Antenna Principles - Directional Arrays and Radiation Fields
February 1947 Radio-Craft

February 1947 Radio-Craft [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 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 III - Directional Arrays and Radiation Fields

By I. Queen

Previous articles described the operation of simple antenna systems suitable for broadcast reception and amateur operation.

More elaborate designs are used for commercial communication and broadcast transmission.

All practical antenna systems and arrays are directional to some extent. They may be grouped in accordance with their characteristics as follows:

1. Those designed to receive or transmit along one or a few narrow beams only. In other directions the system is relatively ineffective.

2. Those producing an irregular pattern.

3. Arrays having circular patterns in the horizontal plane. Actually these types are directional with little propagation upward or downward, but they are commonly termed nondirectional.

Beam Arrays

For point-to-point communication there are many advantages in using an antenna which concentrates power in desired directions only. Secrecy is maintained and very high efficiency is possible. Interference with and from other transmissions is reduced to a minimum. Sharp beams become practical at the higher frequencies because. the radiating systems can be constructed within a reasonable area.

The Amphenol broadside array is an example of a well-designed communications antenna for the 152-162 mc band. These frequencies are assigned to fire departments, police, press, and railroads. The same array also can be used (with slightly lower efficiency) in the neighboring amateur and government bands which extend from 144-198 mc. Its excellent directional characteristics recommend it for fixed or mobile point-to-point service.

The electrical design of the Amphenol broadside array is illustrated in Fig. 1. Four half-wave dipoles are spaced by one-half wavelength and fed at their centers. The feeder system from the array may use RG8U or (for very long lines) RG17U co-axial cable.

The large-diameter tubing lowers the inductance and raises the capacitance of each dipole. The low Q broadens the response curve and accounts for the very wide band over which the antenna is effective. Use of low-impedance cable eliminates difficulty with voltage loops and leakage. Note that the outer dipoles are fed through cables which are one full wavelength longer than the cables which feed the inner ones. Therefore each dipole is fed in phase.

Power is propagated only broadside to the array. Assume that a wave starts out at some instant from one dipole. It reaches the next dipole one-half cycle later because of the half-wave spacing. At this later instant the second dipole tends to radiate a field which is out of phase with that which has jus t reached it from the other. The two opposite fields cancel out along either direction of the array. A receiving antenna located broadside to the array intercepts equal power from each of the four dipoles since in this case all currents are in phase, The total gain in the second case is 7 db over that of a single radiator.

The narrow fields which are possible with the broadside array are shown in Figs. 2-a and 2-b. The first is a cross-sectional view as it might be seen by an observer standing on a level with the array (if radio waves were visible). Little power is lost through upward propagation. The second figure is a view looking down on the antenna.

A sharper beam can be transmitted by using a still more complex array. Photo A shows such a system. This particular antenna is erected above the forty-third story of the building which houses WOR's FM station WBAM, more than 500 feet above street level in the heart of New York City. It has  an effective gain of 60!

This array is beamed toward Washington, D. C., and can be used for transmitting at 47.1 or 106.5 mc. The interesting antenna is of the same type as that which warned its GI attendant of the oncoming Japanese attack on Pearl Harbor. It is also similar to the array used in 1946 to contact the moon by radar.

Irregular Patterns

To properly serve two or more populated centers and to avoid possible interference with nearby transmitters, it is often necessary to design a broadcast antenna so that it radiates an irregular field pattern (Fig. 3). This also makes it possible to reduce power which otherwise might be wasted on mountains, lakes, and wooded sections. An irregular field pattern requires the use of more than one antenna tower. The pattern can be varied by adjusting the magnitude and phase of each tower current and the position of each tower.

Because of the many variables concerned, direct mathematical calculations become quite involved and consume a great deal of time. Several instruments are available for easing the problem, however. At least one mechanical device* has been designed for antenna calculations.

A still more modern and convenient instrument is the RCA Antennalyzer†. An oscilloscope is used to give instantly the field pattern which results from the use of up to five antenna towers. Sixteen dials control the Antennalyzer, four for each tower. Since one tower is taken as the reference, it requires no control. The four dials represent: magnitude and phase of the tower current; distance and angle (in azimuth) of the tower, with respect to the reference tower. To operate this instrument, the desired pattern is drawn with chalk on the face of the oscilloscope tube. Then the dials are manipulated until the same pattern is traced by the electron beam. The necessary tower factors are taken from the dials.

Nondirectional Radiation

Many large and small manufacturers are doing research in the design of high-frequency broadcast antenna systems because of the widespread and increasing importance of FM and television.

Requirements for high-frequency broadcasting present very special problems. First there is the consideration of distance coverage (Fig. 4). Because of the line-of-sight limit, large population centers can be properly served only by locating the transmitting antenna in the heart of a city and high enough so that it overlooks most obstructions. FM, television, and multiple communication require very wide modulation-frequency bands, and consequently special antenna designs. As the carrier frequency increases, the length of a resonant conductor becomes smaller, and as the modulation band increases, the cross section must be made greater.

Many high-frequency antenna systems now in operation take on odd shapes and sizes, often named for the objects they resemble (cloverleaf, turnstile, rocket). Each is designed for circular radiation at low angles, a wide modulation band, and small mounting area.

All these commercial antennas are designed according to the fundamental principles already set forth. They have, however, many interesting special features, arising in most cases out of using different methods of solving the same problem. A description of these antennas is worth an article in itself, and the next number in this series will cover a number of them.

Field Surveys

By mathematical calculations and by the use of precision mechanical computing devices and such instruments as the Antennalyzer, it is possible to determine accurately the field radiated by any antenna. However, the actual field intensity at any point is known only when ideal conditions exist. Especially at the higher frequencies, field strength and contour are determined not only by the radiation characteristics but also by the height of the antenna and the obstructions.

Broadcast stations must supply actual field measurements to the FCC so that possible interference between stations can be eliminated and maximum population coverage provided.

The Federal Communications Commission requires the use of accurate receiving and continuous recording equipment for the survey. Generally the chart may be driven by the same mechanism which actuates the speedometer of the automobile or truck which holds the equipment. Recording is made along eight radials extending from the transmitter, each spaced by about 45 degrees. Highways spaced so conveniently do not generally exist, but it is usually possible to choose streets or roads which run approximately parallel to such radials.

The survey must continue past the points which indicate 1,000 microvolts per meter of field strength so that the required 1,000-microvolt contour may be drawn. A 500-microvolt contour around the station is required also, but due to difficulty with fading at such low field strength it is usually computed from the data for the stronger field, A typical recording is shown in Fig. 5.

* Radio-Craft, August 1943, p.652.

† Radio-Craft, May, 1946, p.536.

Posted November 23, 2020