Rectangular Waveguide Having Shaped External Contour Preventing Internal Deformation During Bending Or Twisting

Abstract

A rectangular waveguide has a cross section such that twisting or bending does not distort the internal cross section, lips being provided at each corner of the waveguide and the wall sections being shaped such that when viewed from a shorter side each corner has a portion with a substantially T-shaped section, and while viewed from a long side each corner has a pronounced asymmetric T-shaped section.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to rectangular waveguides of a flexible nature such that they can be bent, twisted or coiled, and more particularly to such waveguides in which the external contours of the longer and shorter sides of the section are such that, when bent, the interior of the waveguide remains substantially constant in shape.

2. Description of the Prior Art

Generally speaking, waveguides of this general type are of particular use as connecting links between an antenna and the transmit/receive units of radio relay equipment operating at frequencies above 3 GHz. It is well known in the art to construct such waveguide links from rigid sections, namely straight waveguides, elbows and twist members, these being manufactured in a factory and assembled together on site to provide a required configuration. However, the actual local dimensions frequently deviate from those originally specified, or are subject to modifications required for particular cases, so that the use of such prefabricated rigid components may not be possible, since any assembly will not fit, and special lengths of waveguide sections and tailor-made pieces may have to be subsequently produced. In an earlier invention, a waveguide is provided which can be adapted on site to the prevailing requirements of the particular application and has at last the same transmission quality as the rigid waveguide lengths heretofore employed. This prior art waveguide has a cross sectional profile such that when it is bent with respect to, or twisted about, its longitudinal axis, its internal cross section substantially retains its original shape to an optimum extent. Using such a waveguide construction, a guide of substantial length can be manufactured, and delivered conveniently in a coiled state to the required site, where it is then curved, bent and twisted, as required, during assembly to provide the required configuration for the link.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved flexible waveguide of the type just mentioned, so that while retaining optimum integrity of shape for the internal cross section when the waveguide is bent or twisted, the couples or moments required to produce the shaping are reduced.

The invention resides in the provision of a rectangular waveguide in which the internal contours of the longer and shorter sides of the section are in each case drawn in toward the center, preferably in circular or arcuate form, and in which the differing wall thicknesses in the peripheral direction of the waveguide cross section produced by these formations are chosen such that when the waveguide is bent, opposing couples which are located in the plane of section, but outside the internal contour, are developed and act in association with couples producing bending in the same direction to cancel out any turning effect of the couple which produces deformation of the cross section, so that when bent the waveguide interior substantially retains its shape. Consequently, the longer sides of the waveguide section are extended beyond the shorter sides to form lips, one at each corner, and their shape is such that the individual shorter sides and the individual corner sections of the waveguide cross section each form a substantially symmetrical T-section profile, while in relation to the individual longer sides a radically asymmetrical T-section profile is produced at each corner. The lips extend parallel to the longer sides of the inner waveguide section at least in the zone of forming the continuation of the external contour of the longer sides.

By th chosen design of wall thickness profile, it is ensured that the integrity of shape of the internal waveguide section is maintained when the waveguide is curved in the H- and E- planes and when twisted, a very low resisting couple is achieved, in particular when the waveguide is curved in the E- plane. Because of the low height of the cross section in this waveguide, with any bending in the E- plane, for the same mechanical strains in the extreme edges of the material, smaller radii of curvature can be achieved than in known waveguides. By curvature in the E- plane and by twisting, which also involves only a low torsional couple, it is therefore possible to produce waveguide lengths with small radii of curvature throughout. A further advantage is the extremely simple geometrical external contour of the waveguide, which is such that press molds, flanges and attachment elements for the waveguide can be simplified and the processing required when fitting the flanges is facilitated. Furthermore, this kind of shaping, to promote the retention of the waveguide internal cross section, has the result that when the waveguide is coiled, good, stable supporting surfaces are obtained, and such shaping enables simple bending tools to be employed for the waveguide to be bent or twisted on site, in order to route the waveguide in the desired manner.

A still further advantage resides in the provision of a rectangular waveguide constructed in accordance with the invention wherein direct coupling to standard waveguide components, which normally have a rectangular cross section, and to conventional rigid rectangular waveguide sections, can be effected without the need for waveguide junctions. Compared with rigid waveguide structures, it is advantageously provided through the ability to discard elbows, twist sections and intermediate sections of pre-manufactured types, that a substantial economy is achieved in terms of planning time and assembly costs.

Advantageously, the thickness of the waveguide wall in the plane determined by each of the individual shorter sides is such that it satisfies the condition M .apprxeq. h.sup.2 .delta., where:

M is the turning moment to be compensated;

h is the thickness of the waveguide wall; and

.delta. is the maximum permissible bending stress in the material used for the waveguide wall.

With this type of design, the outlay in material for the waveguide is advantageously reduced, while retaining adequate stability, so that the forces required to bend and twist the waveguide are reduced and the assembly of the waveguide is facilitated.

