GB2162693A - Antenna systems - Google Patents

Abstract

A phased array antenna system includes at least three antenna elements 11 disposed equidistantly on the periphery of a circle having a central axis D. The signal passed by each antenna element is fed to a beam former 12, which may be a butler matrix, in which each signal is divided into at least two component signals and respective single component signals are recombined to form at least two mode signals, including a fundamental mode signal M0 and a further mode signal M1. Each mode signal has a radiation pattern with a circular response in azimuth. The fundamental mode signal has a generally constant phase about the central axis D and the further mode signal has a phase distribution which progressively increases with angular displacement about the detecting axis. On reception of a radio signal the relative phase shift between the fundamental and further mode signals is indicative of its angle of arrival. The mode signals are fed to a null steering unit 13 which provides a combined radiation pattern having a null, the unit including a phase shifter whose variation under the control of logic 14 enables the null to be steered. <IMAGE>

Description

SPECIFICATION Antenna systems This invention relates to phased array antenna systems for use for example in sensing the angle of reception of a radio signal. The invention also extends to antenna systems for receiving and/or transmitting radio signals and having a radiation pattern including a steerable null.

According to one aspect of this invention, there is provided a phased array antenna system comprising at least three antenna elements disposed substantially equidistantly about the periphery of a circle having a central axis, beam forming means for dividing the signal passed by each antenna element into at least two component signals and for recombining respective single component signals from each of said antenna elements to form at least two mode signals each of which having a generally circular radiation pattern about said central axis and including a fundamental mode signal having a radiation pattern the phase of which is generally constant about said central axis and a further mode signal having a radiation pattern the phase of which progressively increases with angular displacement about said detecting axis.

In one arrangement adapted for sensing the angle of reception of a radio signal relative to the central axis, the system may further include comparator means for determining the relative phase difference between said fundamental mode signal and said further mode signal on reception of a radio signal whereby the direction of reception of said signal may be deduced.

In another arrangement the system may further include mode signal combining means for combining a selected two mode signals to form a combined mode signal with a radiation pattern having a null in a predetermined angular position with respect to said central axis and phase shifter means operable to impart an adjustable predetermined phase shift between said two selected mode signals whereby the angular position of the null may be adjusted by appropriate adjustment of the magnitude of the phase shift. This arrangement may be adapted for sensing the angle of reception of a radio signal by further including detector means for monitoring the amplitude of said combined mode signal and means for comparing the monitored amplitude with the imparted phase shift whereby the direction of reception of a signal may be deduced.Furthermore the system may further include phase control means operable to cause the phase shift between said selected two mode signals continually to be swept through 21r radians and comparison means for comparing said phase shift and said monitored amplitude whereby the direction of reception of a signal may be deduced.

Advantageously the further mode signal comprises a first mode signal having a radiation pattern the phase of which varies from 0 to 2n radians with angular progression through 2ir radians about said central axis.

The system preferably includes at least one yet further mode signal each having a radiation pattern having a circular response in azimuth and a progressive phase shift between adjacent antenna elements, the phase distribution of said further and said yet further mode signal being defined by the relationship: 2irM N where M is an integer, and N is the number of antenna elements.

According to another aspect of this invention, there is provided a direction finding system for sensing the angle of reception of a radio signal, said system comprising (i) at least three antenna elements disposed equidistantly about the periphery of a circle centred on a detecting axis, (ii) beam forming means for dividing the signal output by each antenna into at least two component signals and for recombining respective single component signals of said antennae to form at least two mode signals each of which having a circular amplitude response pattern in azimuth about said detecting axis and each having a predetermined phase distribution about said axis, and (iii) mode signal processing means for deducing the relative phase shift between said mode signals, thereby to determine the azimuthal angle of reception with respect to said axis.

Further aspects of the invention will become apparent from the following description, which is by way of example only, in which reference will be made to the accompanying drawing which is a block diagram of a first embodiment of this invention.

The system illustrated in Figure 1 can be used for a utility vehicle such as the type known as a 'Landrover' and is designed for for use with VHF radio signals in the 30 to 76 MHz band. The system includes an antenna array 10 consisting of a group of four vertical antenna elements 11 arranged equidistantly on the periphery of a circle centred on a detection axis D. The antenna elements may be either centre-fed dipole or end-fed monopole antenna elements. The diameter of the circle will depend on the vehicle, the number of antenna elements and the bandwidth of operation of the system.In the system illustrated, four antenna elements of unity gain are used; the system may, however, comprise any number of antenna elements greater than three, arranged equidistantly on a circle, and for a system which uses more than eight elements, it is preferred to use directional antenna elements, each boresighted along a radius of the circle.

Signals received by the antenna elements are fed to the beam former unit 12 where each signal is divided equally into a number of components which is equal to the number of mode signals to be produced. In this embodiment, each signal is divided into three equal components. The beam former unit also recombines the components of each antenna element to form three mode signals (M,, M1, M2) each of which having generally circular response pattern in azimuth about the detecting axis D and each having a respective predetermined phase variation in azimuth about said axis.For each mode the beam former unit adds the outputs from each antenna element in the fundamental mode (M,), no phase shift is introduced between the respective component signals but in the higher modes (M1,M2) a progressive phase shift is added between successive antenna elements. The mode signals may be generated by means of a Butler matrix in a manner known to those skilled in the art.

