar to the Cassegrain design except that the secondary reflector is concave, (ellipsoidal) in shape. Aperture efficiency over 70% can be achieved.

3. Parameters

3.1 Gain :
The directive qualities of an antenna are measured by a dimensionless parameter called its gain, which is the ratio of the power received by the antenna from a source along its beam axis to the power received by a hypothetical isotropic antenna. The gain of a parabolic antenna is:


where:
A is the area of the antenna aperture, that is, the mouth of the parabolic reflector
d is the diameter of the parabolic reflector
is the wavelength of the radio waves.
eA is a dimensionless parameter between 0 and 1 called the aperture efficiency. The aperture efficiency of typical parabolic antennas is 0.55 to 0.70.

It can be seen that, as with any aperture antenna, the larger the aperture is, compared to the wavelength, the higher the gain. The gain increases with the square of the ratio of aperture width to wavelength, so large parabolic antennas, such as those used for spacecraft communication and radio telescopes, can have extremely high gain. Applying the above formula to the 25-meter-diameter antennas often used in radio telescope arrays and satellite ground antennas at a wavelength of 21 cm (1.42 GHz, a common radio astronomy frequency), yields an approximate maximum gain of 140,000 times or about 50 dBi (decibels above the isotropic level).

Aperture efficiency eA is a catchall variable which accounts for various losses that reduce the gain of the antenna from the maximum that could be achieved with the given aperture. The major factors reducing the aperture efficiency in parabolic antennas are:.[9]
Feed spillover - Some of the radiation from the feed antenna falls outside the edge of the dish and so doesn't contribute to the main beam.

Feed illumination taper - The maximum gain for any aperture antenna is only achieved when the intensity of the radiated beam is constant across the entire aperture area. However the radiation pattern from the feed antenna usually tapers off toward the outer part of the dish, so the outer parts of the dish are "illuminated" with a lower intensity of radiation. Even if the feed provided constant illumination across the angle subtended by the dish, the outer parts of the dish are farther away from the feed antenna than the inner parts, so the intensity would drop off with distance from the center. So the intensity of the beam radiated by a parabolic antenna is maximum at the center of the dish and falls off with distance from the axis, reducing the efficiency.

Aperture blockage - In front-fed parabolic dishes where the feed antenna is located in front of the dish in the beam path (and in Cassegrain and Gregorian designs as well), the feed structure and its supports block some of the beam. In small dishes such as home satellite dishes, where the size of the feed structure is comparable with the size of the dish, this can seriously reduce the antenna gain. To prevent this problem these types of antennas often use an offset feed, where the feed antenna is located to one side, outside the beam area. The aperture efficiency for these types of antennas can reach 0.7 to 0.8.

3.2 Beamwidth :
The angular width of the beam radiated by high-gain antennas is measured by the half-power beam width (HPBW), which is the angular separation between the points on the antenna radiation pattern at which the power drops to one-half (-3 dB) its maximum value. For parabolic antennas, the HPBW θ is given by:

where k is a factor which varies slightly depending on the shape of the reflector and the feed illumination pattern. For a "typical" parabolic antenna k = 70 when θ is in degrees.
For a typical 2 meter satellite dish operating on C band (4 GHz), like the one shown at right, this formula gives a beamwidth of about 2.6°. For the Arecibo antenna at 2.4 GHz the beamwidth is 0.028°. It can be seen that parabolic antennas can produce very narrow beams, and aiming them can be a problem. Some parabolic dishes are equipped with a boresight so they can be aimed accurately at the other antenna.
It can be seen there is an inverse relation between gain and beam width. By combining the beamwidth equation with the gain equation, the relation is:


4. 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 only radiates toward the reflector, and has a phase pattern that appears to radiate from a single point.

4.1 Feed Patterns :
The radiation pattern of the feed antenna has to be tailored to the shape of the dish, because it has a strong influence on the aperture efficiency, which determines the antenna gain (see Gain section below). Radiation from the feed that falls outside the edge of the dish is called "spillover" and is wasted, reducing the gain and increasing the backlobes, possibly causing interference or (in receiving antennas) increasing susceptibility to ground noise. However, maximum gain is only achieved when the dish is uniformly "illuminated" with a constant field strength to its edges. So the ideal radiation pattern of a feed antenna would be a constant field strength throughout the solid angle of the dish, dropping abruptly to zero at the edges. However, practical feed antennas have radiation patterns that drop off gradually at the edges, so the feed antenna is a compromise between acceptably low spillover and adequate illumination. For most front feed horns, optimum illumination is achieved when the power radiated by the feed horn is 10 dB less at the dish edge than its maximum value at the center of the dish.

