Waveguide to Microstripline Polarization Converter Having a Coupling Patch

1. Field of the Invention

This invention relates to each of two coupling arrangements and, in particular, to arrangements for coupling energy between a transmission line and a waveguide.

2. Description of Related Art

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 polarization, 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 microstrip 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 structure in which both high isolation and good return loss can be achieved simultaneously for orthogonal polarisations.

According to the invention an arrangement for coupling energy between each of two transmission line and a waveguide comprises a conductive patch supported within and normal to the axis of the waveguide, with each transmission line extending transversely through the wall of the waveguide to positions providing coupling between each transmission line and the patch.

Each transmission line preferably extends to a position adjacent to, but not in contact with, the patch.

Each transmission line preferably comprises a microstripline section co-planar with the patch, the end portion of the microstripline section adjacent to the patch having reduced width.

Each transmission line may be one of two similarly arranged with respect to the patch, the two microstripline sections being disposed mutually orthogonally so as to accommodate within the waveguide mutually orthogonal plane polarized signals.

In one embodiment of the invention the transmission line comprises two microstripline branch sections extending from a junction toward the patch from orthogonal directions, means being provided to introduce a quadrature phase difference between signals carried by the branch sections, and thus accommodate a circularly polarized signal within the waveguide.

The means for introducing a quadrature phase difference may be constituted by the branch sections having different lengths.

Alternatively, the means for introducing a quadrature phase difference may be constituted by a hybrid network incorporated at the junction of the branch sections.

The hybrid network may be printed on a common substrate with the branch sections and the patch, the network lying external to the waveguide.

The hybrid network preferably has two first ports connected to the branch sections respectively, and two second ports connected to respective transmission lines.

The patch and the or each microstripline section may be supported on a substrate extending through the waveguide wall.

The wall thickness is preferably a quarter-wavelength at the operative frequency of the waveguide, so as to permit the substrate and the or each microstripline section to extend through the wall without detriment to the function of the waveguide.

A coupling arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG. 1(a) shows an end view and FIG. 1(b) a sectioned side view taken on line of a waveguide coupling arrangement;

FIG. 2 shows a 90.degree. hybrid network for use in the arrangement of FIG. 1 for coupling a circularly polarized signal; and

FIG. 3 shows an alternative feed network for one-coupling a circularly polarized signal.

Referring to the drawings, FIGS. 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, is supported within the cavity 2, transverse to the axis of the waveguide 1 by a dielectric substrate 8. Two microstripline sections 5 are printed on the substrate 8. Each microstripline 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 microstripline 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. Each microstripline section 5 is isolated from the tube 1 by relieving the end face of the tube locally to form a channel 6 in the tube wall through which the microstripline section 5 extends without contacting said wall. Alternatively, an insulating washer may be sandwiched between the end face of the tube 1 and the side of the substrate 8 bearing the microstripline sections 5. The substrate 8 has a conductive around plane 7 on the side opposite the microstripline sections 5. The ground plane 7 is in contact with the waveguide wall, but does not extend within the cavity 2. Although in FIG. 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 microstripline 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 microstripline section 5 to the patch 3. The microstripline 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 microstripline section 5 will require its own transmission line (not shown), which may be a continuous extension of the microstripline 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 microstripline 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 channel 6. 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 microstripline and the ground plane. This means that most of the losses occur in the microstripline sections 5 which feed the strip probes 4. The substrate 8 within the waveguide serves only to support the patch 3 and the microstripline sections 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 (FIG. 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 FIG. 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. FIG. 2 shows in outline one method of achieving circular polarisation by using a 90.degree. hybrid network 12 between the microstripline 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 etched on the same substrate 8 as supports 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.degree. phase difference occurs between the signals coupled to the patch 3 by the two strip probes 4. A left-hand circular polarisation or a right-hand circular polarisation is generated is dependent upon whether the signal is applied to point B or point C.

An alternative method of coupling circular polarisation is illustrated in FIG. 3. Here a single microstrip transmission line 13 is divided into the two microstripling sections 5, which have different lengths to produce the required phase conditions. The hand of the circular polarisation is determined by the microstripline 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 polarized 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.