Right-Angle junction for rectangular waveguides

Abstract

 A right-angle junction for rectangular waveguides, obtained by the 90° coupling of two rectilinear rectangular waveguides of equal cross-sectional dimensions, which comprises two steps arranged symmetrically with respect to the coupling plane between two waveguides, with two dimensions equal to the dimensions of the guides and suitable thickness, and further possible symmetrical pairs of steps of length equal to about a guide quarter-wave at the centre frequency.

Description

[0001] The present invention concerns microwave circuit devices and more particularly it refers to a right-angle junction for rectangular waveguides.

[0002] It is known that devices apt to couple rectilinear waveguides arranged in different positions are necessary to implement microwave circuits or waveguide measuring benches. Namely right-angle junctions apt to couple perpendicular waveguides are required.

[0003] For these devices, as well as for other devices used in microwave circuits, reflection coefficient ought to be minimum, at least within the operative frequency band. That can be achieved by building up arc of circle junctions with large radii of curvature, yet the devices, and consequently the circuits obtained, are very cumbersome.

[0004] This disadvantage is particularly felt in certain applications of microwave circuits for antenna feeders, installed on board of satellites. Among these circuits, beam forming networks (BFN) are widely used.

[0005] Said networks subdivide the incoming power at the input terminal among various output terminals, each of them is to supply a suitable phase-and power signal to a particular feeder. Said networks consist of a great number of directional couplers, phase-shifters, waveguide trunks and right-angle junctions, and connect the input terminal to multibeam or reconfigurable beam antenna feeders. It is clear that in these cases antenna weight and encumbrance must be reduced as much as possible, not to increase launching costs.

[0006] The right-angle junction which results less cumbersome is obtained by the simple coupling of two rectilinear rectangular waveguides, yet the obtained reflection coefficient is very high. An improvement can be reached by replacing the outer corner by a 45° wall, or even better, by a plurality of duly-dimensioned and inclined walls. In the latter case reflection coefficient values of about -30 dB can be attained, after having experimentally optimized the wall characteristics. In fact an accurate equivalent circuit of such devices is unknown, owing to the difficulties of describing electromagnetic field in a structure with walls slanting with each other in a different way. Hence, it is not easy to go on with their design solely in a theoretical way, even though computer-aided. On the contrary, it is necessary to build up a prototype, to measure its parameters and modify its geometric characteristics until, by a method of successive approximations, a satisfactory result is reached. The whole process is then to be repeated to modify the operative bandwidth or the waveguide dimensions.

[0007] It is clear that not only the computer analysis of the single junction, but also of the whole beam forming network is rendered impossible prior to its practical realization, and this is then cause of considerable economic outlay.

[0008] Besides, owing to difficulties depending on experimental junction optimization, this type of solution is feasible only for frequencies inferior to 18 GHz.

[0009] Said disadvantages are overcome by the right-angle junction for rectangular waveguides, provided by the present invention, which presents minimum encumbrance and a very low reflection coefficient over a very wide frequency-band. Besides an accurate equivalent circuit of the junction is available, which allows the designer to obtain the global electrical parameters of the junction, and hence of more complex circuits wherein one or more junctions are used, by using automated computing means.

[0010] The present invention provides a right-angle junction for rectangular waveguides as described in the characterizing portion of claim 1.

[0011] These and other characteristics of the present invention will be made clearer by the following description of a preferred embodiment thereof, given by way of non-­limiting example, and by the annexed drawing, in which:

- Fig.1 is a longitudinal cross sectional view with side projection of a first embodiment of the right-angle junction;

- Fig.2 is a longitudinal cross sectional view of a second embodiment of the right-­angle junction.

[0012] The right-angle junction shown in Fig.1 is obtained by the 90° coupling of two rectilinear rectangular waveguides of identical cross-sectional dimensions a and b, wherein b denotes the smaller side.

[0013] By considering the volume comprised between the inner side walls of the guides and cross-sectional planes c-d and c-f, a square cross-section cavity of side b and height a is delimited, cross-sectional planes c-d and c-f coinciding with the planes containing the wider guide walls. Maintaining the square shape of the cavity, side b is reduced to b1 by introducing in a symmetrical way two steps of thickness S1, that is why S1 =b-b1, and of dimensions equal to a and b. By a suitable choice of thickness S1, and hence of the cavity dimensions, the junction reflection coefficient can be minimized over a certain frequency band.

[0014] A second embodiment of the junction, allowing low values of the reflection coefficient to be obtained over a wider band, is shown in Fig.2.

[0015] Two more steps of thickness S2, less than S1, and of sizes equal to a and m have been introduced into this junction, wherein m in first approximation is equal to a guide quarter-wave at centre frequency.

[0016] Let us see now how the dimensions of steps can be determined to obtain the minimum reflection coefficient. Analyzing by the well known "mode matching" method the right-angle junction with two steps, it can be observed that this structure presents a resonance frequency at which the reflection coefficient is minimum. Since this resonance frequency depends on step thickness S1, it can be made to coincide with the centre frequency by a suitable value of S1, thus obtaining the minimum of the reflection coefficient at the desired frequencies.

[0017] By introducing further steps in a symmetrical way, as in the second example of embodiment, low values of the reflection coefficient can be obtained over wider bands, since they act as quarter-wave impedance matching devices, whose design is already known.

[0018] Further symmetrical pairs of steps with thickness less than S2 can be added, thus widening the operative band of the junction.

[0019] It is evident that what described has been given only by way of non-limiting example. Variations and modifications are possible without going out of the scope of the claims.

Claims

1. A right-angle junction for rectangular waveguides, obtained by the 90° coupling of two rectangular rectilinear waveguides, of equal cross-sectional dimensions (a, b; a>b), characterized in that it comprises two steps placed symmetrically with respect to the couping plane between the two guides.

2. A junction as in claim 1, characterized in that said steps have a first and a second dimensions equal to the cross-sectional dimensions (a, b) of said guides and a first thickness (S1) such that the junction reflection coefficient is rendered minimum over the operative frequency band.

3. A junction as in claim 1, characterized in that it comprises one or more symmetrical pairs of steps, with thickness (S2) decreasing with respect to the first thickness (S1) and length (m) equal to about a guide quarter-wave at the centre frequency.

Drawing