Coaxial Line to Double Ridge Waveguide Transition

This invention relates to apparatus for coupling microwave energy from a coaxial type transmission line to a double ridged waveguide transmission line and, more particularly, to a coax to waveguide transition which provides acceptable impedance matching between input and output transmission lines over a relatively broad bandwidth of microwave frequencies.

It is common practice in microwave transmission systems and in microwave devices, such as traveling wave tubes, to employ different types of microwave frequency transmission lines of known types, such as the coaxial type and the double ridged waveguide type, which have different electrical characteristics, such as characteristic impedance. It is also conventional in such systems to form a microwave energy transmission path for propagating microwave energy from one location to another in which the propagation path is formed of more than one kind of transmission line having different electrical transmission characteristics, such as the coaxial type and the double ridged waveguide type, a subject to which the present invention is directed. The coupling or connection between those two types of transmission lines is accomplished with reasonable efficiency by coupling means that are referred to by those skilled in the art as microwave transitions, or simply transitions.

Heretofore, many different physical structures for accomplishing a good electrical coupling between a coax and a waveguide line performing the function of a transition have been known, such as appears in the patents, believed to be exemplary of the prior art, U.S. Pat. Nos. 3,725,824, 3,737,812, 3,478,282, 3,528,041 and 3,431,515, which have been made known to us, to which the reader's attention is directed for additional background information. In its essentials, the output of the transition contains the configuration of a conventional waveguide type transmission line; the input of the transition contains the structure of the conventional coaxial type transmission line containing a central conductor surrounded by a dielectric; and intermediate the aforedescribed elements is employed an impedance matching section formed of tapered metal section or metal steps, physically resembling a staircase, or modifications and variations thereof.

The present invention is directed to a novel structure in a coax to waveguide transition of the recited type and has as a principal object the coupling of mircrowave energy over the frequency range of 8 to 18 gigahertz with reasonable levels of voltage standing wave ratios of between 1 and 2. An ancillary object of the present invention is to define a physical structure of a transition which is easy to fabricate and adjust.

The transition of the invention includes a rectangular waveguide section having first and second ends, and longitudinally extending ridges depending from each of two opposed walls with an end section of the ridges being outwardly tapered. A step impedance matching section protrudes within the waveguide and is of a geometry of a stack of three short cylinders of decreasing radii, the first cylinder serving as a base protruding into the guide a short distance beyond the outer surface of one ridge; a second cylinder of smaller radius atop the first oriented with a portion of the periphery thereof tangent to the periphery of the first cylinder; and a third cylinder of still smaller radius atop the second mounted coaxial therewith; the overall height of the in-the-guide portion of the step impedance matching member being less than the distance between opposed walls of the waveguide but greater than the distance between ridges; further, a dielectric member having a cylindrical portion extends into the waveguide fom an opposed wall and is in abutting relation with the end of the third cylinder; and a relatively straight electrical conductor extends coaxially through the dielectric member and into the step impedance matching member.

Additionally, the first ridge member abuts the first cylindrical portion and the secnd ridged member has an end spaced longitudinally from the dielectric member and from the third cylindrical portion so as to define a microwave energy propagation gap. The foregoing objects and advantages of the invention, together with the structure characteristic of the invention, as well as alterations and modifications thereof, become more apparent from a consideration of the invention which follows, taken together with the figures of the drawings illustrative thereof.

In the drawings:

FIG. 1 illustrates in perspective a preferred embodiment of the novel microwave transition; and

FIG. 2 is a section view to enlarged scale of the embodiment of FIG. 1 taken along the lines 2--2.

Reference is now made to the embodiment of FIG. 1 in which a coax to waveguide transition, constructed according to the invention, is presented in a perspective view and in which a portion of the waveguide is cut away, revealing the geometry and relationship of the enclosed elements. The structure includes a waveguide section 1 which in cross section is of conventional double ridged geometry, as is visible at the lower right hand side end in the figure, and in which the upper and lower walls contain depending protruding rectanguloid rigdes 3 and 5 that extend over a length of the waveguide, as represented by invisible lines. The upper ridge 3 contains an outwardly tapered extended portion 7 which commences at the end of the uniform ridge section and widens over its length terminating in a flat edge. The portion is of a trapezoidal-like geometry and is of a height essentially equal to that of the ridge section 3. Another tapered ridge section 9 adjoins the end of and forms an extension of the lower straight ridge 5 and is of the same height. The tapered ridge 9 increases in width to the left along its length and has a cylindrical end surface for reasons which become more apparent from the following description.

