US2946999A - Constant beamwidth horn antenna - Google Patents

Description

July 26, 1960 Filed Dec. 16, 1957 K. s. KELLEHER ETAL 2,946,999 CONSTANTBEAMWIDTH HORN ANTENNA 2 Sheets-Sheet 1 I nllll Hull-i INVENTORS KENNETH S. KELLEHER CLAIR Fl PARKER, Jr.

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ATTORNEY July 26, 1960 K. s. KELLEHER ETAL 2,946,999

CONSTANT BEAMWIDTH HORN ANTENNA Filed Dec. 16, 1957 2 Sheets-Sheet 2 .25 m \ucm mx S .M II u m \uemaus .28 I ll INVENTORS ATTORNEY KENNETH s. KELLEHER CLAIR F. PARKER, Jr.

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2. com 03 03 Q8 02. e3. 00 a 3 {on of. on. 02 on m on 02 on. n. 2 n m on a 0 on on o- W 0- am o on on o o w on f v v M M E W W W h h N OHNI M v United States Patent 2,946,999 i CONSTANT BEAMWIDTH norm ANTENNA Kenneth S. Kelleher, Alexandria,- and Clair F. Parker, Jr.,.Falls Church, a., assignors to Melpar, Inc., Falls Church, Va., a corporation of New York Filed Dec. '16, 1957, Ser. No. 702,944

3 Claims. or. 343-786) as impedance transformers between the wave guide sys tems of microwave apparatus and free space. In a conventional electromagnetic horn antenna, the beamwidth of the main lobe is inversely proportional to the aperture in wavelengths. As .frequency is varied the aperture/wavelength ratio varies in direct proportion, pro viding a one to one correspondence between frequency and beamwidth. The inventive concept herein relates to a new type horn antenna which provides an essentially constant beamwidth over a 4 to 1 band instead of the usual 4 to l variation in beamwidth that would result from this frequency change.

Accordingly, it is an object of the present invention to provide ahorn antenna of a particular configuration which maintains a constant beamwidth over a broad frequency band.

Another object of the invention is to provide a horn antenna in which the effective aperture of the antenna is made a function of the frequency so that a constant beamwidth results.

' Another object of'the present invention is to improve upon electromagnetic horns, particularly of sectoral or 2,946,999 Patented July 26, 1960 which is much smaller at the initial aperture than would be true for a straight flare. The higher frequencies have leaked through the pin spacings before the flare angle becomes large, while the lower frequencies, which are constrained within the pins until the final-aperture is reached, are unaffected by the large angle.

Other objects, novel features and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description taken in connection with the accompanying drawing in which:

Figure 1 is a perspective view of the horn antenna constructed in accordance with the present invention;

Figure 2 is an end view of the horn antenna.

Figure 3 is a top view of the horn antenna.

Figure 4 is'a graph showing the measured radiation pattern at a frequency of 9 kmc. in the H-plane for the horn antenna of the present invention; and,

Figure 5 is a graph showing the measured radiation pattern at a frequency of 9 'kmc. in the E plane for the horn antenna of the present invention.

In the following specification there shall be described, and in the annexed drawing shown, an illustrative embodiment of the antenna system of the present invention. It is however, to be clearly understood that the 7 present invention is not to be limited to the details hereinvention illustrated in Figs. 1 to 3, the reference charpyramidal shape, with the'air 'of which it shall be poscan be divided into two parts.

One part produces a pattern in the plane of the magnetic field vector, i.e., the H plane pattern. The other part produces the pat-tern in the plane of the electric field vector, i.e., the E plane pattern.

V The H plane pattern is produced by replacing the, walls of the horn parallel to the electric vector with a series of conductive pins spaced at aiprogressively increasing distance traversing from the throat to the open end of the horn, analogous to a series of waveguides in the sides of the horn at progressively lower cut off frequencies. Thelowest frequencies are guided within the pinwalls to the'open end of the horn, but the higher frequencies are transmitted within the horn only until the aperture is reached which produces the correct beamwidth, and then the energy leaks out between the remaining plus.

