US2784403A - Double cavity primary feed - Google Patents

Description

March 5, 1957 J. A. MARSH ETAL I 2,784,403

DOUBLE CAVITY PRIMARY FEED Filed March 17, 1951 INVENTORS JAMES A. MARSH -GRANT M. RANDALL BY DONALD- F. ZEMKE ATTORNEY nited States Patent DOUBLE CAVITY PRIMARY FEED James A. Marsh and Grant M. Randall, Whittier, and Donald F. Zemke, Harbor City, Calif., assignors to North American Aviation, Inc.

Application March 17, 1951, Serial No. 216,134 8 Claims. (Cl. 343--779) This invention pertains to microwave radiation, and particularly to a microwave antenna designed to radiate a narrow pencil beam of electromagnetic energy from a wave guide system which permits the transmission of microwave energy in two transverse electric modes. This invention is a radiator which is designed for use as a primary feed to illuminate a parabolic reflector. It is particularly applicable to the duomode simultaneous lobing radar disclosed in patent application Serial No. 216,145, filed March 17, 1951, in the name of Robert M. Ashby for Duomode Monopulse Radar System. This duomode radar system employs two hybrid bridges, a section of double rectangular wave guide, this invention, and a parabolic reflector to achieve the monopulse operation desired.

In order for the radio frequency or R.-F. hybrid bridges in this system to extract the information from the radar echoes required for automatic tracking of a target it is necessary for electromagnetic energy to propagate between the R.-F. bridges and free space in the form of two transverse electric modes within each guide of the double wave guide configuration. The double wave guide configuration consists of two rectangular guides laid side-byside so that the wide dimension forms a common boundary between the two guides. One of the two modes which can propagate in this system is the dominant mode for a wave guide of rectangular cross-section. It is called the TE0 mode and is characterized by an electric field which is oriented completely transverse to the direction of energy propagation, has no amplitude variation along the narrow transverse dimension of the wave guide, and a half sinusoid amplitude variation along the wide transverse dimension of the wave guide. The other transverse electric mode which can be propagated is the TEbz mode and is characterized by a field intensity which also has no amplitude variation in the narrow transverse dimension of the wave guide, but has a full sinusoid amplitude variation along the wide transverse dimension of the wave guide.

The function of the present invention is to match the impedance of both wave guides to free space so that efl'icient transfer of energy can take place between the wave guides and free space for both the TEm and TEoz modes. In addition, this invention, being a primary feed, must illuminate efliciently a parabolic reflector in such a manner that the secondary radiation pattern has the desired simultaneous pencil lobes which will yield the data required for accurate automatic tracking.

It is therefore an object of this invention to provide a microwave radiator which is an eflicient impedance transformer between a double wave guide structure and free space, not only for the dominant TEo1 mode, but also for the TEoz mode.

It is another object of this invention to illuminate a parabolic reflector efiiciently, i. e., to direct as much of the energy as possible toward the reflector, and, by

reciprocity, to receive as much of the energy incident upon the reflector as possible, thereby minimizing the spurious side lobe radiation.

It is another object of this invention to provide a primary feed which is minimum in physical size so as to create as small an aperture block, or shadow area, and as high an antenna gain as possible.

It is another object of this invention to provide a symmetrical radiating structure for radiating microwave energy from two colinear rectangular wave guides in two transverse electric modes which will not cross couple energy from one mode to another and which will thereby maintain isolation between the simultaneous lobing operations of the radiator.

It is another object of this invention to provide a primary radiator capable of handling high peak power.

Other objects of invention will become apparent from the following description taken in connection with the accompanying drawings, in which:

Fig. 1 is an elevational view of the invention;

Fig. 2 is a detailed elevational view of a part of the invention;

Fig. 3 is a sectional view of the invention taken at 3-3 in Fig. 2; and

Fig. 4 is a sectional in Fig. 3.

Referring now to Fig. 1, a parabolic reflector 1 carries at its axis of symmetry colinear wave guides 2 and 3 which taper at their outer ends and are connected to cavities 4 and 5. As shown in Figs. 2, 3, and 4, cavities 4 and 5 are box-like in construction, except as modified by the pres ence of channel backing plate 6 which forms a rectangular indentation in each cavity. Guides 2 and 3 are connected to cavities 4 and 5 by plate 7, shown in Fig. 3, which has symmetrical slots 8 and 9 cut in its face and communicating the interior of each cavity to free space. These slots are of the same width as guides 2 and 3. Cavities 4 and 5 are separated by septum 19 which is really a continuation of the common wave guide wall between guides 2 and 3.

