US2421988A - Directive antenna - Google Patents

Description

June 10, 1947. G. H. BROWN ET AL DIRECTIVE ANTENNA Filed Jan. 22, 1944 2 Sheets-Sheet l Patented June 10, 1947 DIRECTIVE ANTENNA George H. Brown and Oakley M. Woodward, Jr., Princeton, N. J assignors to Radio Corporation of America, a corporation of Delaware Application January 22, 1944, Serial No. 519,266

2 Claims.

This invention relates to directive antenna systems, and more particularly to the art of radiating and/or receiving energy directively throughout a broad band of frequencies with a single antenna system.

The principal object of th instant invention is to provide an improved method of and means for receiving directively and/or transmitting directively radio frequency energy.

Another object of the invention is to provide an improved directive antenna system of thetype comprising a single radiator and a parabolic reflector, which is capable of efficient operation throughout a wide frequency band.

A further object is to provide an antenna system of the described type which i imple in design and construction, and may be embodied practically in a structure which is mechanically strong, yet relatively light and inexpensive.

These and other objects will become apparent to those skilled in the art upon consideration of the following description with reference to the accompanying drawing, of which Figure 1 is a schematic perspective diagram of an antenna system according to the present invention, Figure 2 is a sectional elevation of a broad-band radiator element employed in the system of Figure 1, Figure 3 is a graph illustrating variations in the standing wave ratio on a transmission line connected to the system of Figure 1 as a function of frequency, Figure 4 is a graph illustrating the directivity of the system of Figure 1 as a function of frequency, Figure 5 is a graph illustrating variations with frequency in the power received by the antenna of Figure 1, and Figure 6 is a graph of the relationship between standing wave ratio and received power.

It is Well known in prior art practice to employ parabolic reflectors 'for providing antenna directivity. A discussion of parabolic reflectors will be found on pages 837-841 of Radio Engineers Handbook by F. E. Terman, published by Mo- Graw-Hill Book Co., 1943. Such reflectors are used, particularly at very high frequencies, to rovide relatively sharp directive characteristics. Ordinarily the dimensions of the reflector, and its focal length, are large relative to the wavelength of the energy to be transmitted, and so related that the focus lines in or near the plane of the mouth of the reflector. With a reflector which is a large number of Wavelengths in width at its mouth, the beam formed by reflected energy will be very narrow and very concentrated, and the directive pattern will be substantially unaffected by the unreflected energy radiated in a diffuse manner forward from the antenna. At somewhat lower frequencies, it is impractical to use a parabola with a mouth opening very many wavelengths across, because of the prohibitive size of the reflector. In the practice of the present invention, a relatively small parabolic reflector is used, with a broad-band radiator element. The field contributed by the non-reflected direct radiation is of the same order of magnitude as that produced by the energy reflected from the parabola. These fields are combined so as to produce a resultant field pattern which is relatively free of secondary lobes over a wide range of frequency, while the impedance presented by the radiator remains relatively constant.

' Refer to Figure 1. A reflector l, of cylindrical parabolic form, is covered at one end by a conductive bottom sheet or screen 3. A radiator element 5 is supported on the screen 3, relatively close to the surface thereof, and is positioned coaxially with the focal line I of the reflector. The reflector I comprises a framework of pipe or tubing to which the bottom screen 3 and upright surface members 9 and II are welded or otherwise secured. The bottom screen 3 and the rear surface 9 may be constructed of perforated sheet metal. The outer portions ll of the reflector are formed by spaced parallel wires secured at their ends to the tubular frame members. It is to be understood that either solid or perforated sheet material could be used throughout, al-

though in the interests of lightness and strength the described construction is preferred.

Referring to Figure 2, the radiator element 5 comprises a hollow cylindrical body t3 of brass or other conductive material, with top and bottom end walls [5 and I1 soldered or similarly secured thereto. The bottom end I! is secured to a fitting 19 which engages and is electrically connected to the inner conductor 2| of a coaxial coupling member 23. The coupling 23 is adapted to be engaged at its lower end by a complementary coupling device at the end of a coaxial cable (not shown). The outer conductor 25 of the coupling 23 extends through and is secured to a plate 21, which is adapted to be secured to the bottom screen 3 of the reflector assembly.