4.2 Edge Taper :
Almost all feedhorns will provide less energy at the edge of dish than at the center, like Figure 4-4. The difference in power at the edge is referred to as the edge 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 Figure 4-6: 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 losses4,6 versus the energy at the edge of the dish in Figure 4-7, we find that the total efficiency of a dish antenna peaks with an illumination taper, like Figure 4-6, so that the energy at the edge is about 10 dB lower than the energy at the center. This is often referred to as 10 dB edge taper or edge illumination — often recommended but not explained.


4.3 G/T :

When an antenna is receiving a signal from space, like a satellite or EME signal, there is very little background noise emanating from the sky compared to the noise generated by the warm 300K 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 Figure 4-6, the spillover can be reduced by increasing the edge taper, while Figure 4-7 shows the efficiency, and thus the gain, decreasing slowly as edge taper is increased. The best compromise is reached when G/T, the ratio of gain to antenna noise temperature, is maximum.

4.4 Focal Length and ƒ/D ratio :

All parabolic dishes have the same 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 which extends to infinity. A convenient way to describe how much of the parabola is used is the ƒ/D ratio, the ratio of the focal length ƒ 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 ƒ/D ; Figure 4-8 shows the relative geometries for commonly used ƒ/D ratios, typically from 0.25 to 0.65, with the desired and idealized feed patterns for each.

Notice the feedhorn patterns for the various ƒ/D ratios in Figure 4-8. As ƒ/D becomes smaller, the feed pattern to illuminate it becomes broader, so different feedhorns are needed to properly illuminate dishes with different ƒ/D ratios. The feedhorn pattern must be matched to the reflector ƒ/D. Larger ƒ/D dishes need a feedhorn with a moderate beamwidth, while a dish with an ƒ/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, and consequently, it is almost impossible to efficiently illuminate a dish this deep.

4.5 Phase Center :

A well designed feed for a dish or lens has a single phase center, as described in Chapter 1, 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 broadband feed design. Fortunately, we are only considering narrow frequency ranges here.

4.6 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.

4.7 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 E- and H-Planes

• Focal point error

• Feedhorn sidelobes

• Blockage by the feed horn

• Blockage by supporting structures

• Imperfections in parabolic surface.

• Feedline loss

4.8 Practical Feed Systems :

An optimum feed would approximate the desired feed pattern for the ƒ/D of the parabolic reflector in both planes and have the same phase center in both planes.


4.9 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 mountaintop, I certainly want one meter worth of performance!

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.

4.10 Parabolic Reflector :

I have managed to collect a number of parabolic reflectors of various sizes and origins, and wanted to know if they were useful at 10 GHz. First, for each dish I measured the diameter D and the depth d in the center of the dish in order to calculate the focal length and ƒ/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:



f =


D2

16d

The HDL_ANT computer program does the calculation and then generates a PostScript™ plot of a parabolic curve for the specified diameter and ƒ/D ratio. For each reflector, I made a series of plots on a laser printer for a range of ƒ/D 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 parabola9 for optimum performance, although errors up to +/- 3 mm result in only 1 dB degradation. I selected several reflectors with good surfaces, and discarded one that wasn't even close.

Given a choice, a reflector with a large ƒ/D (0.5 to 0.6) would be preferable. As described earlier, dishes with small ƒ/D are harder 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 have 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!

4.11 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 Figure 4-10 shows a dish mounted using a frying pan. WA1MBA uses this technique for a 24 inch dish at his home QTH 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. We do this by keeping the support strut diameter small, by using insulating materials, and by mounting the 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.

4.12 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 beacon or 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.
 
5. Summary

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 ƒ/D of the dish, and, most importantly, accurately locating the phase center of the feedhorn at the focus.
 
6. References

1. B.W. Malowanchuk, VE4MA, "Use of Small TVRO Dishes for EME," Proceedings of the 21st Conference of the Central States VHF Society, ARRL, 1987, pp. 68-77.

2. B.W. Malowanchuk, VE4MA, "Selection of an Optimum Dish Feed," Proceedings of the 23rd Conference of the Central States VHF Society, ARRL, 1989, pp. 35-43.

3. M. Ralston, KI4VE, "Design Considerations for Amateur Microwave Antennas, "

Proceedings of Microwave Update '88, ARRL, 1988, pp. 57-59.

4. H. Reasoner, K5SXK, "Microwave Feeds for Parabolic Dishes," Proceedings of Microwave Update '89, ARRL, 1989, pp. 75-84.

5. D. Turrin, W2IMU, "Parabolic Reflector Antennas and Feeds," The ARRL

UHF/Microwave Experimenter's Manual,. ARRL, 1990.

6. Y. Rahmat-Samii, "Reflector Antennas," in Antenna Handbook: theory, applications, and design, Y.T. Lo and S.W. Lee, editors, Van Nostrand Reinhold, 1988, page 15-42.

Posted in: Antenna