The E plane pattern is produced by providing an exponential flare to the solid metal walls parallel to the acter 10 generally designates the horn antenna which is pyramidal in appearance, having flare angles in both the horizontal and vertical directions. The horn antenna 10 comprises a rectangular wave guide portion 11 having a coupling flange 12 at one end for attachment to other apparatus; for example, receiving or transmitting equipment. The other end of the wave guide portion 11 is connected to the throatof the horn. 10.

The horn 10 is formed of plate members 13 and M, each disposed at an angle'to the longitudinal axis of wave guide 11 and forming the upper and lower surfaces of the horn 10. I

. A plurality of conductive pins or rods 15 connect the upper and lower plate members 13 and 14 on each of the outer edges. of the plate members. For the purpose of explanation, the horn will be considered symmetrical with the effects of both pinwall sides of the horn being identical. I As shown, the pins 15 are spaced at a progressively increasing distance traversing from the throat to the open end of the horn, analogous to a series of wave guides in the sides of thehorn atiprogressively lower cut Joli frequencies. 'I'he lowest frequencies are guided within the pinwalls tothe open end of the horn, but the higher frequencies are transmitted within the horn only until the aperture is reached which produces the correct beamwidth, and then .the energy leaks out between the remaining pins. Thus, the side walls of the horn 10 consist of the conductive pin members 15 which are disposed at an angle with respect to the same axis. By this construct-ion, the horn 10 is pyramidal in appearance, havingflare angles in both. the vertical and horizontal directions.

'As normally excited from a rectangular wave guide po'rtion 11 coupled to flange 12, the linesof electric intensity are vertical; that is, parallel to the pin members 15 throughout the interior of the horn. By a suitable selection of the flare angle of the defining wall-like pin members 15, the radiation pattern may be made sharply directive in the H-plane, perpendicular to the electric 3 vectors within the horn, with a narrow principal lobe and negligible side lobes. The electric field intensity, across the mouth of the horn in this plane is substantially zero at the pin members and maximum on the axis of the horn 10. a The structure thus far described is the construction of the preferred embodiment of the invention. In order to understandmore fully the principal form of the invention, the following discussion of the experimental work that was performed on various modifications of thepin: wall antennas is set forth in detail. i:

To'achieve a 4 to 1 bandwidth, an adaptation of the picture frame principle was investigated. With this antenna the low frequency aperture is formed by a wire picture frame and the higher frequency apertures are formed by smaller picture frames behind the larger, lower frequency picture frames. It'was not apparent how the wave could be supported in the E plane by the frames, but an H plane version of the pinwall antenna was tried in which conductive pins between parallel plates form the walls of an H plane sectoral horn. The theory behind the operation of this antenna is to replace the walls of a waveguide horn with'prins properly spaced to keep the transmitted wave within the horn until it reaches the point where the aperture will produce a degree beam and then allow the wave to leak out between the remaining pins.

There was a problem to determine the proper spacing of the pins and the number of pins required. The first experimental model was constructed with a degree flare, 5.5 inches long, and had removable pins so that spacing and number of pins could be quickly changed. By using a progressively increasing number of pins and comparing patterns it was determined that a minimum of 8 pins, when equally spaced, was required to produce the desired beam with restrained side lobes. The beamwidth was measured to be 20 degrees at 10 kmc. where use of the entire aperture would theoretically produce a 20 degree beam, but the beam was found to become narrower as the frequency was increased. Spacing the 8 pins with a progressivelylarger space between pins toward the aperture end of the horn did not improve the beamwidth variation, and it was next decided to determine the spacing between successive pins analytically as a function of frequency. Pins were spaced a half wavelength apart starting with 16,000 me. and the successive spacing was increased progressively to be a half wavelength for every 1000 me. interval down to 8000 mc. The horn dnce again had a'fiare angle of 30 degrees in the H plane, and was 9 inches in length. The results obtained from this model were similar to the normal waveguide horn, narrowing the beam as frequency increased.