The purpose of slots 8 and 9 is to illuminate parabola l with microwave energy which flows through guides 2 and 3. Each slot illuminates approximately half the reflector and must be placed near the focal point of the reflector. The two slots with cavities 4 and 5 make up the primary feed. This feed acts like a point source located at the focus, radiating spherical waves which are transformed into plane waves at the aperture of the parabola.

The size and shape of the slots as well as the spfiiig between them determines the primary radiation pattern and the power-handling capacity of the primary feed. To illuminate the parabola efiiciently for both the TE and TE, modes it is necessary to choose the slots quite carefully. The length of the slots (b dimension in Fig. 3) must be greater than A the free space wave length of the microwave energy being propagated, by at least 10% for eflicient transmission of the TE mode. However, the maximum length is determined by the f/D (focal length to diameter) ratio of the parabolic reflector to be illuminated. A tapered illumination of the reflector is chosen as a compromise between high gain and high side lobes. The nominal taper figure may be chosen as 10 db, i. e., the intensity of illumination at the edge is down 10 db from the maximum intensity at the center of the reflector. For this duomode antenna, however, the additional factor of angle sensitivity must be considered, and, therefore, a taper of 15 db is used. To determine the value of b it is necessary to proceed as follows;

view of the device taken at 4-4 (1) First determine the half angle subtended by the reflector from the given f/ D ratio (2) Then solve the following equation for b However, this gives only an approximate value of b, since the formula is based on the assumption b 1n the case where f/D=.4l it was found that b should be about 1.11) the minimum value. Final fine adjustments in the value of b may be made experimentally.

The width of the slots and spacing between the two slots (dimensions a and c in Fig. 3-) determines the radiation pattern in the plane perpendicular to the length b. However, if a is small, less than A /ithe mostimportant factor is 0. Whether or not the a dimension is chosen sufficiently small depends upon the f/D ratio .of the reflector. If the f/D ratio is moderately large ,(.2 to .6) there will be difliculty in getting a broad enough primary pattern without making a less than M/ 5. Therefore, in this region the dimension 0 may be obtained from 0.177=cos sin d) where o is the same half angle determined above. 'However, if the a is not less than A /S, c should be obtained by solving the following expression:

we cos (.sin 11 sin /2] M up to 125 kw. peak can be transmitted through a 0.2 x

1.4" slot. For fairly large f/D ratios the value .of a should be as small as the power limitation will permit.

The function of cavities 4 and 5 is to perform an impedance match between the rectangular wave guides and free space and to reverse the direction of energy flow so that the energy radiated by the slots is incident upon the reflector. This impedance match must be performed not only for the TE waves, but also for the TE waves.

The two cavities are identical, and since septum in Fig. 2 isolates cavities 4 and -5 from each other it is necessary to consider only one of them here. The-cavity acts like a tuned L-C circuit which is heavily loaded by the radiation resistance of the slots. The inside cavity dimensions are quite critical in as much as it is required that physical size and shape of the cavity be resonant simultaneously for two modes which have different values of A (wave lengths in the guide). It is found that by moving backing plate 6 in the cavity, the cavity could be resonated for the TE, mode at steps of A /Z for the T13 mode. The exact position of the backing plate 6 with respect to the slots toobtain resonanc is a function of the slot size and shape and the dimensions of the wave guide feeding energy into the cavity. "It is possible to match the impedance of the .wave guide to free spacethrough'this slot and cavity, however, quite readily. This same proposition is true of the TE mode, but the distance between positions where backing plate 6 resonates the TE mode are spaced by A /2 for the TE, mode. Channel backing plate 6 and the dimension d, in Fig. 3 are used to align the resonances for the two modes so that they both occur at common settings of backing plate 6. The points where the TE mode resonates are only every second point where the TE mode resonates, i. c. k for TE is twice k for TE It has been found by experimentation that when the following cavity parameters were obtained, satisfactory operation occurs:

Both the TE and T E02 modes also resonate ,at the same cavity :depth when d, is very close to zero. This cavity depth, however, must be discarded because of the very low powenhandling capacity of the cavity. Still a third cavity depth and many more can be obtained at d,=2)\ (TE 3 (TE etc.; however, the powerhandling capacity does not increase, and the cavity losses do increase. Therefore, the dimensions tabulated above are considered optimum.

