US2512191A - Broad band directional coupler - Google Patents

Description

June 20, 1950 J. M. WOLF BROAD BAND DIRECTIONAL COUPLER Filed Jan. 7, 1946 INVENTOR.

JA M ES M W OLF ATTORNEY.

Patented June 20, 1950 BROAD BAND DIRECTIONAL COUPLER James M. Wolf, Boston, Mass, assignor, by mesne assignments, to. the United States of America, .as represented by the Secretary of War Application January, 7, 1946, Serial No. 639,642

This invention relates to an electrical apparatus and more particularly to a microwave energy coupler.

One of the objects of the invention is to provide a coupler structure in which the shape and size of coupling apertures accomplishes substantially uniform coupling over an operating frequency band. I I

Another object of the invention is to provide a microwave energy coupler in which the magnitude of coupling may be relatively large.

These and other objects and advantages of the invention will be apparent from the following description when read in connection with the drawing, which is an isometric view of the microwave coupler, in part cut away to indicate more clearly the coupling apertures and other internal structure.

The present invention is related to the general class of microwave couplers in which a length of wave guide (main guide) included in a transmission line, and another length of wave guide (secondaryguide) connected to a utilization circuit, have several-paths of energy transfer between them to achieve directionality or wave selectivity in the coupling action. The energy transfer in prior art couplers of this class is relatively small in magnitude, and the amount of energy transfer varies considerably with the frequency of operation.

In accordance with the present invention, a relatively large percentage of microwave energy may be transferred from the main guide to the secondary guide by reason of the aperture form and placement in the component guides. For suitably chosen aperture shapes and sizes, the energy transfer remains substantially constant over a relatively wide band of operation frequencies.

Referring now to the drawing, the coupler embodiment here described includes a main guide 5 and secondary guide 6 which are of rectangular cross section and have a narrow wall I in common. Main guide 5 may be provided with endplates or flanges 8 and 9 as shown which adapt it to be inserted in, or connected in series with, the transmission line of a microwave system. The narrow wall I held in common between the component guides is provided with apertures II and [2 which are elongated in the direction of energy transmission along the guides. The centers of the elongated apertures or slots are spaced by a quarter guide-wavelength along the direction of energy transmission in the guides, and the slots are laterally spaced on each side of the longitudinal axis of secondary guide 6. The slots 4 Claims. (Cl. 17844) may thus overlap, if desired, in the direction of energy transmission.

Each slot in the common wall 1 provides coupling between the component wave guides of the illustrated structure. Because of the quarter guide-wavelength spacing of the coupling slots, very little wave energy is excited in secondary guide 6 in a backward direction with respect to an exciting primary wave in main guide 5. The electrical distances which may be traced along main guide 5, thence through the two coupling slots in a backward direction into secondary guide 6, differs by a half guide-wavelength. The backwardly directed components of energy transferred through slots II and I2 thus interfere destruc tively, so that practically no energy ispropagated in a backward direction. Energy propagation in secondary guide 6 does take place in a forward direction, however, for the forwardly directed components of energy coupled into the secondary guide are-in phase, the path lengths being equal in that direction. Thus, energy excited in secondary guide 6 propagates in the same direction as the corresponding energy wave in main guide 5. v

High selectivity in coupling action is therefore achieved in the coupling structure thus far described, for oppositely directed incident and reflected primary waves in main guide 5 excite proportional and oppositely directed energy waves in secondary guide 6.

Referring again to the drawing, the directional coupler may be so connected in a microwave sys-' tem that incident energy travels through main guide 5 in the direction indicated at l3, toward the right in the drawing. Energy corresponding to the primary incident wave then propagates in the same direction in secondary guide 6. Similarly, an oppositely directed reflected wave in main guide 5 excites an energy wave in secondary guide 6 which travels in the opposite direction, toward the left in the drawing. This latter energy wave, made separately available by the particular disposition of the coupling apertures as set forth, is not utilized in the instant structure and is in fact dissipated as hereinafter described.

The guides may operate in the dominant (TE-0,1) mode, in which the E-vectors (electric field vectors) are normal to the broader walls. A conventional resistive strip termination I4 is supported in an E-plane at the end of secondary guide 6 toward which the coupled reflected wave travels. The resistive strip functions to absorb microwave energy propagated toward it and thus eliminates reflection and interference. An energy pick-up means at the other end is adapted to convey coupled incident energy from secondary guide 6 to an external utilization circuit. This The directional coupler of the present inven- .7 tion is characterized by the fact that it may be designed for relatively large coupling, or for substantially constant coupling over a-broader band than heretofore attained, due primarily tothe elongation of the coupling apertures in the di-- rection of energy transmission in the guides. The magnitude of coupling is determined by the size, shape, and lateral spacing of the apertures. The coupling may therefore be made as loose as desired, for example by using slots of suitably small width. Another feature of the instant direction-a1 coupler is that it may be utilized at any location along a transmission line, for the fraction of power coupled into the secondary guide is substantially independent of the relative phase of standing waves that may be present in the main guide. r

The invention is not limited to a directional coupler having a broad-band characteristic, for it may be desirable in certain instances to provide other relationships of coupling to frequency, as may be accomplished by an appropriate selection of coupling aperture shape. 7

It is apparent that the invention is not limited to the physical construction illustrated in the drawing, but that various changes may be made without departingirom the spirit of the invention.,

Wht is. claimed is: ,1 l. A directional (coupler, including a first length of wave guide wherein primary waves may be propagated, a second length of wave guide, and means interconnecting said lengths oi wave guide comprising a common wall having two spaced apertures therein, said common wall being parallel to the electric field vectors in said second length of wave guidasaid apertures being symmetrically disposed on either side of the longitudinal axis of said wall, being elongated in the direction of, energy transmission and having their centers spaced along said direction by an effective distance of a quarter guide-wavelength, whereby energy waves corresponding to and pro portional to said primary waves .are propagated in the same direction in said second length of wave guide, said directional coupler having a broad-band coupling versus frequency characteristic.

2. A directional coupler, including a first length of rectangular wave guide wherein primary waves may be propagated, a second length of rectangular wave guide, and means interconnecting said lengths of wave guide comprising a common wall having two spaced apertures therein, said apertures being symmetrically disposed laterally with respect to the longitudinal axis of said wall, being elongated in the direction of energy transmission and having their centers spaced along said direction by an eifective distance of a quarter guide-wavelength,

-whereby relatively large energy waves corresponding to, and proportional to said primary waves are propagated in the same direction in said second length of wave guide.

3. A directional coupler, including a first length of wave guide wherein primary waves may be propagated, a second length of wave guide, and means interconnecting said lengths of wave guide comprising a common .wall having two laterally disposed apertures therein, said apertures being elongated in the direction of energy transmission and having their centers spaced along said direction by'an eiiective distance of a quarter guide-wavelength, whereby energy waves corresponding to and proportional to said primary waves are propagated in the same direction in said second length of wave guide, said directional coupler having a broadband coupling versus frequency characteristic.

4. A microwave energy coupling. structure in cluding a-first length of wave guide wherein primary waves may be propagated, a second length of wave guide, means providing two coupling paths between said lengths of wave guide, whereby energy waves corresponding to said primary waves are propagated in said second length of wave guide, said means comprising a pair of laterally disposed apertures elongated in the direction ofenergy propagation, having their centers spaced along said direction by a quarter guide wavelength, and means to absorb'reflected energy in said second wave guide.

JAMES M. WOLF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Great -Bri'tain June is, 1942

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