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Antenna Principles Part IV - Directional Arrays for 300 Megacycles and Higher
April 1947 Radio-Craft

April 1947 Radio-Craft

April 1947 Radio Craft Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

This installment of the multi-month series of articles on antenna principles covers directional arrays for 300 MHz and higher. Keep in mind that in 1947 when this appeared in Radio-Craft magazine, wavelengths of a meter or less were considered to be at the upper end of the operational range. Parabolic reflector antennas were the domain primarily of ground-based installations due to the physical size and weight being prohibitive in airborne platforms, and even then they were rarely used at the time. Most ground and airborne installations were composed of dipole antennas with various configurations of reflector and director elements for desired gain and directivity characteristics. Special applications like for direction finding and longer wavelength radio communications used loop and long wire antennas, respectively. Highly directive dipoles are the focus here with some useful graphs for use in design.

Part II of this Antenna Principles series appeared in the April 1947 issue, Part IV in the January 1947 issue, and Part VI was in the May 1947 issue.

Antenna Principles - Directional Arrays for 300 Megacycles and Higher

Billboard array used by Army radar - RF Cafe

Photo A - Billboard array used by Army radar. Photo by U.S. Army Signal Corps

Part V - Directional Arrays for 300 Megacycles and Higher

By Jordan McQuay

Antennas designed to operate in the u.h.f. region of the radio spectrum - above 300 megacycles - employ most of the basic principles of antenna technique but also introduce some entirely new concepts of radio transmission and reception. Chief among these is the high degree of directivity obtained through use of antenna arrays.

An array - as described in previous articles of this series - is an arrangement of antenna elements. One or more radiating dipoles in conjunction with one or more reflectors, directors, or other dipoles, are used to provide, through their combined action or interaction, considerable directivity and consequent large antenna gain. An array may consist of a large number of elements (Photo A) or a minimum of two elements (Fig. 2).

As in other antennas, a transmitting array is the same as a receiving array, both electrically and structurally. Their functions are reciprocal.

The size of an antenna array is directly proportional to the operating wavelength. Theoretically, an array might be constructed for use at any wavelength. Practically, however, this tends to be impractical for waves longer than about 1 meter (or frequencies smaller than 300 mc) because of the direct relationship between wavelength and the-physical size of the antenna elements.

For instance, an adequate directional array for operation at 10.0. meters might conceivably be a quarter-mile long and almost as high!

Primarily for this reason, use of complex directional arrays is usually con-fined to the transmission and reception of radio waves less than 1 meter in length. And the full range of usefulness of arrays extends down to about 10 centimeters in length.*

Radio waves less than 1 meter in length have quasi-optical characteristics. They act very much like infrared light waves.

With a suitable radiating array, u.h.f waves may be confined and focused into a very narrow beam of r.f. energy, and then directed to ward a similar receiving array. These radio waves travel along direct or semi-optical paths. There is no ground wave. Propagation does not depend upon the sky wave, as in the low frequencies.

Arrays used for either transmission or reception are mounted at least 12 wavelengths above ground in normal practice. Thus, they are considered as functioning in free space and independent of ground effects.

How adding a director or reflector alters the dipole pattern - RF Cafe

Fig. 1 - How adding a director or reflector alters the dipole pattern.

Fig. 2 - Same directivity is provided with either reflector or director.

Dipole elements, whether radiating or parasitic, are usually constructed of conductive tubing. Metal rods can also be used, however, since microwave energy is confined to the outside of such metals.

All elements of an array are mounted in a fixed position. If mobility in any direction is desired, the entire array is moved without disturbing the relative positions of the elements: dipoles, reflector, or directors.

U.h.f. signals transmitted by an array of horizontally mounted dipoles are horizontally polarized, and such signals can be clearly and strongly received only by an array consisting of horizontally mounted receiving dipoles. Similarly, an array of vertically arranged elements will send signals that are vertically polarized and can be received well only by a vertically arranged receiving array.

Horizontally polarized waves are more generally used in u.h.f. practice because, unlike their vertical counterpart, they are not attenuated when passing close to the earth's surface.

Thus, the position (horizontal or vertical) of the various elements of an array in any plane determines the polarity of the microwaves sent or received.

The number and structural arrangement of the elements determine the pattern of field strength or field intensity. Thus they affect the power gain and the degree of directivity of the array. Extremely directional antenna arrays may have directional patterns only a few degrees in width (generally measured at half-power points).

Even though u.h.f. arrays provide a limited range of transmission, this high degree of directivity is a distinct advantage. It permits multiple use of the same wavelength by countless stations having only small geographic separation. The high resolving power of u.h.f. waves has made possible radar and other navigational aids for airplanes and ships at sea. In this uncrowded region of the radio spectrum, wide bands are available for single channels useful to television, facsimile, and carrier telephony.

Directivity provides either an effective increase in transmitter power or receiver sensitivity, depending upon use of the antenna array. The same directional characteristics apply to receiving as well as transmitting arrays, resulting in very large power gain between the two points.

Use of ultra high frequencies simplifies general system design, since the physical dimensions of the components or elements of the circuits are of the same order as the length of the radio waves passing through the equipment.

For this reason, in u.h.f. technique it's desirable to have a visual conception of the actual length of the radio waves being transmitted or received.

Simplest of all antennas is a half-wave dipole isolated completely in free space.

If it were possible to feed energy either to one end or the center of such a theoretical dipole, radiation would take place at right angles to the dipole.

