WAVEGUIDE COUPLING ARRANGEMENT

Abstract: A capacitively coupled printed patch (3) as a high efficiency device to couple orthogonally polarised energy between a stripline (5) and a waveguide (1). Coupling between the stripline (5) and the patch (3) is achieved by the stripline terminating in a narrow strip probe (4), the end of which lies close to, but not in contact with, an edge of the patch (3). Two separate probes (4) arranged mutually orthogonally are used to effect independent polarised couplings to produce independent linear orthogonal signals or independent left- and right-handed circularly polarised signals. The striplines (5) and patch (3) are supported on a common substrate (8) which extends transversely through the waveguide (1). The waveguide wall has a quarter-wavelength thickness (T) so that its inner edge (10) appears continuous to energy passing through the substrate (8). One application is in a DBS satellite TV receiving system where it is required to isolate two signals sharing a common channel but having orthogonal polarisations.

Description: This invention relates to a coupling arrangement and, in particular, to an arrangement for coupling energy between each of two transmission lines and a waveguide. Coupling of energy between a transmission line and a waveguide is usually achieved by the use of one or more wire probes or loops inserted into the waveguide cavity through the wall of the waveguide, the probes lying transverse to its axis. In the case of a waveguide accommodating circular polarisation, or, alternatively, two independent orthogonal polarisations, two such probes are required which must be mutually orthogonal within the cavity and spaced a half-wavelength apart (in the direction of the axis) if high isolation and a good return loss are to be achieved. The first probe would generally be spaced a quarter-wavelength from the short-circuit end of the waveguide. Such an arrangement has two disadvantages: firstly, the probes do not have the same frequency performance, the probe further from the short-circuit having a reduced bandwidth; and, secondly, the probes are not co-planar and hence are not suitable for direct connection to a single microstripline circuit board. Isolation between the two orthogonal polarisations is improved if the structure is deliberately detuned by moving the first probe closer to the short-circuit end of the waveguide. However, in the dual probe structure such detuning results in a seriously worsened return loss because the probes are no longer tuned to the cavity. It is an object of the present invention to provide a waveguide coupling arrangement in which both high isolation and good return loss can be achieved simultaneously for orthogonal polarisations. A waveguide coupling arrangement according to the preamble of claim 1 is known from patent document EP-A2-0071 069. The object of the invention is accomplished by the characterising features of claim 1. Further preferred embodiments of the invention are claimed in claims 2 to 8. A coupling arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1(a) shows an end view and Figure 1(b) a sectioned side view of a waveguide coupling arrangement; Figure 2 shows a 90° hybrid network for use in generating a circular polarisation of either hand in the arrangement of Figure 1; and Figure 3 shows an alternative feed network for generating one hand of circular polarisation. Referring to the drawings, Figures 1(a) and 1(b) show a standard waveguide structure in the form of a conductive tube 1 of circular section having a resonant cavity 2. A conductive patch 3, such as is commonly used in microwave antennas, is supported within the cavity 2, transverse to the axis of the waveguide 1 by a dielectric substrate 8. Two stripline sections 5 are printed on the substrate 8. Each stripline section 5 is reduced in width at one end to a narrow conductive strip probe 4, the end of the probe lying adjacent to, but not in electrical contact with, an edge of the patch 3. The two strip probes 4 and their associated stripline sections 5 lie mutually orthogonal, both co-planar with the patch 3. The substrate 8 extends through the whole circumference of the waveguide wall, i.e. it is sandwiched between two sections of the conductive tube 1. The stripline sections 5 are isolated from the tube 1 by relieving the adjacent end face of the tube locally, as indicated by reference numeral 6 on Figure 1. Alternatively, an insulating washer may be sandwiched between the end face of the tube 1 and the side of the substrate 8 bearing the stripline sections 5. The substrate 8 has a conductive ground plane 7 on the side opposite the striplines 5. The ground plane 7 is in contact with the waveguide wall, but does not extend within the cavity 2. Although in Figure 1 the ground plane 7 is shown on the face of the substrate 8 closest to the short-circuit end 11 of the waveguide tube 1, it will be appreciated that the ground plane 7 may equally be provided on the opposing face of the substrate 8, the patch 3 and the stripline sections 5 then being formed on the face nearest the short-circuit 11. The substrate 8 provides a convenient printed circuit board for mounting circuitry associated with the waveguide. For this reason, the substrate 8 and its ground plane 7 may extend substantially beyond the periphery of the waveguide. The wall thickness T of the waveguide tube 1 is made a quarter-wavelength at the operative (i.e. tuned) frequency. At the discontinuity due to the substrate 8 the outer edge 9 of the tube 1 constitutes an open-circuit (or at least a very high impedance) to energy travelling through the substrate 8. By making T a quarter-wavelength this open circuit is transformed to an effective short-circuit at the inner edge 10 of the tube 1. Thus, at the tuned frequency, the inner edge 10 of the waveguide wall will appear continuous to signal energy, and the wall provides a choke that effectively enables the substrate to interrupt the waveguide wall without detriment to the waveguide function. The gap between the end of the strip probe 4 and the edge of the patch 3 provides capacitive coupling of signal energy from the stripline section 5 to the patch 3. The stripline sections 5, with their associated strip probes 4, are capable of separately coupling signals to the waveguide to produce independent orthogonal polarisations with a high degree of isolation. If two such independent signals are to be accommodated within the waveguide, each stripline section 5 will require its own transmission line (not shown), which may be a continuous extension of the stripline section 5 in the form of a printed track on the substrate 8. Alternatively, the transmission lines may comprise coaxial cables, in which case a connector is required at the transition from the stripline to the cable. The connector can be mounted as close to the waveguide as desired, provided the outer screen of the cable does not bridge the relieved portion 6 of the waveguide tube. The outer screen of the cable is connected to the ground plane 7 on the substrate 8. The use of the conductive patch 3 as the coupling element ensures low loss and high isolation between the two polarisations. Loss is minimised because the energy propagating along the strip probes 4, once inside the waveguide, is mainly in air, i.e. no longer trapped between the stripline and the ground plane. This means that most of the losses occur in the striplines 5 which feed the strip probes 4. The substrate 8 within the waveguide serves only to support the patch 3 and the striplines 5 and so should be as thin as practical to minimise losses further. The substrate 8 is positioned a distance L (say, one-eighth of a wavelength) from the short-circuit end 11 of the waveguide 1 to deliberately detune the structure (Figure 1(b)). This detuning improves isolation between the orthogonal polarisations. The incorporation of the patch 3 between the strip probes 4 maintains good return loss even when the cavity is detuned; hence both high isolation and good return loss can be achieved simultaneously. Other orthogonal polarisations, such as circular polarisation, can be generated within the waveguide using the structure shown in Figure 1. To achieve a circular polarisation, the signals applied at the strip probes 4 must have a quadrature phase difference in addition to their orthogonality in space. Such a phase difference can be achieved in a number of ways. Figure 2 shows in outline one method of achieving circular polarisation by using a 90° hybrid network 12 between the stripline sections 5 and a single transmission line (not shown), which may be connected to a point B or a point C. The hybrid network consists of a simple arrangement of signal paths, which may be conductive tracks formed on the same substrate 8 as carries the patch 3, but external to the waveguide. A signal applied to point B or point C by the transmission line reaches the strip probes 4 via two separate paths of different length. The difference in the path lengths is such that a 90° phase difference occurs between the signals coupled to the patch 3 by the two strip probes 4. The hand of the circular polarisation generated is dependent upon whether the signal is applied to point B or point C. An alternative method of generating a circular polarisation of one hand only is illustrated in Figure 3. Here a single microstrip transmission line 13 is divided into the two striplines 5, which have different lengths to produce the required phase conditions. The hand of the circular polarisation is determined by the stripline which provides the longer signal path. Although the above description of embodiments has generally referred to applications in which the waveguide is used as a radiating element fed by one or two transmission lines, the coupling arrangements are equally suited to configurations for receiving polarised signals. One such application is in a DBS satellite TV receiving system where two broadcast signals sharing a common frequency channel may be isolated by virtue of their having independent orthogonal polarisations. The choice of programme may then be made without adjustment to the antenna by switching the transmission line carrying the desired signal to the receiver input.

