Field of the Invention The invention relates generally to antennas and in particular to antennas of base stations of cellular communications systems.

Background Cellular base station antennas are typically implemented as phased-array antennas that use stripline-or microstrip-circuit technology or cable harnesses to feed radiating elements, such as dipoles or patches. A disadvantage of this technology is that, as higher frequencies are used, losses increase.

If the beam is desired to be tilted electrically, phase shifters are required.

Disadvantageously, this further increases the complexity and loss in such antennas. Also, as the size of apertures of such antennas is increased, the gain increases. However, losses increase with the size of the antenna, leading to a diminishing return of gain as length is increased.

Thus, a need clearly exists for an improved antenna for use with a base station in a cellular communications system.

Summary A waveguide antenna is provided for use with a cellular-communications base station. This antenna includes: parallel plate waveguides, one or more portions of the waveguides forming a curved backwall reflector; one or more radiating elements coupled to the parallel plate waveguides to shape a radiation beam pattern; a feed having at least one source of electromagnetic energy located within the parallel plate waveguides; and a polarizer for rotating a plane of polarization of a radiation beam.

The shape of the curved backwall portion may approximate a portion of a parabolic curve.

The curved backwall portion may be shaped to achieve a particular antenna pattern textures or characteristics.

Optionally, the feed has two sources of electromagnetic energy located therein in the form of two probes, but may have other numbers of probes. Alternatively, other sources of electromagnetic energy can be used, such as loops or slots.

The feed is moveably coupled to the parallel plate waveguide for adjusting the tilt of the radiation beam.

At least two waveguide antennas can are used in combination to provide dual polarization. Each waveguide antenna may have a rectangularly shaped portion of waveguide, with the rectangularly shaped portions of waveguide arranged in parallel with each other. The antenna portions containing the curved backwall portions may be located adjacent each other or at opposite ends from each other. Alternatively, each waveguide antenna has a curved shape portion of waveguide.

Brief Description of the Drawings A small number of waveguide antenna configurations are described hereinafter with reference to the drawings, in which: Figs 1A and 1B are side elevation and top plan views of a waveguide antenna for use in cellular communications in accordance with a first configuration; Fig. 1 C is a side elevation view of the movable feed 130 of Fig. 1A ; Fig. 2 is a feed pattern for the antenna of Figs. 1A and 1B ; Fig. 3 is a top plan view of a waveguide antenna for use in cellular communications in accordance with a second configuration ; Fig. 4 is a top plan view of a waveguide antenna for use in cellular communications in accordance with a third configuration; Fig. 5 is a top plan view of a waveguide antenna for use in cellular communications in accordance with a fourth configuration;

Fig. 6 is a top plan view of a waveguide antenna for use in cellular communications in accordance with a fifth configuration ; and Figs. 7A and 7B are side elevation and top plan views of a waveguide antenna for use in cellular communications in accordance with a sixth configuration.

Detailed Description A number of waveguide antenna configurations are described hereinafter. The antennas implement high gain, low loss waveguide antennas. Generic features of these antenna configurations are as follows: . A parallel plate waveguide region comprising a pair of conducting plates supporting the TEM mode with a shaped reflecting wall is used to form a beam with desired characteristics. The spacing of the parallel plates is less than half a wavelength so that the only waveguide mode that can propagate is one in which the electric field is uniform between the plates.

A feed, typically comprising probes or slots, is used to illuminate the reflector from a focal region. The feed normally directs a signal towards the reflecting wall.

If a narrow beam is required the optimum reflector shape approximates a parabola.

'Typically, cellular base station antennas require a narrow beam in the elevation plane and a wide beam in the azimuth plane. The plates of the parallel plate waveguide region are therefore mounted vertically. The parallel plate region is expanded into a horn to form the desired azimuth beam shape.

The antenna radiates horizontal polarization, since this is the orientation of the field emerging from the parallel plate region. Cellular base station antennas are normally required to radiate vertical polarization or slant polarization (linear polarization with the electric vector inclined at 45 degrees to the vertical). Where a polarization other than horizontal is required, a polarizer is placed in front of the horn to effect a rotation of the plane of polarization.

Steering of the beam over a limited range of angle such as is required to provide adjustable electrical downtilt of the beam may be accomplished by physically changing the location if the feed, typically in a vertical direction. The motion of the feed can, if desired be derived from a motor and be remotely controlled.

The polarizer may consist of spaced layers of printed patterns designed to produce different phase shifts for the transmission of different polarizations.

To provide a dual-polarisation capability for an antenna system, two of the waveguide structures of the type described above can be arranged in any of a number of arrangements shown in the drawings. These arrangements involve the notion of"wrapping"or interlocking the waveguide cavities to provide a more compact structure.

The following variations are foreseen: Offset feeding of the reflecting wall to prevent blockage by the feed.

Use of radiating elements such as dipoles or patches fed from probes or slots in the parallel plate waveguide.

Shaping of reflector to compensate for phase characteristics of feed.

Shaping of reflector to modify sidelobe structure for example to reduce the sidelobe level above the main beam at the expense of those below the main beam or to fill certain nulls in the pattern below the main beam.

A waveguide antenna 100 for use with a cellular-communications base station is shown in Figs. 1A and 1B. The corresponding feed pattern is depicted in Fig. 2. The antenna includes parallel plate waveguides 110, a vertical horn 120 coupled to the parallel plate waveguides, a feed 130 having at least one probe 132, and a polarizer 140 for rotating the plane of polarization of a radiation beam. The waveguides 110 have a curved backwall reflector, which may have a parabolic or semi-parabolic shape. Optionally, the profile of the curved waveguide backwall may be varied for beam shaping.

The probe 132 is located within the parallel plate waveguides 110 in the focal region of the curved backwall reflector. As shown in Fig. 1C, the feed 130 may have two probes 132. The feed is moveably connected to the parallel plate waveguide 110 for adjusting the tilt of the radiation beam.

The antenna 100 uses an extremely low-loss parallel plate waveguide region to form the required beam. As stripline-and microstrip-circuit components are dispensed with, losses are consequently reduced compared with existing technology.

Figs. 7A and 7B illustrate a different configuration for the antenna in which the reflector portion is semi-parabolic in shape.

Figs. 3 and 5 respectively illustrate a pair of waveguide antennas 300 and 500.

Notably, a portion of each waveguide pair 310A and 310B is curved, outwardly or inwardly from or to each other, respectively. Moveable probes 330 and 530 are located in the waveguides 310 and 510.

Figs. 4 and 6 respectively illustrate a pair of waveguide antennas 400 and 600. A portion of the waveguide 410 and 610 is rectangularly shaped with the reflectors in parallel at one end and at opposite ends, respectively. The reflectors in Fig. 6 may be angled from the central axis of each rectangular portion of waveguide.

Only a small number of configurations described herein. However, in view of this disclosure, variations and changes may be made by those skilled in the art without departing from the scope and spirit of the invention.