Iris coupled coaxial transmission line to waveguide adapter

Abstract

An adapter for coupling a coaxial transmission line to a waveguide, wherein the center conductor of the coaxial line passes via a back short of the waveguide through an iris and that terminates to the inside wall of the waveguide.

Claims

1. An apparatus for coupling a coaxial transmission line to a waveguide comprising: a coaxial section having a center conductor, a waveguide having an inside wall, a shorted end, and an iris, and wherein the center conductor of the coaxial section passes through the iris adjacent the shorted end of the waveguide, and electrically terminates to the inside wall of the waveguide.

2. The apparatus of claim 1 wherein the iris is formed as a circular, square, or irregularly shaped opening in a conductive shim.

3. The apparatus of claim 1 wherein the iris is fabricated as a separate part and affixed to the coaxial transmission line and waveguide.

4. The apparatus of claim 1 wherein the coaxial section is provided by a launch end of a coaxial connector.

5. The apparatus of claim 1 wherein the waveguide is rectangular in cross section.

6. The apparatus of claim 1 wherein the inside wall of the waveguide is a broadwall.

7. The apparatus of claim 1 wherein the iris is a circular hole formed in a metal disk.

8. The apparatus of claim 1 wherein the iris is configured to provide an impedance discontinuity between the coaxial section and the rectangular waveguide.

9. The apparatus of claim 1 additionally wherein the center conductor terminates at a ridge connected to the inside wall of the waveguide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Additional novel features and advantages of the approaches discussed herein are evident from the text that follows and the accompanying drawings, where like features are denoted by the same reference numbers throughout the detail descriptions of the drawings, and in which:

(2) FIG. 1 is a cross-sectional view of a prior art coaxial line to waveguide transition.

(3) FIG. 2 is a cross-sectional view of another prior art coaxial line to waveguide transition.

(4) FIG. 3 is a cross-sectional view of a novel coaxial transmission line to waveguide adapter where a center conductor of a coaxial line section passes through a back short of the waveguide and through an iris.

(5) FIG. 4 is a Smith chart resulting from a computer model of the novel adapter.

(6) FIG. 5 a zoomed in view of the same Smith chart as FIG. 4, centered on 50Ω impedance.

(7) FIG. 6 is another arrangement of the novel adapter.

(8) FIG. 7 is an embodiment where the coaxial section is provided by a coaxial connector.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT(S)

(9) Referring to FIG. 3 and FIG. 6, an illustrative embodiment may be comprised of a coaxial transmission line 110 with center conductor 103, dielectric 101 (FIG. 3), and outer conductor 108 (FIG. 3), a thin conducting wall 105 (FIG. 3) with a hole forming an iris 100, possibly fabricated as a metal shim 109 (FIG. 6) with a circular hole, wherein the center conductor passes through the iris 100. The thin conducting wall 105 is placed across the waveguide 102 to form a shorting plane or back short to the waveguide TE10 mode. (In other words, the waveguide is shorted in some way). The center conductor 103 terminates into a ridge 107 in the waveguide 102 that forms a magnetic loop 104 (FIG. 3).

(10) The iris dimensions may be adjusted to vary the reactance of the iris. This in turn is used to match the impedance of the coaxial section to the waveguide section. A selection of thin conducting walls with varying iris dimensions can therefore be prepared prior to the manufacture and/or test of a coaxial transmission line to waveguide transition. Examples of iris dimensions to vary include their thickness, and if circular, their radius. Using this prepared selection, irises of various dimensions can be swapped in and out during testing in order to tune the match of the transition.

(11) In one embodiment, the coaxial transmission line, which may be any TEM mode transmission line, is provided by another structure, such as a coaxial connector 120. FIG. 7 illustrates this case, where the coaxial transmission line section shown in FIG. 3 is provided by a launch end of a coaxial connector 120. In the example illustrated in FIG. 7, the iris 100 is formed in a thin metal shim 109 and a center conductor 103 terminates into the launch end of the coaxial connector 120, passes through the iris 100, and terminates into the stepped ridge 107 inside the waveguide 102.

(12) The iris 100 aperture closes tightly around the center conductor 103 of the coaxial transmission line such that the coaxial impedance formed by the center conductor 103 and iris 100 is less than the impedance of the coaxial transmission line. This allows back short currents to flow largely unobstructed in the end wall 105 of the waveguide 102 resulting in a more effective magnetic loop return path and waveguide back short.

(13) Results of a computer model, with and without the iris, shows the effect of the iris on return loss. Referring to FIG. 4, the solid line on the Smith chart shows the improved match with the iris, whereas the dashed line shows the performance without the iris. FIG. 5 shows a more detailed view of the Smith chart shown in FIG. 4. (The scale of the horizontal axis in the Smith charts represents normalized impedance). The example computer model used a 50 ohm impedance coax section, WR-15 rectangular waveguide and a wall thickness of 0.001 inches. The frequency limits on the plot are 50 GHz to 75 GHz. Other waveguide bands and wall thicknesses are envisioned as are similarly functioning geometries. For example, a circular waveguide TE11 mode adapter would make an efficient transition in this same manner. Furthermore, the iris could be square or irregular in shape.

(14) An advantage over prior art is that the iris 100 can be easily fabricated into flat sheets of conductive material, or shims 109, using processes such as machining, chemical milling or laser cutting, and attached to the waveguide section to form the back short as shown in FIG. 6. This eliminates overhanging material allowing the waveguide section to be fabricated using simple manufacturing processes such as CNC machining or casting. The entire transition can therefore be manufactured with only one complex part that can be made using simple manufacturing processes.

(15) The preferred embodiments are not dependent on the thin conducting wall, or shim 109 itself. The shim is therefore but one convenient implementation of the iris. In a more general conceptualization, an arbitrary TEM transmission line may be utilized, coupled to a waveguide wherein the center conductor of the coaxial line passes through the back short of the waveguide through an iris and where the center conductor terminates at one of the inside walls of the waveguide. The center conductor need not be rotationally symmetric around the TEM transmission line's axis of propagation as shown in FIGS. 3 and 6.

(16) Other types of transmission lines, such as a microstrip or coplanar waveguide may have TEM and TM modes. Therefore, an iris, as described above, may also be used to launch these other types of transmission lines into rectangular or circular waveguides.

(17) Furthermore, the TEM mode need not be the sole significant propagating waveguide mode in the transmission line. The inside walls of the waveguide need not have a feature for the center conductor to make contact with. The center conductor may instead contact with a broadwall 106 of the waveguide as shown in FIG. 1. The dielectric 101 (FIG. 3) between the center and outer conductors of the arbitrary TEM transmission line could be occupied by vacuum or air with a dielectric constant approximately equal to 1.0.

(18) The above description has particularly shown and described example embodiments. However, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the legal scope of this patent as encompassed by the appended claims.