US3145356A - Different sized waveguides coupled by a narrow tapered dielectric rod - Google Patents

Description

Aug. 18, 1964 P. J. B. CLARRICOATS Q 3,145,356

DIFFERENT SIZED WAVEGUIDES COUPLED BY A NARROW TAPERED DIELECTRIC ROD Filed Oct. 6. 1961 United States Patent ce 3,145,356 DIFFERENT SIZED WAVEGUIDES COUPLED BY A NARROW TAPERED DIELECTRIC ROD Peter John Bell Clarricoats, Belfast, Northern Ireland,

assignor to National Research Development Corporation, London, England, a British corporation Filed Oct. 6, 1961, Ser. No. 143,499 Claims priority, application Great Britain Oct. 11, 1960 3 Claims. (Cl. 333-34) This invention relates to methods of coupling two electromagnetic waveguides of dilfering cross-section without substantial impedance mis-match so that the apparatus may be used over a broad frequency band.

When two Waveguides of different cross-section and therefore different characteristic impedance are joined together, the discontinuity in the boundary conditions causes reflections in an electromagnetic wave travelling along the guides. Whereas it is relatively straight-forward to match the two guides for any one frequency so that there is no reflection at that frequency, it is not easy to do this over a broad band of frequency. Known methods of doing this include the tapering of the waveguide dimension over a distance of several wavelengths, the forming of stepped transitions at the junction, and the inserting of a dielectric transformer in one of the waveguides to match the characteristic impedances of the two waveguides. The first method requires accurately made taper transitions, while in the latter two methods the dimensions of the apparatus are critical and need to be calculated separately for each individual case.

It is an object of this invention to provide a simplified device which will couple together two wave-guides of different cross-section without substantial impedance mismatch.

It has now been found that if an electromagnetic Wave is propagated along a dielectric rod in free space, and if the rod dimensions are suitably chosen, virtually all the energy associated with the wave passes along within the rod. This being so, it is obvious that the introduction of metallic bounding surfaces in the free space outside the dielectric rod will have virtually no effect on the wave, provided that these surfaces are not too near the dielectric. Hence, if a dielectric member is supported within a waveguide out of contact with the walls thereof when an electromagnetic wave is propagated along the Waveguide, virtually all the energy associated with the Wave will be concentrated in the dielectric. This means that the cross section of the bounding waveguide may be changed abruptly without appreciably affecting the wave within the dielectric, or, in other words that the dielectric rod acts as a guide for the waves as they pass through the region of abrupt waveguide change. The energy will be confined within the rod provided only that the dimensions of the rod and the permittivity of the dielectric be so chosen that the wavelength in the rod, A is less than the wavelength in free space, A 7

Since the concentration of energy within a dielectric rod can with appropriate choice of rod dimensions be made broad band, it follows that such a rod may be used to provide a broad band junction between two waveguides of differing size and/ or shape. It is of course necessary to ensure that the energy in the waveguide is transferred to the dielectric rod with adequate efiiciency, and this may be achieved by any of the known methods, such as by the 3,145,356 Patented Aug. 18, 1964 tapering of the rod over a distance of as many wavelengths as is possible, or, Where a more compact form of junction is required, by the formation of stepped transitions of known form. The only requirements of the dielectric rod are that its dimensions and permittivity should be sufiiciently large for k to be less than M, while the cross-section of the rod should not approach too closely that of the smaller waveguide. Although the cross-sectional shape of the rod is not a critical consideration, it is preferred to use a rod of the same shape as that of the coupled waveguides.

One embodiment of the present invention will now be described by Way of example and with reference to the accompanying diagram in which the single figure represents in longitudinal cross-section a waveguide junction between two waveguides of different dimensions.

In the drawing 1 and 2 are two circular waveguides which are joined together end to end. 3 is a dielectric rod made of, for example, polystyrene, which is tapered to points at 4 and 5, and which is supported centrally in the guide at 6 in a block of a substance, such as foamed polystyrene, which is substantially transparent to microwave radiation but mechanically quite rigid. The diameter of the dielectric rod is large enough for k to be less than M but less than that of the smaller waveguide; for optimum working it should be about equal to the mean of the value for which a i and the smaller waveguide diameter.

