US3508273A

US3508273A - Dispersion correcting antenna feed

Info

Publication number: US3508273A
Application number: US623523A
Authority: US
Inventors: Allan C Schell
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1967-03-14
Filing date: 1967-03-14
Publication date: 1970-04-21
Anticipated expiration: 1987-04-21

Description

ig am; I SEARCHROOM April 21, 1970 I A. c. SCHELL 3,508,273

DISPERS ION CORRECTING ANTENNAFEED Filed March 14, 1967 2 Sheets-Sheet 1 INVENTOR. MA 6! GUI/44 I vBYZiggy;{ i g 2 April 21,1970 A. c. SCHELL DISPERSION CORRECTING ANTENNA \FEED 2 Sheets-Sheet 2 Filed March 14, 1967 4 J .4 r m A, M MM am mawfwm United States Patent 3,508,273 DISPERSION CORRECTING ANTENNA FEED Allan C. Schell, Winchester, Mass., assignor to the United States of America as represented by the Secretary of the Air Force Filed Mar. 14, 1967, Ser. No. 623,523 Int. Cl. H01q 19/06 us. or. 343-754 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to antenna feeds and more particularly to a feed utilized with a specialized antenna of multiple zones wherein the frequency dispersive properties of zoned antennas are compensated for by a dispersion correcting feed.

Inherently zoned antennas have frequency dispersion limitations thus making them narrow band devices. This frequency dispersion occurs, for example, because electromagnetic rays emanating from the feed to the reflector and thence in the desired direction do not have equal electrical path lengths (inherent in the design). The conventional zoned antenna is designed so that the path length to the focus has been chosen such that there is coherent addition of a received wave at a specified frequency. With a shift in the specified frequency, the unequal path length from the wave front to the focus destroys the in phase relationship of the received wave. The present invention provides means to retain the in phase relationship over a wider frequency bandwidth by compensating for the unequal path lengths.

The present invention provides a device that corrects for the frequency dispersive properties of zoned antennas, such as the multiple reflector type of antenna. It may be utilized as a feed, or primary illuminator, for very large antennas. The device consists of a line feed or a planar array in which the individual radiating elements are interconnected by transmission lines whose lengths have been chosen to compensate for the external path length differences across the antenna aperture.

The novel feature of the present invention is the use of a prearranged dispersive characteristic of an extended feed to correct for the dispersive character of an antenna. Feeds have previously been built with special illumination patterns and means for removing aberrations, but this invention provides means for compensating for frequency characteristics over a band. It is to be noted that antennas that consist of zones, whether reflective or refractive, are inherently limited in their frequency range when operated with a conventional feed. This invention can greatly widen the frequency range of a zoned antenna by compensating for dispersion.

An object of the present invention is to provide an antenna having a prearranged dispersive characteristic to correct for the dispersive characteristic of a zoned antenna.

Another object of the present invention is to provide an antenna feed in which the individual radiating elements are interconnected by transmission lines whose lengths have been chosen to compensate for the external path length differences across the antenna aperture.

3,508,273 Patented Apr. 21, 1970 These and other objects and advantages of the present invention will be better understood when considered with the following description taken in connection with the accompanying drawings made a part of this specification wherein embodiments are illustrated by way of example. The device of the present invention is by no means limited to the specific embodiments illustrated in the drawings since it is shown merely for purposes of description.

In the drawings:

FIGURE 1 illustrates in schematic form the feed of the present invention and the associated zoned reflector;

FIGURE 2 illustrates one embodiment of the feed of the present invention including dipoles and associated preselected balanced transmission lines; and

FIGURE 3 shows another embodiment of the feed of the present invention showing slotted waveguides.

Now referring to FIGURE 1, there is shown a schematic of a zoned reflector 10 and its associated feed 11. A zoned reflector of this type is conventional and is described in IRE Trans on Antennas and Propagation, vol. AP-6, pp. l29133, January 1958, by L. Ronchi and G. Toraldo di Francia. It is also described in IRE Trans on Antennas and Propagation, vol. AP-9, pp. 48, January 1961, by L. Ronchi, V. Russo, and G. Toraldo di Francia.

Feed 11 and its associated transmission line 12 which comprises the present invention are shown in this instance, for purposes of a detailed exposition of its theory and mode of operation. Feed 11 is a line source of length L, consisting of a number of radiators connected by large electrical path lengths by means of transmission line 12, that is aligned vertically with its base at a height h above the aperture plane, as shown in FIGURE 1. Zoned reflector 10 extends outward from the point underneath the feed for a distance R =h tan a There is also shown preselected antenna 13. The bottom section of the line source feeds the farthest part of the reflector as shown by line 15, while the top of the line source illuminates the near part as shown by line 16. The electrical path length correction in feed 11 that is needed for rays leaving feed 11 at an angle a with the downward vertical is where z is measured along the vertical line feed and a and on are shown in FIGURE 1.

The phase difference between adjacent radiating elements of the line feed determines the direction of the maximum of the radiation from the elements by the relation The desired phase difference between radiating elements can be obtained by appropriate taps on the path length sections. In general, the transmission line connecting any two elements is longer than the straightline distance between elements, and thus the condition for correction by a conventional line feed,

does not hold.