BRIEF DESCRIPTION OF THE DRAWING

Other objects, features and advantages of the invention, its organization, construction and operation will best be understood from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawing which carries a single FIGURE illustrating the cross section of a waveguide constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment illustrated in the drawing has a rectangular internal cross sectional profile, defined by longer sides 1, and shorter sides 2. The waveguide is profiled to provide the required external contour, and is preferably constructed of soft material having good electrical conductivity, in particular aluminum. The waveguide may be produced, for example, by an extrusion process. When bending the waveguide about the E- point, indicated in the drawing as an X----X bending plane, tensile and compressive stresses are generated parallel to the bending plane and these result in forces which are directed perpendicular to the bending plane. These forces depend upon the radius of curvature, upon the cross sectional area and upon the cross sectional shape, and are responsible for generating turning moments M which might distort the internal waveguide profile. As a calculation can show, the cross sectional distortion can be controlled by suitable dimensioning of the cross sectional area. Within a given rectangular waveguide, or for that matter any other arbitrarily shaped waveguide internal section, a minimum cross section distortion can be achieved if the external contour is so chosen that during bending of the waveguide, turning moments M are produced which act together with turning moments M.sub.0 to cancel any deforming action of the moments M.

In the case of the exemplary embodiment of a waveguide according to the invention and illustrated in the drawing, the external contours of the longer sides 1 and the shorter sides 2 are in each case drawn in toward the center, preferably in circular arcuate form, so that differing wall thicknesses are produced in the waveguide section in different circumferential directions. The longer sides 1 of the waveguide each extend beyond the shorter sides 2 to form the lips 3. These extensions are so designed and dimensioned that, in relation to the associated shorter side 2 at that particular corner of a waveguide section, a substantially symmetrical T-profile is created, while in relation to the individual longer sides 1, a highly asymmetrical T-profile is formed. Two conditions should be considered in the design and dimensioning of the lips 3. First of all, the bending stresses produced by the moments in the lips 3 should not exceed the permissible value for the material. Secondly, the longer sides 1 of the waveguide section should not deflect beyond the permissible limit during or after any bending or twisting. The lips 3 have longer sides extending in the direction of the longer sides 1 of the waveguide section, and in each case at least that side which constitutes the extension of the external profile of the associated longer side 1 should be disposed parallel to the longer side 1 of the internal waveguide cross section. At their ends, the lips 3 are radiused and preferably merge arcuately into the centrally inwardly dished external contour of the associated shorter side 2. This construction then provides a kind of compensation of materials, since an amount of material equal to that removed or absent from the end of the lip is present at the points of transition from the longer side to the lip itself, so that the effective cross sectional area available for resisting turning moments is maintained. In the neighborhood of the lip 3, when the waveguide is bent, opposing turning moments M located in the cross sectional plane, but outside the internal contour, are produced, and these, together with the turning moments M.sub.0 acting in the same sense, cancel out the turning action of the other moments M which are responsible for producing the cross sectional distortion. The waveguide wall has a thickness h for the longer sides 1 of the section in the planes determined by the individual shorter sides, and this is preferably fixed at a minimum permissible value to advantageously provide a reduction in the amount of material, and also in the reduction in the requisite bending moments. In the present example, the thickness of the waveguide wall is so chosen that it satisfies the condition M .apprxeq. h.sup.2 .delta.; where:

M is the turning moment which is to be compensated;

h is the thickness of the waveguide wall at the longer side of the section in the plane of each shorter side; and

.delta. is the maximum permissible bending stress in the material of which the waveguide wall is made.

A proportionality factor C should be included in this equation, and depends, among other things, upon the bending radius, generally having a value C = 1/6, for example. The thickness of the waveguide wall should then be selected to be at least sufficiently large, but preferably not substantially larger than, that required to satisfy the condition M = h.sup.2 .delta. /6.

The rectangular section waveguide illustrated in the drawing has been drawn substantially to scale so that the dimensions correspond to the true size ratio.

Although we have described our invention by reference to a particular illustrative embodiment thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. We therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of our contribution to the art.

Claims

We claim:

1. A rectangular waveguide, comprising: four sides including a pair of parallel longer sides and a pair of parallel sides shorter than said longer sides, said longer and shorter sides including external contours, as viewed in section, which are in each case drawn toward the center of the respective side, said sides having differing wall thicknesses in the peripheral direction of the waveguide cross section and dimensioned such that when the waveguide is bent, opposing couples located in the plane of a section and outside the internal contour of the waveguide are developed and act in association with couples producing bending in the same direction to cancel out any turning of a resulting couple which would produce deformation of the cross section, so that when bent the waveguide interior substantially retains its rectangular shape, said longer sides of said waveguide section extending beyond said shorter sides to form lips at each corner of the waveguide, said lips having a shape such that the individual shorter sides and the individual corner sections of the waveguide cross section each form a substantially symmetrical T-section profile, while in relation to the individual longer sides a radically asymmetrical T-section profile is produced at each corner, said lips extending parallel to said longer sides of said inner waveguide section at least in the zone of formation of the continuation of the external contour of said longer sides.

2. A rectangular waveguide as claimed in claim 1 wherein said external contours are drawn in toward the center of the respective sides in a circular arcuate form.

3. A rectangular waveguide as claimed in claim 1, wherein the thickness of the waveguide wall at a longer side, measured in each plane determined by an individual shorter side is such that it satisfies the condition M .apprxeq. h.sup.2 .delta., where:

M is the turning moment to be compensated;

h is the thickness of th waveguide wall at a longer side; and

.delta. is the maximum permissible bending stress in the material used for the waveguide wall.