US3230482A

US3230482A - Compact directional coupler

Info

Publication number: US3230482A
Application number: US319625A
Authority: US
Inventors: Seymour B Cohn
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-10-28
Filing date: 1963-10-28
Publication date: 1966-01-18
Anticipated expiration: 1983-01-18

Description

Jan. 18, 1966 5, COHN 3,230,482

COMPACT DIRECTIONAL COUPLER Filed Oct. 28, 1963 FIG.I

nel

Hil A CUJQQQQ INVENTOR,

SEYMOUR B. COHN ATTOR N EY.

United States Patent 3,230,482 COMPACT DIRECTIONAL COUPLER Seymour B. Cohn, Calabasas, Califi, assignor to the United States of America as represented by the Secretary of the Army Filed Oct. 28, 1963, Ser. No. 319,625 7 Claims. (Cl. 333-) This invention relates to directional couplers and more particularly to a compact 3 db directional coupler of the waveguide type.

A waveguide directional coupler comprises a pair of waveguides which share a common wall. Energy is transferred between the two waveguides by means of one or more apertures in the common wall. A simple method of obtaining directional coupling of two such waveguides consists of two holes in the common wall spaced along the longitudinal axis by a distance equal to one quarter of the guide wavelength. When power flows through one of the guides in a given direction, part of the microwave power is coupled through each of the holes, exciting waves in the second waveguide which propagate in both directions from each hole. The waves from each hole travelling in the same direction as the energy in the first waveguide reinforce each other while the backward waves cancel each other. The first waveguide is called the main guide and the second the auxiliary guide. The performance of a directional coupler is measured by its directivity and coupling, the directivity being the ratio of power flowing in the auxiliary guide in the desired forward direction to that flowing in the backward direction and the coupling being the ratio of power input to the main guide to power output of the auxiliary guide. The directivity of a two-hole coupler falls off rapidly as the operating frequency depart from the design frequency. Wideband operation as well as increased coupling can be achieved by utilizing one or more arrays of holes in which the coupling through the various holes of the array is made proportional to the coefiicients of a binomial expansion. In a so-called Tchebycheif array, the coupling of the successive holes is made such that the amplitude of the backward wave in the auxiliary waveguide varies as a function of frequency as does the Tchebycheif polynomial of order one less than the number of coupling holes. Multiple Tcheby-cheff or binomial arrays may be overlapped with some of the central holes forming parts of two or more arrays and hence being of larger size than the holes at either end.

The directional coupler of the present invention was designed in response to the need for an X-band directional coupler of minimum length. In order to achieve the required compactness as well as a coupling ratio of 3 db, it was found necessary to utilize a relatively small number of large coupling holes. In order to achieve the required tight coupling with a minimum length coupler, it was necessary to use coupling holes substantially larger than those of either a Tchebychefi? or binomial coupling array. However, it was found that these short arrays of large coupling holes caused several undesired resonances within the operating range of the waveguide system. These resonances can be eliminated by reducing the height or b dimension of both of the waveguides in the coupling region. In order to match the impedance of the reduced height section to the rest of the waveguide system, a stepped impedance transformer is utilized. A further size reduction is accomplished by incorporating this impedance transformer within the coupling region.

It is therefore an object of this invention to provide a compact 3 db directional coupler.

It is a further object of the present invention to provide a novel and useful X-band 3 db directional coupler.

3,230,482 Patented Jan. 18, 1966 Other objects and advantages will become apparent from the following detailed description and drawing, in which:

FIG. 1 is an isometric view of the one embodiment of the invention, and

FIGS. 2 and 3 are end and side views thereof respectively.

FIG. 4 is a plan view of the common wall of the two waveguides.

In FIG. 1 the directional coupler 5 comprises a pair of

waveguides

8 and 9 which share a common broad wall 10. The common Wall includes two longitudinal arrays of relatively

large coupling holes

12 and 14, arranged on opposite sides of the longitudinal axis of the common wall.

