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Antenna Principles Part VI - Directive Arrays with Metal-Screen Reflectors
May 1947 Radio-Craft

May 1947 Radio-Craft

May 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.

Part VI of the multi-month series of articles on antenna principles which appeared in Radio-Craft magazine covers directive arrays with metal-screen reflectors. Metal-screen, wire, and mesh reflectors are discussed as reflector surfaces for broadside array, the collinear array, and billboard array collections of dipole elements. An interesting statement by author Jordan McQuay is, "It is more practical and efficient to use a reflector screen [as opposed to reflector dipole elements], particularly if there are a large number of dipoles. Such a non-resonant reflector is easier arid cheaper to construct, and provides a better broad-band response than a resonant reflector." I don't know enough about antenna deign to determine whether with modern methods of simulation and construction, if that still holds.

Part II of this "Antenna Principles" series appeared in the January 1947 issue, Part III in February, Part IV in March, Part V in April, and Part VI in the May 1947. I do not yet have Part I from the December 1946 issue.

Antenna Principles Part VI - Directive Arrays with Metal-Screen Reflectors

Antenna Principles: Directive Arrays with Metal-Screen Reflectors, May 1947 Radio-Craft - RF CafeBy Jordan McQuay

The reflector elements considered in our previous article on the subject were single pieces of rod or tubing, dipole-like in construction and slightly longer than the radiating dipole.

A prominent characteristic of u.h.f. waves is that they are reflected by almost any type of metal screen, object, or surface. The metal functions much as an ordinary mirror when light waves impinge on it.

The reflector may be a flat screen - RF Cafe

Fig. 1 - The reflector may be a flat screen. 

Thus, when desired, the dipole-like reflector element can be replaced by a metal screen or surface of suitable area, properly spaced behind the radiating dipole. Length of the metal screen or surface should be such that the reflector extends about a half wavelength beyond the extremities of the radiating dipole. Height of the metal screen or surface is not critical, but should be at least half the length of the reflector. See Fig. 1.

At u.h.f, operating wavelengths of less than 1 meter, the metal reflector need not be a solid surface, It may be perforated with holes no larger than λ/8. Or the reflector may employ a screen of wire mesh, again providing that openings are no larger than λ/8. Many types of ordinary fencing material are satisfactory for the construction of reflectors for arrays.

Metal-screen reflectors are spaced in the same manner as the dipole-like reflectors. The reflectors are not connected to the electrical circuit, since their operation is parasitic in nature, as in the case of rod or tube reflectors.

Typical uses of metal-screen, wire, or mesh reflectors are shown in Fig. 1, and photos A, B and C.

Phased Arrays

The simple horizontal arrays previously described provide various amounts of directivity of the field intensity pattern in the horizontal plane. The vertical plane also is unidirectional, but the pattern of radiation is extremely wide and not too useful.

Such arrays are adequate for low-power or limited-range applications, where extreme directivity in both horizontal and vertical planes is not required.

But for high-power operation, extreme directivity in both planes, and general increased efficiency - upright and much larger arrays (consisting of many radiating dipoles) are used for the transmission and reception of u.h.f. waves.

Included in this group of important microwave antennas are: The broadside array, the collinear array, the billboard array. Differences in the arrays are primarily those of arrangement and number of radiating dipoles.

Billboard antenna's screen reflector - RF Cafe

Photo A - Billboard antenna's screen reflector.

Photos by U.S. Army Signal Corps

In general, the half-wave dipoles are constructed of conventional metal rod or tubing. They may be center-fed or end-fed, but all dipoles must be fed in phase - by suitable spacing and arrangement of feed or transmission lines.

The dipoles .are arranged within the same plane with respect to the earth. They may be stacked parallel, or mounted end-to-end. The position of all dipoles within that plane determines the polarization of the u.h.f. waves being transmitted or received. Horizontal polarization - used in most u.h.f. applications - is obtained by mounting the dipoles in a horizontal position. For vertical polarization, the dipoles are mounted vertically.

For unidirectional operation, individual and separate reflector elements can be used behind each radiating dipole.

It is more practical and efficient to use a reflector screen, particularly if there are a large number of dipoles. Such a non-resonant reflector is easier arid cheaper to construct, and provides a better broad-band response than a resonant reflector.

The wire mesh of the reflector is often made the main support of the entire array by mounting the radiating dipoles on quarter-wave metallic insulators which are short-circuited at the reflector screen. This rigidity of construction permits use of larger, heavier radiating dipoles - in turn providing operation over a broader band of frequencies.

Directors are seldom used with large, phased arrays. This is mainly because of mechanical difficulties of construction. Any added benefit of directivity can be equaled - if not surpassed - by careful design and arrangement, spacing, and phasing of dipoles.