Claims: A waveguide coupling arrangement for coupling energy between each of two transmission lines (5) and a waveguide (1), the transmission lines (5) being carried on a substrate (8) disposed normally with respect to the waveguide axis and spaced from a short-circuit end (11) of the waveguide (1), said two transmission lines (5) comprising mutually orthogonal strip conductors (5) which extend transversely through the wall of the waveguide (1), the arrangement being characterised by a conductive patch (3) disposed on said substrate (8) so that said strip conductors (5) terminate at respective positions adjacent to, but not in contact with, said patch (3) for coupling respective orthogonal plane-polarised signals. An arrangement according to Claim 1, characterised in that each said strip conductor (5) terminates in a section of reduced width (4). An arrangement according to Claim 1 or Claim 2, characterised in that said strip conductors (5) are connected at a junction with a common transmission line (13), and by means (12) for introducing a quadrature phase difference between said signals so as to couple a circularly-polarised signal between the waveguide (1) and said common transmission line (13). An arrangement according to Claim 3, characteristed in that said quadrature phase difference is achieved by said strip conductors (5) having different length. An arrangement according to Claim 3, characterised in that said means for introducing a quadrature phase difference comprises a hybrid network (12) incorporated at said junction. An arrangement according to Claim 5, characterised in that said hybrid network (12) is carried on said substrate (8). An arrangement according to any one of the preceding claims, characterised in that said strip conductors (5) and said conductive patch (3) are formed in stripline on a microstripline circuit board. An arrangement according to Claim 7, wherein said waveguide (1) comprises two sections sandwiching said circuit board, the arrangement being characterised in that the waveguide wall (1) has a thickness equivalent to a quarter wavelength at the operative frequency of the arrangement so as to permit said circuit board to interrupt said wall without detriment to the waveguide function.

Inventors: HOWARD, KEVIN RICHARD
Applicants: GEC-MARCONI LIMITED
Application year: 1989
Main classification: H01P1
IPC codes: H01P1, H01P5