End flanges

6 and 7 are provided for inserting the coupler into a waveguide system. The illustrative directional coupler was designed for use in a microwave system comprising rectangular waveguides having a and b dimensions of .9 and .4 inch respectively and operating over the frequency range of 8.2 to 12.4 kmc. In accordance with the invention, the height or b dimension of each of the waveguides comprising the coupler is reduce-d from that of the rest of the waveguide system. This height reduction was found to move the undesired resonances caused by the relatively large coupling holes upward in frequency beyond the operating range of the system. Referring to FIG. 3, the b dimension of the flanges is made the same as the b dimen sion of the waveguide system, which is .4 inch in the illustrative embodiment. The height reduction is accomplished by increasing the thickness of the top and bottom

broad walls

24 and 25 of the coupler in three

steps

20, 21 and 22 in the top wall and 11, 15, and 16 in the bottom wall. The total reduction in height amounts to .065 inch in each of the

waveguides

8 and 9. This amounts to a reduction of approximately 16% in the b dimension. Each series of steps constitutes a Tche'by-cheif stepped impedance transformer, so-called because the Tche'bycheff polynomial is used in its design. Such a transformer matches the impedance of the reduced height coupler to the rest of the system with a minimum number of steps and minimum standing wave ratio. The design of such transformers is described in an article authored by the present inventor on pp. 1621 of IRE Transactions on Microwave Theory and Techniques, vol. MTT3, #3, April 1955. The steps are spaced .38 inch apart, which is approximately one quarter of the waveguide wavelength at the center of the operating frequency range in the illustrative embodiment. In accordance with the design principles given in the cited article, the step heights are .020, .027 and .018 inch from left to right in FIG. 3 for both of the illustrated series of steps. Similar impedance transformers (no-t shown) are formed in the right hand ends of the top and bottom broad walls.

FIG. 4 is a plan view of the common wall of the directional coupler of FIGS. 13. The common wall 10 contains two similar longitudinal arrays of

coupling holes

12 and 14. Each array consists of six generally rectangular holes 29 with longitudinal dimensions of .360 inch and a transverse dimension of .320". The six large holes of each array are symmetrically disposed on either side of the transverse center line 2727 of the common wall and the centers of the six holes are approximately onefifth of the a dimension or width of the common wall from either edge. The penultimate holes 30 at each end of each array are slightly smaller than the six largest holes, being .335 inch along the longitudinal direction of common wall 10 and .305 inch in the transverse direction thereof. Each of the four end holes 31 of each array are .264 inch by .228 inch with the long dimension parallel to the long dimension of the common wall 10 as is the case with all of the coupling holes. The hole centers of each array are spaced by approximately one quarter of the guide wavelength at the center of the operating frequency range. This leaves only a space of approximately .020 inch between the six large coupling holes 29 and approximately the same space between

holes

29 and 30, but a space of about .070 inch between the end holes 31 and the penultimate holes 30. The entire coupler measures approximately four inches in length.

While the invention has been described in connection with a specific embodiment, the inventive concepts disclosed herein are of general application and hence the invention should be limited only by the scope of the appended claims.

What is claimed is:

1. A compact 3 db directional coupled comprising, a pair of waveguides having a common broad Wall, two similar arrays of relatively large coupling holes extending along the longitudinal dimension of said common wall on either side of the longitudinal center line thereof, a flange at either end of said pair of waveguides, said flange being adapted to connect said coupler to a waveguide system of dimensions a and b, means to reduce the b dimension of each of the waveguides comprising said coupler by increasing the thickness of the top and bottom broad Walls thereof in step fashion, said steps comprising a Tchebychefi impedance transformer.

2. A compact 3 db directional coupler comprising, a pair of Waveguides having a common broad wall, two similar arrays of relatively large coupling holes extending along the longitudinal dimension of said common wall on either side of the longitudinal center line thereof, means to connect said coupler to a waveguide system of dimensions a and b where a is the interior width of said common Wall, means to reduce the b dimension of both of said waveguides relative to the b dimension of said waveguide system by increasing the thickness of the top and bottom broad walls of both of said waveguides by means of a series of steps, said series of steps comprising a Tchebycheff impedance tranformer.

