Multi-mode cavity for waveguide filters, including an elliptical waveguide segment

Abstract: The cavity (1) comprises at least one waveguide segment with elliptical cross section whose axes are arranged at a given inclination angle (α) with respect to the polarisation of the incident TE field. Thus a dual-mode cavity is realised, with the ability to let resonate two transverse fields (TE) with polarisation planes orthogonal to each other. By adding a waveguide element (CR2, CR3) able to introduce a non-axial discontinuity, a triple-mode cavity is obtained, allowing for an additional longitudinal mode to resonate as well.

Description: The invention described herein relates to a multimode cavity, free from coupling and tuning screws, for wave-guide filters, the cavity comprising at least one wave-guide segment having a longitudinal main axis; and one longish iris at a first end of the cavity, having an iris axis parallel to a reference plane passing through the longitudinal main axis of said wave-guide segment, said iris having a predetermined displacement different from zero with respect to said longitudinal main axis. A cavity with the displacement being different from zero is known from Beyer R et al: "Field-Theory Design of Circular Waveguide Dual-Mode Filter by a Combined Mode-Matching Finite Element Method", 24th European Microwave Conference Proceedings, Cannes, Sept. 5 - 8, 1994, Vol. 1, No. Conf. 24, 5th September 1994, European Microwave Management Committee, pages 294 - 303, XP000643177. In the prior art, this cavity is a single cavity of a dual-mode filter, with asymmetrically located irises. A dual-mode cavity is also described, for example, in EP-A-0 687 027. That previous document can usefully serve as a reference to illustrate the general problems inherent to manufacturing such cavities, particularly with regard to the possibility of making waveguide filters suitable for being completely designed through computer aided design techniques, with no need for specific calibration operations like the ones required by conventional cavities fitted with tuning and coupling screws. In particular, EP-A-0 687 027 discloses a cavity comprising three coaxial waveguide segments arranged in cascade along the main axis of the cavity. The two end segments (with circular, square or rectangular cross section) allow for two modes to resonate, which modes have linear polarisation parallel and respectively perpendicular to a reference plane essentially identified by the diameter plane parallel to the major dimension of the iris used to couple the modes into the cavity. The intermediate segment consists of a waveguide with rectangular cross section whose sides are inclined by a given angle with respect to the aforesaid reference plane. Such a cavity can be included in a microwave band-pass filter to be used, for instance, in satellite communications. A dual-mode cavity without tuning and coupling screws is disclosed in JP-A-60 174501. Elimination of the screws is made possible by the cavity having a rectangular cross section bevelled in correspondence with a corner, or a similarly deformed elliptical cross section. The structure is apparently simpler than that disclosed in EP-A-0 687 027, yet the cross-sectional deformation with respect to an exactly rectangular or elliptical shape results in very great numerical difficulties in analytically modelling the behaviour of the cavity itself. Thus it is very difficult to obtain the required accuracy in the design of the cavity and hence, once the cavity is manufactured, its operation will not be satisfactory. Arranging a cavity inclined with respect to a reference plane is also known in the art. Examples are disclosed in US-A 3,235,822 (De Loach) and US-A 4,513,264 (Dorey et al.). Both documents disclose a filter comprising a plurality of cavities each made by a single rectangular waveguide segment, where the waveguide segments may be inclined with respect to one another. In detail: In US-A-3,235,822 inclination is used to vary the amount of coupling between two adjacent cavities between a maximum and a minimum value. The cavities are strictly single-mode cavities. Increasing the shorter dimension of the rectangular cross section so as to give a nearly-square cross section (as it would be required for dual-mode operation) would result in a loss of control over the transmission characteristics of the filter, making it impossible to obtain useful electrical responses from the filter. Moreover, for very narrow bandwidths, such as the ones the present invention is concerned with, tuning screws are to be provided. In US-A-4,513,264, a dual-mode filter is described which comprises three cavities each comprising a respective iris or tuning and coupling screws. One of the cavities could be ellipsoid. The first cavity is aligned with the input field and inchnation of the second cavity is used to generate diagonal couplings between adjacent cavities. Coupling between the two modes and tuning is obtained by the screws. The purpose of the present invention is to provide a multi-mode cavity which: allows for three electromagnetic modes to resonate (with the consequent possibility of using the