Multi-mode cavity for waveguide filters

Abstract: A multi-mode, typically triple-mode, cavity (1), for waveguide band-pass filters is disclosed, which cavity does neither require tuning or coupling screws nor rounded edges. The cavity comprises a waveguide element such as an iris (IR1) or a waveguide segment (CR2) arranged in a generically eccentric position with respect to the general development of the cavity and in particular to the main axis (Z) of the same. The cavity may be used to make narrowband filters for satellite communications. A filter comprising cavities (1) of the type described can be designed entirely using an electronic computer and requires no calibration procedure.

Description: The invention described herein relates to a multimode cavity with the characteristics stated in the preamble of Claim 1. A dual-mode cavity with such characteristics is described, for example, in EP-A-0 687 027 in the name of the same Applicant. 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, the solution described in EP-A-0 687 027 comprises three coaxial waveguide segments arranged in cascade along the main longitudinal 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 diametral 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. US-A 3,235,822 (De Loach) and US-A 4,513,264 (= CA-A 1153432) (Dorey et al.) disclose filters 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 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 inclination of the second cavity with respect to the first one is used to generate diagonal couplings between adjacent cavities. Coupling between the two modes and tuning is obtained by screws. Elimination of the screws in the filter according to US-A-4,513,264 would destroy any possibility of operation of the filter since it would cancel coupling between the modes, thus making impossible for the energy to propagate towards the output. None of the above documents disclose a cavity having a non homogenous cross section along its axis, this being the feature allowing tuning and coupling screws to be dispensed with in the above mentioned EP-A-0 687 027. A dual-mode cavity without tuning and coupling screws is also 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 comer, or a similarly deformed elliptical cross section. The cavity has homogeneous cross section throughout its length. 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. The purpose of the present invention is further to develop the solution according to EP-A-0 687 027, in particular with regard to the possibility of making a cavity allowing for three electromagnetic modes to resonate (so-called "triple-mode" cavity), i. e. two resonant transverse modes and an additional resonant longitudinal mode: this gives the possibility of using the same cavity several times in making filters, with obvious benefits stemming from the reduction of the overall number of cavities and therefore of the overall size of the filter, while obviously maintaining the advantages concerning the complete designing by CAD techniques. According to the present invention, a multi-mode cavity for waveguide filters is provided as defined in claim 1. 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 cavity according to the invention, Figure 2 is an ideal cross-sectional view taken along line II-II in Figure 1, Figure 3 is a schematic representation, from a viewpoint essentially similar to that of Figure 2, of a possible variant embodiment of the cavity shown in Figure 1. Figure 1 is an ideal perspective view of a cavity included in a microwave band-pass filter for use, for instance, in satellite communications. 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, cavity 1 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. In the exemplary embodiment depicted in Figure 1, cavity 1 comprises four waveguide segments arranged in cascade along main axis Z. The first three waveguide segments (starting from the left in Figure 1) correspond essentially to the three waveguide segments forming the cavity illustrated in EP-A-0 687 027. They include: a first waveguide segment CC1 with circular cross section, a second waveguide segment CR1 with rectangular cross section, and a third waveguide segment CC2 again with circular cross section. The fourth waveguide segment CR2 is another segment with rectangular cross section and is arranged in cascade with the segments previously described. IR1 indicates an iris provided at the input end of the first waveguide segment CC1. Iris IR1, whose task is to allow coupling of the modes into the cavity, is diametrically arranged with respect to the cross section of waveguide segment CC1. Its major dimension defines, with main axis Z of cavity 1, a reference plane with respect to which the sides of segment CR1 are inclined by an angle β. The criteria and the purposes of this arrangement are described in greater detail in the above mentioned EP-A-0 687 027. Said reference plane, indicated by π, is identified in Figures 2 and 3 by its intersection trace with the plane of the sheet. IR2 indicates an iris for coupling multiple modes simultaneously, for instance a cross-shaped iris, whose horizontal element is parallel to IR1. Iris IR2 allows coupling with an additional cavity 1' arranged in cascade with cavity 1. The possible cascaded arrangement of multiple cavities such as cavity 1 described in detail herein (whether identical to or differing from each other) allows obtaining microwave filters with the desired transfer functions: here too the manufacturing criteria are well known by the technician skilled in the art and need not be described specifically in this document. As can be better appreciated by the cross-sectional view in Figure 2, the characteristic of the second rectangular waveguide segment CR2 is its generally eccentric (i.e., dissymmetric or off-axis) arrangement with respect to main axis Z of cavity 1 and in particular with respect to reference plane π. The amount of eccentricity (or dissymmetry or spacing from the axis) defines an "offset" a off . In particular, in Figure 2, offset a off corresponds to the distance between the main diametral plane of the cross section of waveguide segment CC2 (thus plane π) and the ideal section plane which divides in half the minor sides, of length a, of rectangular waveguide segment CR2. The sides of rectangular waveguide segment CR2 have lengths a, b which usually, but not necessarily, differ from each other. Therefore, for the purpose of defining the scope of the invention, the term "rectangular" must be taken to include the square shape, seen as a particular case of the rectangular shape. The same applies for segment CR1. The Applicant's experiments have demonstrated that, thanks to the presence of the additional rectangular waveguide segment CR2, which defines a wave-guide element introducing a non-axial discontinuity, cavity 1 depicted in Figure 1 is able to make resonate a TM longitudinal mode, with polarisation of the electrical field directed along longitudinal axis Z of cavity 1, in addition to two transverse TE modes with polarisations respectively parallel and orthogonal to reference plane π; thus cavity 1 behaves as a triple-mode cavity. By operating on the amount of offset a off and on lengths a and b of the sides of rectangular waveguide segment CR2 (in particular on the ratio between the same, the so called "aspect ratio") it is possible independently to control the resonance frequencies of the resonant modes and the degree of coupling, so as to attain the required operating characteristics. The embodiment depicted in Figure 1 constitutes only one amongst several possible embodiments of the invention. For example, segment CR2 may be placed along the body of the cavity, instead of constituting an end segment. The end segment can then be an additional segment with circular cross section similar to CC1 and CC2. Figure 3 shows how one or both waveguide segments CC1, CC2 with circular cross section could be replaced by waveguide segments with square or rectangular cross section, while maintaining the eccentric location of rectangular segment CR2. It is also possible to merge rectangular segments CR1, CR2 into a single rectangular segment which is at the same time tilted with respect to reference plane π and eccentric with respect to the main axis of the cavity. This solution however gives rise to some analytical difficulties in the design phase. In addition, the eccentricity of segment CR2, which here is represented as an offset a off with respect to the diametral plane (defined by iris IR1) of the circular waveguide segments, could be an offset in two directions: that is, with reference to Figure 2, CR2 would exhibit not only offset a off , but also a corresponding offset, of identical or different amount, of the ideal median plane which divides in half the major sides b. It should also be noted that, if the rectangular cross sections of segments CR1 and CR2 are larger, at least locally, than those which can be inscribed in the respective reference cross sections (circular, square, rectangular or elliptical) of the other segments in the cavity, such rectangular cross sections can be replaced by rectangular cross sections with comer portions adapted to the contour of the reference sections. In addition, according to a variant not specifically illustrated here, eccentric waveguide segment CR2 can have circular or even elliptical cross section. The elliptical cross section could also be adopted for segment CR1.