If the above-described combination wereto be used at a single frequency, with a parabolic reflector of relatively small mouth width W (see Figure 1) of, for example, two Wavelengths, the focal length F would be made an odd number of quarter wavelengths so that energy reflected from the parabola would add, along the axis of the beam, to that radiated directly from the antenna. At

other frequencies, particularly those at which the focal distance F is an even number of quarter wavelengths, the direct energy will subtract from the reflected energy along the beam axis, diminishing the main pattern lobe and producing relatively large side lobes. We have found that by making the focal distance F a single quarter wavelength, rather than a large number of quarter wavelengths, the frequency may be varied over a total range of approximately two to o e without producing seriously large secondary pattern lobes. larger mouth width (measured in wavelengths) at higher frequencies than at lower frequencies the range of effective operation may be extended much further toward the higher frequencies than toward the lower frequencies. tem designed to operate over the range 350 to 725 megacycles, the optimum focal distance was found to be seven inches, or one quarter wavelength at 423 megacycles. The focal length is 0.208 wavelength at 350 megacycles, and 0.43 wavelength at 725 megacycles.

Broad-band radiator elements of the type having relatively large surface areas in proportion to their lengths are well known, such elements being used in television antennas and other applications where broad-band radiation is required. We have discovered that in the system shown in Figure 1, a cylindrical radiator having approximately equal dimensions of length and diameter, of approximately one quarter wavelength at the upper frequency limit, will provide the minimum variation in impedance over the frequency band. For the band of 350 to 725 megacycles, the best values for the dimensions D and L (Figure 2) are 4%. inches and 4 inches respectively. Figure 3 illustrates the variations of impedance with variation in frequency, in terms of the standing wave ratio measured on a transmission line connected between the radiator element and a source of radio frequency power.

The performance of the described antenna system when used for reception is illustrated by the curve of Figure 5. Designating as P1 the power received under operating conditions, with constant field strength, and as P2 the power which would be received if a perfect impedance match existed between the antenna and a, receiver connected to it, the ratio Pl/PZ varies with frequency as shown by Figure 5. The curve of Figure is related to that of Figure 3 as follows:

where R is the standing wave ratio. This relationship is shown by the curve of Figure 6.

Figure 4 illustrates the width of the beam provided by the system of Figure 1, as a function of frequency. The curve of Figure 4 was obtained with a reflector having a mouth width of 87 inches, about 2.4 wavelengths at 350 megacycles. Owing to reflection from the bottom screen, the axis of the beam is tilted upward. If a horizontal beam is desired, the assembly is tilted forward from the vertical axis through a corresponding Thus for a sys- However, since the parabola has a angle or, as illustrated in Figure 1. This angle is approximately 20 degrees, with a reflector of the dimensions described herein.

The invention has been described as a broadband directional antenna system, comprising a relatively small parabolic reflector and a short, large diameter cylindrical radiator. The focal length of the parabola is approximately one fifth wavelength at the low frequency end of the band. The radiator element has a diameter approximately equal to its length, which is approximately one eighth wavelength at the low frequency end of the band. With the described dimensions, which are given by way of example rather than by way of limitation, the system will provide eflicient directive operation up to a frequency of approximately twice the low frequency limit.

We claim as our invention:

1. A directive antenna system including a reflector of cylindrical parabolic shape, with a focal length substantially equal to one fifth wavelength at the lower limit of the band of frequencies over which the system is to operate and a mouth width of approximately two wavelengths at the lowest frequency at which the system is to operate, and a broad-band radiator element positioned substantially in the focal line of said refiector, said radiator element comprising a cylindrical conductive member of length and diameter approximately equal to each other and approximately equal to one eighth'wavelength at the lowest frequency at which the system is to operate.

2. A directive antenna system including a reflector of cylindrical parabolic shape, with a focal length substantially equal to one fifth wavelength at the lower limit of the band of frequencies over which the system is to operate and a mouth width REFERENGES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,275,646 Peterson Mar. 10, 1942 1,990,649 Ilberg Feb. 12, 1935 OTHER REFERENCES Radio Engineers Handbook, by F. E. Terman, first ed., 1943. Published by McGraw-Hill Book Co., New York, N. Y., pages 838 and 839.

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