same cavity several times in making filters, thus reducing the number of geometrical shapes involved); does not require coupling and tuning screws and can be easily and very precisely designed and manufactured with computer aided design techniques. This purpose is reached thanks to a cavity which, starting out from a cavity as defined at the beginning, is characterized in that said wave-guide segment has elliptical cross section; and said iris is arranged so that the axes of said elliptical cross section are inclined by a given angle, different from zero, with respect to said reference plane, whereby three resonant modes, orthogonal to each other, resonate in the cavity. In the invention, the offset and inclination of the iris with reference to the cavity is one of the features allowing generation and control of coupling between different modes within the cavity without need of using coupling and tuning screws, and without destroying the possibility of operation of the filter which would take place in case of cancelling the coupling between the modes, then making it impossible for the energy to propagate towards the output. The invention shall now be described, purely by way of non limiting example, referring to the enclosed drawings, wherein: Figure 1 is a perspective view of a prior art cavity according to EP-A-0 687 027, Figure 2 is a perspective view of a cavity according to the invention. Figure 3 shows a modified cavity according to the invention. The formalism adopted to represent the cavity, indicated as a whole by 1, is wholly similar to that adopted in EP-A-0 687 027. As is evident to the technician skilled in the art, such a representation shows the geometry of the volume of the cavity itself, which usually is manufactured within a body of conducting, typically metallic, material, with working processes such as turning, electrical discharge machining, etc. The related manufacture criteria are widely known to the technicians skilled in the art and do not require to be illustrated specifically herein, especially since they are not in themselves relevant for the purpose of understanding the invention. It will also be appreciated that, for the sake of clarity, the cavity has been represented in the perspective views by enhancing its extension along the main longitudinal axis (axis Z) with respect to the actual constructive embodiment: differently stated, in practice, the cavity will usually be longitudinally "squashed" with respect to the shape shown. It should in any case be specified that the lengths of the individual sections of the cavity constitute design parameters for the cavity itself, as is well known. Figure 1 depicts a dual-mode cavity for making microwave band-pass filters, like that disclosed in EP-A-0 687 027. In short, that cavity comprises three coaxial waveguide segments arranged in cascade along the main cavity axis Z. Specifically, there is a first waveguide segment CC1 with circular cross section followed by a second waveguide segment CR1 with rectangular cross section and then by a third waveguide segment CC2, again with circular cross section. Reference IR1 indicates an iris allowing coupling of the modes into cavity 1, and reference IR2 indicates an iris arranged so as to couple multiple modes simultaneously (for instance a cross-shaped iris), located at the opposite end of cavity 1. Iris IR2 allows coupling cavity 1 with a cavity (identical or different, not shown), arranged in cascade, to make a microwave filter. The presence of waveguide segment CR1 with rectangular cross section, the sides of which are inclined by a given angle with respect to a reference plane which passes through axis Z and is parallel to the major dimension of iris IR1 and of the horizontal element of iris IR2, makes the cavity shown in Figure 1 able to allow for two electromagnetic modes to resonate: such modes are transverse with respect to axis Z and have polarisation planes respectively parallel and orthogonal with respect to the aforesaid reference plane. The non-homogeneous cross-sectional shape of the cavity along axis Z (and the resulting discontinuity) allows tuning and coupling screws to be dispensed with. For a more precise description of the manufacturing criteria of this known cavity, particularly in regard to the possibility of replacing circular segments CC1 and CC2 with segments having square or rectangular cross sections, reference can be made to the specification of EP-A-0 687 027. The solution according to the present invention is based on the ascertainment of the fact that a triple-mode operation with some similarity to the dual-mode operation attained in the prior art solution depicted in Figure 1 can be obtained with the cavity having the structure shown in Figure 2. That cavity, still denoted by reference numeral 1, comprises a waveguide segment with elliptical cross section, with semiaxes a, b arranged at an angle with respect to a reference plane denoted π, which is identified by the trace of its intersection with the plane of the sheet. Cavity 1 can be coupled, for example through iris IR2, with another cavity 2, also with elliptical cross section (whose profile is sketched in dashed