GB2249873A - Antenna system - Google Patents

Abstract

An antenna system transmits or receives a beam of microwave radiation. The system includes a waveguide (5) and a feed circuit which couples a feed signal to or from the waveguide (5). Components, such as phase shifters (6, 7) are included in the feed circuit. Those generate an imbalance in the food signal. The waveguide (5) is arranged to generate asymmetric higher order modes in response to the imbalance feed signal. As a result the beam of microwave radiation is deflected through a predetermined angle. Transmission line structures other than waveguides, such as coaxial lines, may be employed. <IMAGE>

Description

ANTENNA SYSTEM The present invention relates to antenna systems, and in particular to a system enabling precisely controlled deflection of a beam of microwave radiation.

The use of high performance antennas for communications systems means that accurate pointing of the antenna beam is becoming increasingly critical. RF sensing systems exist which permit the derivation of pointing error information, but they are generally either mechanical systems (such as step-track or conscan) with limited speed, inherent RF degradation and only modest accuracy, or comparatively expensive, albeit extremely accurate, systems such as static split or multimode monopulse systems which require dedicated tracking receiver sub-systems. As a further alternative, electronic beam scan systems have been used. Such systems offer a compromise between tracking accuracy and cost but suffer the serious disadvantage of offering only a narrow bandwidth.

According to the present invention, an antenna system for transmitting or receiving a beam of microwave radiation comprises a waveguide, means to couple a feed signal to or from the waveguide, and means to generate an imbalance in the feed signal, the waveguide being arranged to generate asymmetric higher order modes in response to an imbalanced feed signal, thereby deflecting the beam of microwave radiation through a predetermined angle.

It will be understood that the term "waveguide" as used in the present context encompasses transmission line structures such as coaxial lines, as well as conventional hollow waveguides.

The present invention provides a system which enables a controlled deflection of the microwave beam with an accuracy comparable to the known electronic beam scan system, but which, at the same time is capable of operating over a wide bandwidth.

The waveguide may receive or transmit radiation directly but, preferably illuminates a reflector or lens system.

Preferably the means to couple a feed signal comprise at least one pair of microwave coupling ports or probes positioned in diametrically opposed regions of the waveguide and branches of a microwave feed circuit connected to respective ports or probes. Preferably the means to generate an imbalance comprise switchable signal modifying means connected in each of the branches and arranged to introduce a phase shift and/or attenuation in one of the branches to deflect the beam through the predetermined angle in a first direction and to introduce a phase shift and/or attenuation in the other of the branches to deflect the beam through the predetermined angle in an opposite direction.

With an appropriate design for the rest of the feed system, the differential phase shift between the higher order asymmetric mode or modes generated by imbalanced coupling ports or probes and the fundamental mode can be set so that the induced asymmetric mode results in a beam deflection away from the boresite in a fixed direction. If instead the same imbalance is introduced into the opposing branch of the feed circuit the beam is deflected by an equal amount in the opposite direction and such deflection can be used to provide fine beam-pointing control.

Preferably when the system is used to receive microwave radiation it includes means to derive a tracking signal from the deflected beam.

If the antenna is misaligned then differing levels of received signal will be obtained from the boresite and deflected beam positions. This difference between the signals can be used to derive the approximate angular misalignment in the direction of the scan and, where necessary, to drive the antenna using a feedback loop to correct the misalignment.

Preferably the means to couple a feed signal include a second pair of diametrically opposed coupling ports or probes and further branches of the feed circuit connected to the second pair of coupling ports or probes, the second pair of coupling ports or probes being arranged orthogonally with respect to the first pair, and the means to generate an imbalance including further switchable signal modifying means in the branches connected to the further pair of coupling ports or probes, the means to generate an imbalance being arranged to switch the beam sequentially in four directions.

As a further alternative rather than using a balanced divider followed by a variable or switched attenuator or phase shifter, the imbalance may be caused by an imbalanced divider, such as a variable power divider.

The signal modifying means may include PIN diode phase shifters, ferrite phase shifters or varactor diodes.

A further advantage of the present invention is that the network used to modify the feed signal is compatible with that used on, for example, orthomode transducers (OMTs) and so can be readily incorporated into such systems, thereby, for example, providing tracking capability over the full received band. The resulting system can operate in both linearly and circularly polarised antenna feed systems and can, with some increased complexity, provide dual polarisation tracking capability.

The system may include filters associated with the coupling ports or probes so that the beam deflection occurs for only a limited bandwidth. Such a system may, for example, be arranged so that deflection occurs at a received bandwidth but not at the transmit band, or just at a beacon frequency band. Although highest tracking accuracy is generally achieved by balanced beam deflections a simpler and cheaper system may be constructed using one imbalancing unit only for each of two orthogonal directions instead of one for each of four.A system in accordance with the present invention will now be described in detail with reference to the figures of the accompanying drawings in which: Figure 1 is a schematic of an antenna and feed network; Figure 2 is a graph showing the derivation of a tracking signal from the antenna response; Figure 3 is a graph illustrating four-position beam switching; and Figure 4 is a graph illustrating two-position beam switching.

