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A Circular Antenna for U.H.F.
November 1942 QST Article

November 1942 QST

November 1942 QST Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from ARRL's QST, published December 1915 - present. All copyrights hereby acknowledged.

Long before there were computer programs to instantly plot antenna radiation patterns, there were engineers who used slide rules to generate tables of values for power levels based on fundamental equations, and then plotted those points by hand on graph paper. Any copies were either hand generated like the original, or were run off on a mimeograph machine with its characteristic purple ink. Such was the case for the antenna radiation patterns published in the November 1942 edition of QST that describes the virtues of a circular antenna in the UHF band. It is too bad that the author did not include the equations for the antennas presented; that would really give you an appreciation for computers!

A Circular Antenna for U.H.F.

Technical Review

One of the problems that keeps life from getting dull for the radio engineer is that of designing antenna systems suitable for u.h.f. broadcasting. What is wanted is a horizontally-polarized system which radiates equally well in all horizontal directions, has relatively little vertical radiation and at the same time is as simple as possible both mechanically and electrically.

  A four-bay circular antenna system model - RF Cafe
A four-bay circular antenna system model. The bays are fed in pairs from two main transmission lines.

The 90·degree V antenna and horizontal pattern - RF Cafe


The 90·degree V antenna and pattern - RF Cafe

Fig. 1 - The 90·degree V antenna and pattern.

The cubical antenna for television broadcasting - RF Cafe
The cubical antenna for television broadcasting. The eight half-wave elements are arranged in two groups of four, each group forming a horizontal square.

Circular antenna for the f.m. band - RF Cafe
Circular antenna for the f.m. band. It gives a substantially circular horizontal pattern and does not need to be insulated from the supporting pole.
An interesting type of antenna which fulfills these requirements was described by M. W. Scheldorf of the General Electric Company in a paper presented at the I.R.E. Summer Convention. It might be called a "circular end-loaded folded dipole" were it not for the contradictions inherent in such a description. Designed for f.m. broadcasting, the antenna gives a substantially circular radiation pattern in the horizontal plane, is a simple mechanical structure (at least from the commercial viewpoint), and can be mounted without insulation on a grounded metal pole. The latter feature is of course highly desirable from the standpoint of lightning protection. Individual antennas can be stacked to form a multi­unit system giving an increase in field strength over a single unit. While its resonance characteristic is not broad enough for good television transmission it is amply broad for wide-band f.m. The resonant frequency of the antenna is readily adjustable after installation.

The final antenna design was a product of evolution, starting with the cubical antenna shown in the photograph. This antenna, the first one used for television broadcasting at the G. E. Helderberg station, consisted of two horizontal sets of four half-wave elements each, the elements of a set being arranged in the form of a square. Sub­sequent work showed that the same effect could be secured by replacing the square set of four elements by a pair of elements arranged in the form of a V having a 90-degree opening, as shown in Fig. 1. This gave the horizontal pattern also shown in Fig. 1; the shape could be controlled by altering the angle between the arms of the V, an angle smaller than 90 degrees giving an improvement over the pattern shown. However, the antenna was still bulky and the elements had to be insulated from the support.

The next step is shown in Fig. 2, where the antenna consists of two quarter-wave sections each bent in the form of a U having sides of equal length, the two sections being fitted together in the form of a square with two of the sides overlapping. This gives a cir­cular radiation pattern, since the currents in the overlap­ping sections are in phase and the resultant" effective" cur­rent tends to be uniform around the square. This type of antenna also is obviously much smaller than the V or cubical arrangements. Be­cause of the capacity between the adjacent sections of the antenna, the overlapping square antenna is practically the equivalent of a loop an­tenna having capacity loading, as shown in Fig. 3.1

The final system used is shown in Fig. 4. Because the radiation resistance of a circular antenna such as that shown in Fig. 3 is quite low, a second element was added to provide a step-up impedance transformation, using the principle of the folded dipole.2 The effective length of the elements including the loading of the end capacity C, is one-half wavelength overall.

Overlapping square antenna horizontal pattern - RF Cafe


Overlapping square antenna - RF Cafe

Fig. 2 - Overlapping square antenna.

The actual physical arrangement is shown in the close-up photograph. Point D, Fig. 4, is at ground potential and the antenna therefore can be mounted directly on a metal supporting pole at this point, without insulation. In the practical antenna the elements are made of steel pipe formed into a circle having a diameter of 33 inches, for a center frequency of about 46 Mc. This compares with a length of slightly over 10 feet for a half-wave dipole at the same frequency.

Fig. 5 shows the development of the antenna from the plain folded arrangement. In the top drawing, the current distribution is close to that characteristic of an ordinary half-wave antenna.

Simple loop antenna radiation patterns - RF Cafe


Simple loop antenna - RF Cafe

Fig 3 - Simple loop antenna. To obtain a truly circular horizontal pattern the total length
of the loop must be small enough in comparison to a half wavelength so that
the current is substantially the same in all parts.

By adding end capacity, Stage 2, the current distribution is made more uniform because an appreciable current flows into the end capacitors. In the final stage the antenna system is formed into a circle with the end capacitors facing each other to form a condenser.

The relative diameters of the two elements A and B determine the magnitude of the impedance step-up. It has been found experimentally that a wide range of impedance change can be obtained. In the commercial design the terminal impedance is about 35 ohms, at resonance at 46 Mc., when the antenna is mounted on a 4-inch diameter steel pole. With poles of larger diameter the radiation resistance decreases because of out-of-phase currents induced in the surface of the pole.

The circular antenna described in the text radiation patterns - RF Cafe


The circular antenna described in the text - RF Cafe

Fig. 4 - The circular antenna described in the text.

Since the antenna is appreciably smaller than an ordinary dipole, some loss of signal strength is to be expected as compared to the latter. However, it turns out that this loss is only one decibel as compared to a vertical dipole (which also has a uniform horizontal pattern). The antennas can be stacked vertically to increase the field strength, and it has been found that optimum gain is obtained when the spacing between units is about one wavelength. The gain in decibels over a vertical half-wave antenna, as a function of number of antennas or "bays," is shown in Fig. 6. It can be seen that doubling the number of elements results in approximately 3 db. gain. This is to be expected in view of the fact that the mutual impedance between antenna units or bays has been determined experimentally to be very low, when the spacing is one wavelength, hence the bays act almost independently of one another.

A four-bay antenna for the f.m. broadcasting band is shown in the third photograph. Each bay is provided with a quarter-wave matching section which matches the antenna terminal impedance to that of the concentric transmission line used. The matching sections are lengths of concentric line so constructed that the inner conductor and the spacing insulators both can be removed after installation. This makes it possible to vary the size of the inner conductor and also the number of spacing insulators used when it is desired to bring about an exact match. It has been found that the surge impedance and velocity of propagation in the line are both inverse functions of the square root of the average dielectric constant, regardless of the shape of the insulators used. This relationship makes it possible to predetermine the performance of the matching section.

Evolution of the circular antenna from a folded dipole - RF Cafe

Fig. 5 - Evolution of the circular antenna from a folded dipole.



Gain of circular an term a over a vertical half-wave dipole - RF Cafe


Fig. 6 - Gain of circular an term a over a vertical half-wave dipole. Bay spacing is 1 wavelength.

1  A. Alford and A. G. Kandoian, "Ultrahigh-Frequency Loop Antennas," AIEE Transactions Supplement, 1940.

2   P. S. Carter, "Simple Television Antennas," RCA Review, October, 1939.

Posted 8/9/2011

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