lines in Figure 2), with a different inclination angle α from that of cavity 1. Thus, a microwave filter comprising multiple resonant cavities coupled with each other can be made according to criteria known in themselves. Applicant's experiments have demonstrated that the coupling and tuning of two resonant modes, i.e. the TE resonant modes of the cavity, orthogonal to each other, can be defined with a high degree of precision in the course of the design (typically by using a computer) and then directly obtained during manufacturing, without need for adjustments, by controlling the value of the inclination angle (α), the ratio between semiaxes a and b ("aspect ratio") and the length of the waveguide segment with elliptical cross-section. The invention illustrated in Figure 2 is further developed to give rise to a triple-mode cavity, i.e. a cavity with the ability to make resonate, in addition to the two TE modes mentioned previously, also a third mode, i.e. a TM mode with electrical field polarisation directed along the main axis Z of cavity 1 and orthogonal to the previous ones. This result is obtained by the waveguide element that introduces a non-axial discontinuity being the iris IR1 which is arranged eccentrically (i.e. dissymmetrically or off-axis) with respect to axis Z, that is to say (as can be seen in the drawing) in such a way that the intersection point of the diagonals of the iris is displaced by a predetermined amount a off with respect to the main axis of the elliptical cavity. In a further embodiment of the triple-mode cavity according to the invention, shown in Figure 3, this is obtained by providing, at one or both ends of the elliptical waveguide segment 1 like the one constituting cavity 1 shown in Figure 2, a rectangular waveguide segment CR2 and CR3 (the term "rectangular" also includes, as a particular case, a square cross section) arranged eccentrically (i.e. dissynunetrically or off-axis) with respect to axis Z. By way of example, Figure 4 shows the case of both waveguide segments CR2, CR3 with rectangular cross section located at the two ends of the elliptical waveguide segment 1. Segment CR2 is arranged in such a way that at least one of the ideal median planes dividing in half the sides of the cross section of the segment CR2 is spaced apart by the predetermined offset amount (a off ) from main axis Z of the cavity, and in particular from reference plane π. Should the application make it advisable, one of the rectangular segments might be arranged along the body of cavity 1, in an intermediate position between two elliptical segments. The or each rectangular waveguide segment can be oriented so that its sides are respectively parallel and perpendicular to reference plane π. It is also possible to load the cavity with a dielectric element in order to reduce the resonance frequency or the volume of the cavity. In any case, coupling the orthogonal modes by means of a waveguide segment with elliptical cross section allows easy modelling and mechanical manufacturing of the cavity and of the related filter. In particular, very accurate computation algorithms exist to analyse the cavity elements described herein as a function of the related parameters (aspect ratio a/b, inclination angle α, etc.). Thus it is possible to use algorithms to obtain the complete design of the dimensions of the cavity, with no further need for tuning the device thus manufactured.

Claims: Multimode resonant cavity free from coupling and tuning screws, for wave-guide filters, the cavity comprising at least one wave-guide segment (1) having a longitudinal main axis (Z); and one longish iris (IR1) at a first end of the cavity, having an iris axis parallel to a reference plane (π) passing through the longitudinal main axis (Z) of said wave-guide segment (1), said iris (IR1) having a predetermined displacement (a off ) different from zero with respect to said longitudinal main axis (Z); characterised in that said wave-guide segment (1) has elliptical cross section; and said iris (IR1) is arranged so that the axes of said elliptical cross section are inclined by a given angle (α), different from zero, with respect to said reference plane (π), whereby three resonant modes, orthogonal to each other, resonate in the cavity. Cavity as per claim 1, characterised by at least a further wave-guide segment (CR2, CR3) arranged eccentrically with respect to said longitudinal main axis (Z), having a rectangular cross section and arranged so that its sides are respectively parallel and orthogonal with respect to said reference plane (π). Cavity as per claim 2, characterised in that said further wave-guide segment (CR2, CR3) arranged eccentrically is located at least at one end of said wave-guide segment (1) with elliptical cross-section. Cavity as per claim 2, characterised in that said further wave-guide segment arranged eccentrically is located in an intermediate position between wave-guide segments with elliptical cross section.

Inventors: ACCATINO LUCIANO, BERTIN GIORGIO, ACCATINO, LUCIANO, BERTIN, GIORGIO
Applicants: TELECOM ITALIA SPA, TELECOM ITALIA S.P.A.
Application year: 1997
Main classification: H01P1
IPC codes: H01P1, H01P7