Claims: Multi-mode cavity for waveguide filters, free from coupling and tuning screws, comprising at least three waveguide segments (CC1, CR1, CC2) directly arranged in cascade along a common longitudinal main axis (Z) and having different cross-sections, wherein two resonant modes having polarisation planes orthogonal to each other and transverse with respect to the longitudinal main axis (Z) of the cavity resonate, characterised in that it comprises at least one further waveguide segment (CR2) arranged in cascade to said at least three wave-guide segments (CC1, CR1, CC2) in an eccentric position (a off ) with respect to said longitudinal main axis (Z) of the cavity (1), so as to introduce into the cavity (1) itself a non-axial discontinuity, which determines that a further resonant mode, having a polarisation plane along the longitudinal main axis (Z) and constituting a third resonant mode, resonates in the cavity (1). Multi-mode cavity for waveguide filters, free from coupling and tuning screws, comprising at least three waveguide segments (CC1, CR1, CC2) directly arranged in cascade along a longitudinal main axis (Z) common to at least two of the three waveguide segments and having different cross-sections, wherein two resonant modes having polarisation planes orthogonal to each other and transverse with respect to the longitudinal main axis (Z) of the cavity resonate, characterised in that one of said three waveguide segments is arranged in an eccentric position (a off ) with respect to said longitudinal main axis (Z) of the cavity (1), so as to introduce into the cavity (1) itself a non-axial discontinuity, which determines that a further resonant mode, having a polarisation plane along the longitudinal main axis (Z) and constituting a third resonant mode, resonates in the cavity (1). Cavity as per claim 1 or 2, characterised in that said further waveguide segment (CR2) arranged in eccentric position has rectangular cross section, with sides (a, b) respectively parallel and orthogonal with respect to a reference plane (π) defined by said longitudinal main axis (Z) of the cavity and by a major dimension of an iris (IR1) coupling the modes into the cavity (1). Cavity as per any of claims 1 to 3, characterised in that said further wave-guide segment (CR2) arranged in eccentric position has rectangular cross section with both its pairs of sides (a, b) offset with respect to said longitudinal main axis (Z) of the cavity (1). Cavity as per claim 1, characterised in that said further waveguide segment arranged in eccentric position has circular or 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: H04B7, H01P1, H01P7