Although the invention has been described and illustrated indetail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim:

1. Means for illuminating a parabolic radar reflector with microwave energy in two transverse electric modes of transmission comprising a rectangular wave guide of sufficient width to support transverse electric mode propagation having full sinusoidal variation in transverse electric field across the width thereof for transmitting said energy to the vicinity of the focus of said reflector, and a parallelepipedal closed cavity resonant for 'both said modes and of depth greater than the wave length in guide of said energy propagated in the dominant transverse electric mode connected to the end of said guide and having a rectangular output slot of width greater than the wave length .of said energy for reflecting said microwave energy onto said reflector.

2. A device as recited'in claim 1 and further comprising a rectangular indentation in the reflective surfac of said cavity centered in a direction parallel to the short cross-sectional dimension of said guide whereby microwave energy propagated in said guide in two transverse electric modes may be caused to illuminate said reflector with good impedance match for both said modes.

3. A radar antenna structure for radiating and receiving electromagnetic energy transmitted in rectangular wave guide in two transverse electric modes comprising a parabolic reflector, a rectangular wave guide of width greater than the wave length of said energy projecting through said reflector and terminating near the focus thereof, and .a parallelepipedal cavity resonant in both said modes of depth and width at least equal to that of said guide, connected to the .end of said guide and .having a slot of width equal "to "that of said guide for illuminating said reflector with microwaves, said .slot facing said reflector to thereby illuminate said reflector with microwave energy propagated in said guide.

4. A device as recited in claim 3 and further comprising a rectangular channel-like indentation in the reflecting surface of said cavity of width and depth approximatelyone fourth of said wave length to thereby illuminate said reflector with microwave energy propagated in said guide in two transverse electric modes with good impedance match 'for'both said modes.

5. A device as recited in claim 3 and further comprising a second rectangular wave guide colinear with said guide, a second parallelepipzdal cavity similar to said first cavity connected to said second guide and having a slot similar to that in said first-named cavity to thereby illuminate said reflector with microwave energy throughout its area.

6. Microwave radiation apparatus for radiating and receiving electromagnetic energy in two transverse electric modes of transmission in guide comprising a parabolic reflector, a pair of symmetrical rectangular wave guides of width greater than the wave length of said energy laid with their Wide sides together and projecting through the center of said reflector to the focus thereof, a pair of symmetrical parallelepipedal cavities resonant for microwave energy in both said modes of depth and width at least equal to that of said guide connected to the ends of said guide and having slots of width equal to that of said guide opening toward said reflector in symmetrical relationship to thereby illuminate said reflector with microwave energy propagated in said guides.

7. Microwave radiation apparatus for radiating and receiving electromagnetic energy transmitted in rectangular wave guide in two transverse electric modes comprising a parabolic reflector, a pair of identical rectangular wave guides of Width at least equal to the wave length of said energy laid with their wide sides together and projecting through the center of said reflector to the vicinity of the focus thereof, a pairof identical pa-rallelepipedal cavities resonant to the frequency of the microwave en ergy propagated in said guides for both said modes of transmission of depth and width at least equal to the width of said guides connected to the ends of said guides and having slots of width equal to said guides widths opening toward said reflector in symmetrical relationship to thereby illuminate said reflector with microwave energy propagated in said guides with eflicient impedance match in both said modes.

8. A device a recited in claim 7 in which said cavities include a channel-shaped indentation centered in the reflective ends of said cavities of depth and width approximately one fourth the width of said guides and normal to the wide sides of said wave guides to cause resonance of said microwave energy simultaneously in two transverse electric modes to thereby radiate microwave energy in two transverse electric modes.

References Cited in the file of this patent UNITED STATES PATENTS 2,356,414 Linder Aug. 22, 1944 2,436,408 Tawney Feb. 24, 1948 2,483,575 Cutler Oct. 4, 1949 2,496,643 Smith Feb. 7, 1950 2,545,472 Kline Mar. 20, 1951 2,566,900 McArthur Sept. 4, 1951

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