An antenna system for transmitting or receiving microwave radiation comprises a waveguide 5, feed circuit 8 and ports 1,2,3,4 arranged to couple a feed signal to the antenna 5 from the feed circuit 8. In the present example, the waveguide 5 is formed as a hollow cylinder but the present invention is equally applicable to antennas using other waveguide configurations such as square cross-section waveguides or coaxial lines. The cross-section of the waveguide 5 is chosen to be large enough to support higher-order asymmetric modes (such as the TM 01 and/or TE 21 modes. So long as the ports 1,2,3,4 are fed in balance no such asymmetric modes are generated.

The branches of the feed circuit 8 connected to the ports 1,2 include phase shifters 6,7 which in the present example are formed by PIN diodes. So long as the ports are fed in balance no asymmetric modes are generated. However, when one or other of the phase shifters 6,7 is switched on to create an imbalance in the network then the resulting asymmetric feed to the waveguide 5 excites the asymmetric modes in addition to the fundamental mode of the waveguide.

The actual asymmetric mode induced is influenced by the geometry of the coupling ports. For a circular waveguide, for example, the TM o, mode will be predominant if the ports are circumferential slots. Longitudinal slots give rise mainly to the TE 21 mode. The phase shift generated by the phase shifters 6,7 is chosen so that the differential phase shift between the higher order asymmetric modes generated by the imbalanced coupling ports and the fundamental mode results in a deflection of the beam away from the boresite of the waveguide 5 in a fixed direction. By introducing the phase shift in one or other of the branches the beam is deflected by an equal amount in opposite directions and this deflection is used to provide fine beam-pointing control.

In an alternative embodiment, the antenna is used to receive microwave radiation and a tracking signal is generated. By reciprocity the shift in the response of the antenna to a received beam is the same as that generated by the equivalent phase shift on a transmitted beam.

Therefore as the phase shifters 6,7 are switched to scan the beam, differing levels of received signals are obtained from the different beam positions if the source is off boresite. The variation in the signal is monitored to derive a difference signal as shown in Figure 2. This difference signal which varies with the angular misalignment can then be used to drive a tracking system.

In the preferred example, the accuracy of control over the system is increased by using a second pair of ports 3,4 arranged orthogonally with respect to the first pair and including a further pair of phase shifters (not shown) in the respective branches of the feed circuit 8. These are used to deflect the beam in opposing directions orthogonal to the deflection generated by the first pair of ports.

The overall system is then capable of being switched in four directions as shown in Figure 3.

For applications where cost is more critical than the ultimate degree of control over the deflection of the beam then a cut-down version of the system may be used, again having four ports as shown in Figure 1 but with a phase shifter in one only of each pair of branches feeding the ports. The deflection of the beam is then possible in two directions only as shown in Figure 4.

Claims (8)

1. An antenna system for transmitting or receiving a beam of microwave radiation comprising a waveguide, means to couple a feed signal to or from the waveguide, and means to generate an imbalance in the feed signal, the waveguide being arranged to generate asymmetric higher order modes in response to an imbalanced feed signal, thereby deflecting the beam of microwave radition through a predetermined angle.

2. A system according to claim 1, in which the waveguide is arranged to transmit or receive radiation via a reflector or lens system.

3. A system according to claim 1 or claim 2, in which the means to couple a feed signal comprise at least one pair of microwave coupling ports or probes positioned in diametrically opposed regions of the waveguide and branches of a microwave feed circuit connected to respective ports or probes.

4. A system according to claim 3, in which the means to generate in imbalance comprise switchable signal modifying means connected in each of the branches and arranged to introduce a phase shift and/or attenuation in one of the branches to deflect the beam through the predetermined angle in a first direction and arranged also to introduce a phase shift and/or attenuation in the other of the branches to deflect the beam through the predetermined angle in an opposite direction.

5. A system according to any of the preceding claims, in which the antenna is arranged to receive radiation and includes means to derive a tracking signal from the deflected beam.

6. A system according to claim 3 or claim 4 or 5 when dependent on claim 3, in which the means to couple a feed signal include a second pair of dimetrically opposed coupling ports or probes and further branches of the feed circuit connected to the second paid of coupling ports or probes, the second pair of coupling ports or probes being arranged orthoganally with respect to the first pair, and the means to generate an imbalance including further switchable signal modifying means in the branches connected to the further pair of coupling ports or probes, the means to generate an imbalance being arranged to switch the beam sequentially in four directions.

7. An antenna system according to any one of the preceding claims further comprising an orthomode transducer system (OMT).

8. An antenna system substantially as described with respect to the accompanying drawings.