This invention may also be used to join two waveguides of different shape e.g. rectangular and circular when a rod shape intermediate between the shapes of the two Waveguides may be used, e.g. polygonal in the case of rectangular and circular. Alternatively a rod of varying crosssection may be used, provided that any change in the shape of the rod be introduced gradually in order not to cause additional reflections.

When the two waveguides to be joined are of grossly differing cross-section, a high permittivity material may be used for the dielectric rod, in order that the rod should be small enough to be compatible With the smaller waveguide, while still satisfying the condition that be less than ho.

Furthermore while the arrangement illustrated employs a rod having tapered ends, a rod having stepped transi tions at its ends could be used, or else a series of rods of differing diameters and permittivities.

I claim:

1. Apparatus for coupling together two electromagnetic waveguides of different cross-sectional dimensions comprising a rod of dielectric material extending between the two waveguides, the rod having such cross-section and permittivity that the wavelength of electromagnetic energy propagated in the rod is less than the wavelength of such energy in free space and in which the cross-section of the rod is sufficiently large to permit substantially the whole of the energy of a wave transmitted across the coupling to be concentrated in the rod, without being so large relative to the cross-section of the smaller waveguide as to alloW the walls of said smaller waveguide to appreciably affect the propagation characteristics of the energy concentrated in the rod, whereby wave energy may be propagated through said coupling in either sense.

2. The arrangement as claimed in claim 1 in which the ends of the rod are tapered.

3. The arrangement as claimed in claim 1 in which the cross-section of the rod is about equal to the mean be- 1 3 4 tween the cross-section of the smaller Waveguide and that 2,503,549 Fox Apr. 11, 1950 cross-section of rod for which the Wavelength of energy 2,625,605 Chadler Jan. 13, I953 propagated therein would equal the wavelength in free 2,742,612 Cohn Apr. 17, 1956 space. 2,802,994 Ober et al Aug. 13, 1957 5 2,906,974 Reggia et al Sept. 29, 1959 References Cited in the file of this patent 3,010,083 Weiss Nov. 21, 1961 UNITED STATES PATENTS OTHER REFERENCES 2,207,845 WOHT July 1940 Weiss: IRE Transactions on Microwave Theory and 2,292,496 Von Baeyer F" 1942 Techniques, January 1960, pages 120-121.

2,422,191 Fox June 17, 1947 10

Claims (1)

1. APPARATUS FOR COUPLING TOGETHER TWO ELECTROMAGNETIC WAVEGUIDES OF DIFFERENT CROSS-SECTIONAL DIMENSIONS COMPRISING A ROD OF DIELECTRIC MATERIAL EXTENDING BETWEEN THE TWO WAVEGUIDES, THE ROD HAVING SUCH CROSS-SECTION AND PERMITTIVITY THAT THE WAVELENGTH OF ELECTROMAGNETIC ENERGY PROPAGATED IN THE ROD IS LESS THAN THE WAVELENGTH OF SUCH ENERGY IN FREE SPACE AND IN WHICH THE CROSS-SECTION OF THE ROD IS SUFFICIENTLY LARGE TO PERMIT SUBSTANTIALLY THE WHOLE OF THE ENERGY OF A WAVE TRANSMITTED ACROSS THE COUPLING TO BE CONCENTRATED IN THE ROD, WITHOUT BEING SO LARGE RELATIVE TO THE CROSS-SECTION OF THE SMALLER WAVEGUIDE AS TO ALLOW THE WALLS OF SAID SMALLER WAVEGUIDE TO APPRECIABLY AFFECT THE PROPAGATION CHARACTERISTICS OF THE ENERGY CONCENTRATED IN THE ROD, WHEREBY WAVE ENERGY MAY BE PROPAGATED THROUGH SAID COUPLING IN EITHER SENSE.

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