In the third experimental model, which was designed to cover the full range from 4,000 me. to 16,000 mc., the spacing between pins along the horn was increased for every 1000 me. increment, and, in addition, the'aperture was increased to theoretically pro'vide a 20 degree beam at the frequency for which the spacing was a half wavelength. This design resulted in a horn approximately 10 inches in length with curved edges. The results of this horn were most encouraging. A satisfactory beam shape was obtained over the full 4 to 1 bandwidth ratio, with side lobes morethan 10 db down, and the beamwidth variation less than 11 degrees.

- In the fourth pinwall antenna, the apertures were again chosen to provide a 20 degree beam at 500. mc.'

increments from 4000 me. to 9000 mc., but between 9.0 kmc. and 16.0 kmc. the increments were chosen every 1000 me. using a total of 17 pins. The spacing between successive apertures was again a half wavelength at the frequency for which the smaller aperture was calculated. The results obtained were similar to those obtained from the previous horn in variation over the band, but were better at the low end of the band and worse at the high end.

The fifth modified antenna, in which the spacing of antenna number four was adjusted to be equal to that of number three at the frequencies for which number three provided the better beamwidth, proved mainly that the pins could not be moved to adjust the beamwidth of a single frequency without affecting the beamwidths at all the other frequencies. It also indicated that successive pins could not be very far out of line.

The sixth horn was designed to put the pins in a straight line, while still maintaining the apertures for which the beam would theoretically be 20 degrees at the frequency providing a half wavelength spacing to the next aperture. This was accomplished by choosing the frequencies logarithmically to provide equal percentage increase in the spacing between the 12 pins. The results were quite encouraging; a beamwidth of minus 6 and plus 4 degrees over a 4 to 1 band was achieved.

The next antenna was identical except that the top and bottom were flared instead of parallel, and the pin spacing was reduced to be a half wavelength between pins insteadof between apertures. The H plane patterns had a beamwidth variation :4 degrees from 22 degrees.

I The H plane beam shape was good and the highest side V correcting the E plane patterns of the pinwall antenna.

' moved, patterns" were taken in both planes.

lobes more than 10 db down. However, the E plane patterns were poor and the beam split above 10,000 me. The next model in which 7 longitudinal stringers in the top and bottom supported twelve picture frames, spaced as in the previous antenna, was designed in hopes of bettering E plane patterns. However, the resulting patterns were an improvement only in that the beam did not split. .The E plane beamwidths turned out to be very wide at some frequencies (up to 40 degrees), and the beam shape was irregular. The H plane beamwidths varied :5 /1 degrees about 18 /2 degrees. Despite these adverse results, the fact that the beam did not split indicated that making the top and bottom plates ofthe horn mo're transparent at the higher frequencies is a means Investigating this theory further, the top and bottom plates of antenna were covered with a copper screen which in effect made the walls solid. As expected, this produced a split beam. Next, sections of the screen were removed, and as each successivesection of screen was re- 7 As mo're screen was removed, and the plates became more transparent, the E plane beam shape steadily improved. The H plane beam shape remained good but the beamwidth variation increased. From this information it was concluded that two solid plates are better for H plane patterns (more nearly 20 degrees beamwidth), and the more transparent plates produce the better E plane patterns. This suggested a compromise. However, removing the screen in this particular configuration did not yield a good compromise. l

Another pinwall antenna was designed with two solid wallsand 24 pins per side in the other two walls. This produced a horn 25 inches in length. The purpose of this antenna was to investigate the effect of an increased number of pins. The number of pins also adjusts the flare angle of the horn; more pins produce a smaller angle. Since the phase error across an aperture is decreased by a moregradual taper, it was conceivable that the split beam observed in the B plane pattern at the high end'of the band could be eliminated. The results of these tests indicate that, although the E plane pattern did not split, the H plane beamwidths were adversely affected. ."The next attempt to reduce the E plane split was a strip of polystyrene, with a square cross section onefourth wavelength wide at 10,000 mc., which was placed along'the centerline of the, two solid plates of the pinwall antenna. at 16,000'mc. and E plane side lobes were completely eliminated for directions beyond70 degrees to either side of center, but the-majorlobe was still split; at

This was tested at only two frequencies:

9,000 mo. in the E plane many lobes appeared nearly equal in magnitude to the major lobe.