Broadside Array

Characteristics of broadside and collinear arrays - RF Cafe

Fig. 2 - Showing how characteristics of broadside and collinear arrays are combined in the billboard to give excellent sharpness and gain.

When any number of half-wave dipoles (or pairs of half-wave dipoles) are stacked one above the other in parallel, the result is known as a broad-side array. It is essentially an arrangement in height, and may consist of two or more dipoles.

Vertical spacing between parallel dipoles should be close to a half-wave length. To preserve phase relationships without unnecessary lengths of transmission line, polarity is reversed be-tween alternate dipoles as shown by antennas A and B in Fig. 2. Thus the array is fed with equal currents in the same phase.

The broadside array is used to obtain extreme directivity in the vertical field. Sharpness of the radiation pattern in the vertical plane is primarily a function of the number of stacked dipoles. The greater the number of dipoles, the greater the directivity in the vertical plane with no regard for the horizontal plane.

This relation is illustrated by antennas A and B and their relative radiation patterns in the vertical field, where antenna A provides greater directivity and greater power gain. This is an outstanding characteristic of the broadside array.

Collinear Array

When any number of half-wave dipoles are placed end-to-end along a horizontal line, We result is known as a collinear array. It is essentially an arrangement in width, and provides extreme directivity in the horizontal field. Typical example of the collinear array is shown in Fig. 2.

Quarter-wave stubs are used between adjacent dipoles. Thus current is in phase in each radiating section of the array.

Simple horizontal four-element collinear array - RF Cafe

Photo B - A simple horizontal four-element collinear array with a wire-screen reflector. 

Sharpness of the radiation pattern is primarily a function of the number of half-wave radiating dipoles arranged in a horizontal line. The greater the number of dipoles, the greater the horizontal directivity - with no regard for the vertical directivity pattern.

This relation is shown in Fig. 2 by antennas C and D with their relative radiation patterns plotted in the horizontal plane, where antenna C provides greater directivity and consequent increase in power gain.

This is the outstanding characteristic of the collinear array.

Billboard Array

When a considerable number of half-wave dipoles are arranged geometrically both in height and width, the result - a combination of the broadside and collinear types - is known as a billboard array.

It may consist of 4 or multiples of 4 dipoles. Some months ago when radar contact was made with the moon, Signal Corps engineers used a billboard array consisting of 64 half-wave dipoles. Another arrangement is shown in Photo B. In general, the greater number of dipoles in a billboard array, the greater the power gain and directivity.

High-elevation 32-element billboard - RF Cafe

Photo C - High-elevation 32-element billboard. 

Vertical spacing between parallel dipoles is about a half wavelength, and feed points along the transmission line (Fig. 2) are chosen to place the dipoles a half-wave apart. By reversing connections on alternate dipoles, they are effectively fed in phase.

The billboard array exhibits many directional characteristics of both the collinear and broadside arrays. It combines the directivity and power gain of antennas A and C - resulting in an extremely narrow, directional beam in the horizontal field of intensity. It also exhibits similar high directivity in the vertical plane. But, except for radar and certain types of navigational equipment, the horizontal field of intensity is of prime importance.

Feeder Systems

Maximum efficiency of the u.h.f. antenna system requires a low-loss, non-radiating feeder system between the output of the transmitter and the actual antenna array and between the array and the input of the u.h.f. receiving equipment.

At fairly low frequencies in the u.h.f. band - from 300 to 600 megacycles - it is possible to use rigid, spaced, open-wire transmission line. Such feeder lines consist of metal tubing. They must be non-resonant, otherwise leakage current will damage the insulators.

Polystyrene can be used for all insulators, attached to the feeder line at voltage nodes. However, a much more satisfactory insulator is the metal stub support, or metallic insulator, which also helps keep the feeder line rigid. A stub support is a quarter-wave section of line, short-circuited at one end by any kind of metal frame or surface. The opposite end - connected to the line - represents a very high impedance. Thus no energy is lost through use of such an insulator at ultra-high frequencies.

The feeder line is matched to both antenna array and the transmitter output, with matching stubs placed anywhere, along the feeder line.

The principal disadvantage of the open-wire feed line is a sporadic tendency to radiate because of the spacing between conductors. U.h.f. feeder lines must be non-resonant. The best remedy is to employ a concentric line or coaxial cable.

The concentric line may contain ceramic or polystyrene insulators between inner and outer conductors. Often the line is sealed shut - after injecting an inert gas. This prevents collection of moisture inside the concentric lien and thus raises the breakdown-voltage.



Posted April 8, 2020

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