Since the line feed contains large path lengths, the phase difference between radiators shifts rapidly with frequency, changing the direction of radiation of the sections of the feed. If the line feed is to correct for disper sion, the variation of path length along the feed must be chosen so that the phase difference between points in the aperture is approximately constant with frequency.

The change of direction versus frequency of the radiation from a section of the feed is fdD 50!- dz csco: (2)

At the frequency f, a section of the line feed at z illuminates plate 14 at an angle a; at the frequency f+6f, these plates are illuminated by a feed section located at z+6z. This feed section has a path length that differs from that at z by dD EZBZ and a path length to the plates that is greater by cos 1x52. If the relative phase across aperture 13 is to remain unchanged with frequency, then must be independent of Assuming h L, and using the approximation the relative phase dependence on it becomes dD dz dz 2 dd a+cos a m I: h see a 5111 a-l-cos a 50: Substituting (2) and requiring independence of a,

dz da 2 2 l: it see a sin a+cosa :|sec a K The relation between z and a is found by integrating (3) to yield K log (K-sec 001e,,

The constant 2 can be determined from the property Z=O when a=a,,, yielding K log Elie tan a see a dd 16 5 integrating (5) yields 3 3 z- (sec 01 sec at) and applying the condition a=0 when z=L,

h K- -(see 05 -1) Substituting (6) into 1) with the approximation h L,

Z 1/3 z s a D(z) h sec 01 h[see 0: (sec 01 1) (7) The path length variation of the line feed given by (7) corrects for the aperture path length variation within the accuracy of the approximations (h+z)-h and K sec a. A more involved analysis along the lines of the above would yield a more accurate expression. At frequencies other than the design center for the feed, the illumination pattern will be displaced in angle due to the phase change with frequency. This can be minimized by using a long feed, oriented at an angle other than vertical.

The theory and mode of operation of the present invention is also described in a paper by Allan C. Schell entitled The Multiplate Antenna, by A. C. Schell, published in IEEE Transactions on Antennas and Propagation, vol. AP-14, No. 5, pp. 550-560, September 1966.

It is emphasized that the dispersion correcting feed described for FIGURE 1 functions by using electrical path lengths in the feed to compensate for differences in path lengths for rays from different parts of the aperture. In FIGURE 1, there is shown a feed for a ground reflector on a plane. It should be clear that the zoned reflector need not be on a plane; it may follow any curved surface desired.

There are many physical forms that an embodiment of this invention may take. However if they are to perform the described function, each must contain the two essential parts: first, a series of basic radiating elements, such as, horns, waveguide slots, or dipoles, and second, a series of electrical path length delays, such as, waveguide sections, loaded transmission lines, or appropriately spaced subreflectors.

FIGURE 2 shows one representative embodiment of the feed of the present invention wherein line source feed 11 of FIGURE 1 is shown as a set of dipoles 20-27 along the Z axis. Each of the dipoles being a radiating element. Insulating structure 28 is utilized to hold dipoles 20-27. There is provided antenna feed terminals 29 to which is connected balanced transmission line 30. Associated with transmission line 30 is a series of transmission line sections 31--37 which are representative of transmission line 12 of FIGURE 1. The length of each transmission line section is cut so that the electrical path length of the section obeys the desired phasing law given by Equation 7.

It is to be noted that in one method of dispersion compensation, a reference point such as terminals 29 may be selected. Then there may be selected the electrical path length from the reference to each successive radiator dipole element such that the electrical path length variation along the Z axis obeys Equation 7.

Now referring to FIGURE 3 which shows a zoned antenna feed being another embodiment of the present invention. Structure 40 is shown and is utilized for supporting waveguide 41. Waveguide 41 includes terminating load 42. There is provided antenna input 43. The radiating elements are longitudinal offset slots 44-51 which are cut in the broad face of waveguide 41. Waveguide 41 has been wound in a manner such that the electrical path through the guide from slot to slot provides the correct variation of electrical phase along the line feed axis as specified by Equation 7.

While in accordance with the provisions of the statutes, there have been illustrated and described some preferred forms of the invention. It will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometime be used without corresponding use of other features.

What is claimed is:

1. In a zoned antenna having a zoned reflector with a preselected aperture, a feed therefor of predetermined length L comprising a series of radiating elements aligned vertically, a common input for said series of radiating elements serving as a reference point, said reference point being at a preselected height h from said zoned reflector, a transmission line section connected from said reference point to each of said radiating elements, the electrical path length D from said reference point to each successive radiating element being of a different preselected length to compensate for differences in electrical path lengths from said preselected aperture to said radiating elements in accordance with the equation Z 1/3 D(z) h sec a-h [S003 OIO-(SGG3 015-1) 2. In a zoned antenna having a reflector with a preselected aperture, a feed therefor as described in claim 1 wherein said radiating elements consists of dipoles.

3. In a zoned antenna having a reflector with a preselected aperture, a feed therefor as described in claim 1 wherein said radiating elements is comprised of a series of longitudinal offset slots cut in the broad side of a waveguide, said waveguide being wound so that the electrical path length through the waveguide from slot to slot pro- References Cited UNITED STATES PATENTS 6/1962 Strumwasser et al. 343--771 12/1968 Wong 343771 X 10 HERMAN KARL SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner US. Cl. X.R.