3. A compact 3 db directional coupler comprising a pair of X-band waveguides arranged to share a common broad wall, a relatively short array of large coupling holes extending along said common wall on either side of the longitudinal center line thereof, means to connect said coupler to an X-band waveguide system with interior dimension of a and b, means to reduce the height or b dimension of each of said waveguides by approximately 16% relative to the b dimension of said waveguide system, said last-named means comprising a Tchebycheff stepped impedance transformer formed in the top and bottom broad walls of each of said waveguides.

4. A compact 3 db directional coupler comprising, a pair of waveguides having a common broad wall, two similar arrays of coupling holes extending along the longitudinal dimension of said common wall on either side of the longitudinal center line thereof, said coupling holes being substantially larger than those of either a Tchebychelf or binomial array of equal coupling, means to connect said coupler to a waveguide system, the a dimension of said waveguide system being the same as that of said waveguides and the b dimension of said Waveguides being less than that of said waveguide system, and means within said coupler to match the impedance of said coupler to the impedance of said waveguide system.

5. A compact X-band directional coupler comprising, a pair of waveguides having a common wall, two similar arrays of generally rectangular coupling holes extending along the longitudinal dimension of said common wall on either side of the longitudinal center line thereof, said coupling holes being substantially larger than those of either a Tchebycheff or binomial array of equal coupling, means. to connect said coupler to a waveguide system, the a dimension of said system being the same as that of said coupler, the b dimension of said coupler being approximately 16% less than that of said waveguide system, and a Tchebychetf stepped transformer at either end of both of said waveguides.

6. A compact directional coupler for use in a waveguide system having a frequency range of 8.2 to 12.4 kilomegacycles comprising, a pair of waveguides having a common broad Wall, two similar arrays of relatively large coupling holes extending along the longitudinal dimension of said common wall, said waveguide system having a ance of said coupler to that of said waveguide system.

7. A compact directional coupler for use in a waveguide system having a frequency range of 8.2 to 12.4 kilomegacycles comprising, a pair of waveguides approximately four inches in length having a common broad wall, two similar arrays of coupling holes extending parallel to and on either side of the longitudinal center line of said broad wall, said coupling holes being substantially larger than those of either a Tchebychelf or binomial array of equal coupling, said waveguide system having a and b dimensions of .9 and .4 inch respectively, said waveguides having an a dimension the same as that of said waveguide system and a b dimension reduced approximately 16% from that of said waveguide system, the reduction in said b dimension being accomplished by a series of steps in the thickness of the top and bottom walls at either end of said waveguides, said series of steps comprising a Tchebychetf impedance transformer.

References Cited by the Examiner UNITED STATES PATENTS 12/1957 Adcock et al. 33310 l/1958 Sferrazza 33-10 OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner.

G. TABAK, Assistant Examiner.

Claims

1. A COMPACT 3 DB DIRECTIONAL COUPLED COMPRISING, A PAIR OF WAVEGIUDES HAVING A COMMON BROAD WALL, TWO SIMILAR ARRAYS OF RELATIVELY LARGE COUPLING HOLES EXTENDING ALONG THE LONGITUDINAL DIMENSION OF SAID COMMON WALL ON EITHER SIDE OF THE LONGITUDINAL CENTER LINE THEREOF, A FLANGE AT EITHER END OF SAID PAIR OF WAVEGUIDES, SAID FLANGE BEING ADAPTED TO CONNECT SAID COUPLER TO A WAVEGUIDE SYSTEM OF DIMENSIONS A AND B, MEANS TO REDUCE THE B DIMENSION OF EACH OF THE WAVEGUIDES COMPRISING SAID COUPLER BY INCREASING THE THICKNESS OF THE TOP AND BOTTOM BROAD WALLS THEREOF IN STEP FASHION, SAID STEPS COMPRISING A TCHEBYCHEFF IMPEDANCE TRANSFORMER.