The left side end of waveguide 1 does not contain ridges but includes a positionable shorting plunger 11 or shorting wall which fits within the walls of the waveguide. The end surface of the plunger, as represented in dotted or invisible lines is of a concave cylindrical configuration and serves to block this end of the waveguide interior and define a cavity to one side of impedance transition section 9. A coaxial input member 13 includes an elongate center conductor 15 and an intermediate cylindrical shaped dielectric member 17. The dielectric member 17 contains an end geometry which is of a right cylinder and protrudes within the enclosed regions of the waveguide. An impedance matching member, generally represented as 19, extends though a opening in one wall into the waveguide. Member 19 is machined metal part, suitably copper, unitary in structure and contains a portion which protrudes within the interior of the waveguide which resembles three right cylinders, 21, 23 and 25, stacked one atop the other to form a series of steps. In order to describe more fully the structural relationship of the foregoing elements, reference is made to the cross section view presented in FIG. 2 which is drawn to a slightly greater scale than that of FIG. 1 but in which identical parts are identically labeled. As is apparent from this view, it is seen that the cylindrical portion 21 of element 19 is inserted into the waveguide so that it protrudes from the upper wall to a greater depth than the height of the adjacent ridge 9 so as to form a step 12 therebetween. Moreover, the second cylinder portion 23 is of a smaller radius than the corresponding radius of first cylinder portion 21 and the axis of this portion is displaced from the axis of cylinder 21. However, the second cylinder is oriented so that the left hand periphery is tangent to the same bisecting plane normal to the plane of the paper through which the left most peripheral portion of the first cylinder is tangent. The third cylinder portion 25 is of a radius which is less than the radius of cylinder portion 23 but has its axis coaxial with that of cylinder portion 23 so as to form a step on both the left and right hand sides. As is noted from the figure, the overall height of the portion of member 19 located within the waveguide section is less than the distance between the opposed upper and lower waveguide walls but greater than the spacing between the outer edges of ridges 9 and 7. The cylindrical portion of dielectric 17 extends from an opening in an opposed waveguide wall within the waveguide. The end surface thereof abuts the top of cylindrical portion 25. The elongate conductor 15 is seen to extend through a passage in dielectric member 17 and through a corresponding coaxial passage in member 19. As seen in this figure, the member 19 contains a hollowed out cylindrical portion containing a threaded wall 22, a threaded metal plug 26 and a circular spring 27 and cylindrical guide 28 adapted to apply an axial pushing force to the end of conductor 15. As is presented, the end of tapered ridge 9 abuts the outer peripheral surface of cylindrical portion 21. However, the opposed tapered ridge 7 terminates at a distance from the cylindrical portions 25 and dielectric member 17 so as to define a gap 31 through which microwave energy may pass. It is recognized that the outer surface of members 15 and 17 is configured as a female connector which is adapted to mate with any suitable RF coaxial transmission line connector 13 as shown in an exploded view in FIG. 1 and by dash lines in FIG. 2. By way of specific example, the waveguide used is type WRD-750-D24 of a dimension 0.691 inches by 0.321 inches, containing ridges 0.173 inches by 0.0925 inches. The in-the-guide height of cylindrical sections 21, 23 and 25 is 0.125 inches, 0.055 inches and 0.095 inches, respectively. Opening 30 is of a 0.250 inch diameter and dielectric member 17 of "TEFLON" is approximately 0.165 inches in diameter. The gap between the end of tapered ridge 7 and the peripheral surface of cylindrical portion 25 is approximately 0.020 inches.

In one practical embodiment of the invention the transition was operated on test equipment over a frequency range of 8 to 18 gigahertz and the VSWR at any frequency within that range did not appear to exceed a VSWR of 1.6.

All of the foregoing elements are constructed of electrically conductive metal, suitably copper, unless an element is otherwise described as a dielectric.

It is believed that the foregoing description of a preferred embodiment of the invention is sufficient in detail to enable those skilled in the art to make and use same. However, it is expressly understood that the details presented for the foregoing purpose are not intended to limit the invention inasmuch as modifications or substitution of equivalent elements or even improvements become apparent to those skilled in the art upon reading this specification, all of which embody the invention. Accordingly it is respectfully requested that the invention be broadly construed within the full spirit and scope of the appended claims.