A fourth method of E plane side lobe reduction, the insertion of the best polyrod into the mouth of pinwall antenna, yielded good results. However, this excellent showing was achieved only for a narrow band, and above 6,000 mc. multilobe characteristics and irregular shapes appeared.

Prior changes in the E plane dimensions from parallel plates to a sectorial flare had no effect on the H plane patterns, and it was likely that curving the top and bottom would still have no effect on the H plane. On the other hand, it could possibly eliminate the phase error producing the split beam at the high end of the band by permitting the flare angle to be small where the high frequencies are constrained by the walls, while still providing the large aperture required of the lower frequencies; i.e., the curved shape results in a steadily increasing flare angle which is much smaller at the initial aperture than would be true for a straight flare. The higher frequencies have leaked through the pin spacings before the flare angle becomes large, while the lower frequencies, which are constrained within the pins until the final aperture is reached, are unaifected by the large angle. The first tests were conducted with four curved copper side plates placed against the pins of an improved pinwall antenna to determine any effect on the H plane patterns before adding the curved top and bottom. The results on the E plane patterns were negative as expected, but the H plane patterns were unexpectedly improved. The total beamwidth variation was reduced from the original 7 degrees to degrees. After several curves were tried with increasing degree of success, the preferred exponential flared horn shown in Figure 1 which produced good patterns in both planes, was adopted (see Figures 4 and 5).

Thus, a horn antenna with essentially constant beam- Width was developed, and has been called a pinwall antenna, which provides a constant 20 degree beam in each plane over a 4 to 1 frequency band.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A horn antenna for providing a pattern of constant beamwidth comprising, a waveguide transmission line having a rectangular cross section; upper and lower plate members forming extensions of the respective walls of said transmission line, said plate members producing an E plane pattern; a plurality of conductive pin members joining said plate members on their longitudinal outer edges and being spaced at progressively increasing distances traversing from the throat to the mouth of said horn antenna, said conductive pin members producing an H plane pattern; said plate and pin members defining a tubular structure flared from the throat to the mouth of said horn antenna; said conductive pin members allowing the higher transmitted frequencies to radiate outwardly therefrom while constraining the lower frequencies.

2. A horn antenna for providing a pattern of constant beamwidth comprising, a wave guide transmission line having a pair of extension wall members, said extension members producing an E plane pattern; a plurality of conductive pin members joining said extension walls on their longitudinal outer edges and being spaced at progressively increasing distances traversing from the throat to the mouth of said horn antenna, said conductive pin members producing an H plane pattern; said extension walls and pin members defining a tubular structure flared from the throat to the mouth of said horn antenna; said conductive pin members allowing the higher transmitted frequencies to radiate outwardly therefrom while constraining the lower frequencies.

3. An electromagnetic horn antenna for providing a pattern of constant beamwidth comprising, a wave guide transmission line having upper and lower extensions of its walls, said extension walls producing an E plane pattern; a plurality of conductive pin members joining said extension walls on their longitudinal outer edges and being spaced at progressively increasing distances traversing from the throat to the mouth of said horn antenna, said conductive pin members producing an H plane pattern and allowing the higher transmitted frequencies to radiate outwardly therefrom while constraining the lower frequencies; said extension walls and conductive pin members providing a constant 20 degree beam in each plane over a 4 to 1 frequency band.

References Cited in the file of this patent UNITED STATES PATENTS 2,283,935 King May 26, 1942 2,317,464 Katzin Apr. 27, 1943 2,603,749 Kock July 15, 1952 2,692,336 Kock Oct. 19, 1954 2,794,185 Sichak May 28, 1957 FOREIGN PATENTS 238,928 Switzerland Nov. 16, 1945 1,091,260 France Oct. 27, 1954 OTHER REFERENCES Constant Beamwidth Broadband Antennas, by Parker and Anderson, IRE National Convention Record, part I, Mar. 18-21, 1957 pp. 87-98.

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