Since, in normal u.h.f. practice, the radiating dipoles are usually situated in a horizontal position with, respect to the earth, this theoretical dipole (and all arrays that follow) will be considered in terms of the horizontal position. (All dipoles and arrays discussed transmit or receive horizontally polarized waves.)

The complete shape of the radiation pattern of the theoretical dipole in free space resembles a doughnut, with the dipole passing through the center (Radio-Craft, December 1946, p. 23). A horizontal cross section of the pattern resembles a figure eight in shape, and is bidirectional (Fig. 1).

Reflectors

Radar antenna in Beaufighter nose - RF Cafe

Photo B - Radar antenna in Beaufighter nose. British Official Photo

4-element Yagi array under RAF night-fighter nose - RF Cafe

Photo C - A 4-element Yagi array under RAF night-fighter nose. British Official Photo

This bidirectional radiation of a half-wave dipole may be affected by reflectors or directors, parasitic elements assisting in the unidirectional concentration of energy.

A reflector is placed behind a radiating dipole, in a position opposite in direction to the desired field of maximum intensity. But a director is placed before, or in front of, the radiating dipole, in a position toward the desired field of maximum intensity. Neither type of element is electrically connected to the radiating or receiving circuit.

The simplest type of reflector consists of a single piece of rod or tubing, very similar in shape and general appearance to the radiating dipole. However, the reflector is slightly longer than the radiating dipole.

Such a reflector is mounted parallel to and about one-quarter wave behind the dipole. A typical arrangement (Fig. 2) employs a reflector 5 percent longer than the center-fed half-wave dipole. spaced 0.2λ behind the radiator. The reflector is entirely parasitic in nature. It absorbs power from the dipole and then reradiates it, acting somewhat like a second dipole. Length and spacing of the reflector cause the re-radiation to have a phase and polarity relation with the original radiation such that the two fields of intensity add in the desired direction of power gain and cancel in the opposite direction.

Only a, small amount of energy travels beyond the reflector, because the two fields cancel when they are of opposite polarity and phase. However, reflected energy arrives back at the dipole with the same polarity and in phase with the radiating dipole, adding to the field intensity in a direction opposite to the reflector. The resultant field-strength pattern (Fig. 1) reveals pronounced directivity a right angles to the dipole.

Directors

A director is similar in shape and construction to a reflector, but is slightly shorter than the radiating dipole. The director is placed parallel to and about one-tenth wave in front of the dipole. It is a parasitic element, unconnected to a source of circuit energy, and consists of a single piece of rod or tubing.

A typical arrangement (Fig. 2) employs a director 5 percent shorter than the center-fed half-wave dipole, spaced 0.1λ in front of it.

The director acts as a second dipole by absorbing power from the radiating dipole and then reradiating it. However, due to length and spacing of the director the reradiation has a phase and polarity relation with the original radiation such that the two fields of intensity add in one direction and cancel in the opposite direction. The resultant field-strength pattern is similar to the pattern with a dipole and a reflector.

An example of the practical use of a radiating dipole and a director is the radar antenna (Photo B) used on many airplanes, where economy of space is a factor.

Effect of spacing of parasitic element on antenna gain - RF Cafe

Fig. 3 - Effect of spacing of parasitic element on antenna gain.

Directivity is increased by increasing number of elements - RF Cafe

Fig. 4 - Directivity is increased by increasing number of elements.

In summary, directors and reflectors exert somewhat similar influences on a radiating dipole when used separately. When used in combination, directional radiation and consequent power gain are almost doubled because both reflector and director influence radiation similarly. The reflector element is generally 5 percent longer than the half-wave radiating dipole; the director 5 percent shorter in length. The important factor of phase is controlled by these dimensions plus the structural spacings between the parallel elements.

Spacing is important. And optimum spacing - in terms of relative field strength, or power gain - may be determined from the design chart, shown in Fig. 3.

For maximum power gain use of a reflector with a dipole requires a spacing between 0.2λ and 0.25λ. When a director is used, the spacing is more critical, the optimum value being about 0.1λ.

Combining a director and a reflector to influence the radiation of a half-wave dipole causes a two-fold increase in both field intensity, directivity, and power gain. Because the three elements are arranged parallel and in a horizontal plane, they are known as horizontal arrays. The array (Fig. 4) produces a horizontal radiation beam with a width of about 60 degrees, measured at half-power points.

Addition of a second director provides greater power gain and more directivity. Dimensions of the second director are the same as those of the primary director, but the greater spacing between the two directors should be noted. Beam width of 40 degrees is typical.

Such an array is standard equipment for radar-equipped RAF night-fighters (Photo C), and is also used for other types of radar installations on aircraft; where available space is limited.

Use of three directors with a dipole and reflector further improves the directional effects of the horizontal array and provides a radiation beam approximately 15 degrees in width.

Four directors with a dipole and reflector are used on each of four "legs" of the extremely directional array of a U.S. Army combat radar set. Consisting of 4 phased sets of horizontal arrays, the antenna can be considered as an array of arrays. The combined radiation pattern provides a very narrow beam less than 8 degrees in width.

Almost any number of directors can be used with a reflector and single radiating dipole. Some radio amateurs have used as many as 8 or 10 directors in a horizontal array. The practical limit is about 4 or 5 directors, all of the same dimensions.

* At wavelengths of less than 10 centimeters, arrays are replaced by parabolic reflectors, lens systems, horns, and other radiating devices which will be discussed in the next issue of Radio-Craft.

*For further study of the electret, its principles and practical applications, see Radio-Craft, November, 1945, page 88. - Editor.

 

 

Posted April 2, 2020

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