The phase variation of the radiation pattern of each mode signal in azimuth may be expressed algebraically as a phase shift of N radians between successive antenna elements, where M is the mode number and N is the number of antenna elements. Thus, the fundamental mode (Mo) has a constant phase in azimuth about the detecting axis; the first Mode (M1) has a phase variation which increases progressively from 0 to 2ar radians in direct correspondence with the azimuthal angle; and the second mode (M2) has a phase variation which increases progressively from 0 to 4r radians in correspondence with the azimuthal angle.

Since each mode signal has an equal amplitude circular response, and assuming that the first and second mode signals each have zero phase differences in a datum direction, then a signal arriving from an azimuth direction 0 with respect to the detecting axis will produce mode signals having relative phases 0 radians (Mo) 0 radians (M,) and 28 radians (M2).

In addition, if any mode signal (M*0) is com bined with the fundamental mode signal (M = 0) there will be azimuth angles at which the two re sponse patterns will be in antiphase and a deep well will result if the amplitudes are equal. In the arrangement described, the combined response pattern of the fundamental and first mode signals will have a single null at r radians with respect to the datum direction; the combined response pattern of the fundamental and second mode signals will have two nulls at tern2 and -rl2 respectively.

The mode signals are supplied from the beam former unit 12 to the null steering unit 13 where a cyclical relative phase shift of 0 to 2r radians is imparted between a selected two mode : signals by means of a phase shifter which is driven by a drive signal supplied by a null steering control unit 14. This effectively causes the null to rotate contin uously around detection axis D. The combined two mode signals are supplied to a receiver 15 which outputs an amplitude signal to a null position de coder 16. The null position decoder 16 is also sup plied with the drive signal output by the null steering control unit 14, and compares the phase of the amplitude signal with the phase of the drive signal to deduce the angle of arrival of the radio signal.The angle of arrival is displayed on display 17 as a direct measure of the azimuth angle of reception of the signal, and supplied to input/output interface 18.

The interface 18 also supplies to the null steering control unit 14, the receiver 15 and the null position decoder 16 a channel select signal which adjusts the system forthe reception of different radio channels.

In this arrangement, the fundamental and first mode signals may be used to give a first indication of the direction of a radio signal, and the fundamental and second mode signals used to give a more precise reading. The arrangement will give an accuracy of typically + 20, though the accuracy achieved in practice is a function of the number of antenna elements and the diameter of the array.

The receiver should have a sufficient dynamic range for the system, together with an allowance of 15 dB to detect the null with sufficient accuracy.

In a second embodiment, not illustrated, but operating on the same principles as described above, the direction of a radio signal may be determined by introducing between the fundamental and first modesignals, or the fundamental and second mode signals,as the case may be, a phase shift sufficient to null the resultant output from the combination of the mode signals. The phase shift required to obtain a null is then an indication of the direction of the radio signal. In this arrangement, the phase shift is merely adjustable,rather than continuously sweeping through 2r radians as before.

In a third embodiment, again not illustrated, the phases of the two mode signals may simply be measured or compared thereby to determine the relative phase difference and thus the angle of reception.

In the above described embodiments, the objective has been to determine the direction of arrival of a received signal. The system described may however simply be modified to prevent 'noise' jammers from interfering with the reception of radio signals, by combining at least two of the mode signals to produce a null as hereinbefore described and then adjusting the phase of one of the mode signals to steer the null to the appropriate azimuth direction so as to suppress interference from a jamming source. With a group of three or more antenna elements it will be appreciated that by making use of the mode signals more than one null can be created, each null being independently steerable.

It will also be apparent to those skilled in the art that this latter technique may also be employed for controlling active radio transmission so as to direct signals predominantly towards friendly areas thereby rendering the monitoring of communications traffic by third parties more difficult.

Claims (9)

1. A phased array antenna system comprising at least three antenna elements disposed substantially equidistantly about the periphery of a circle having a central axis, beam forming means for dividing the signal passed by each antenna element into at least two component signals and for recombining respective single component signals from.each of said antenna elements to form at least two mode signals each of which having a generally circular radiation pattern about said central axis and including a fundamental mode signal having a radiation pattern the phase of which is generally constant about said central axis and a further mode signal having a radiation pattern the phase of which progressively increases with angular displacement about said detecting axis.

2. A phased array antenna system according to Claim 1 for sensing the angle of reception of a radio signal relative to the central axis and which further includes comparator means for determining the relative phase difference between said fundamental mode signal and said further mode signal on reception of a radio signal whereby the direction of reception of said signal may be deduced.

3. A phased array antenna system according to Claim 1, which further includes mode signal combining means for combining a selected two mode signals to form a combined mode signal with a radiation pattern having a null in a predetermined angular position with respect to said central axis and phase shifter means operable to impart an adjustable predetermined phase shift between said two selected mode signals whereby the angular position of the null may be adjusted by appropriate adjustment of the magnitude of the phase shift

4. A phased array antenna system according to Claim 3, for sensing the angle of reception of a radio signal and which further includes detector means for monitoring the amplitude of said combined mode signal and means for comparing the monitored amplitude with the imparted phase shift whereby the direction of reception of a signal may be deduced.

5. A phased array antenna system according to Claim 4, which further includes phase control means operable to cause the phase shift between said selected two mode signals continually to be swept through 2rr radians and comparison means for comparing said phase shift and said monitored amplitude whereby the direction of reception of a signal may be deduced.

6. A phased array antenna system as claimed in any of the preceding claims wherein the further mode signal comprises a first mode signal having a radiation pattern the phase of which varies from O to 2n radians with angular progression through 27r radians about said central axis.

7. A phased array antenna system as claimed in any of the preceding claims which includes at least one yet further mode signal each having a radiation pattern having a circular response in azimuth and a progressive phase shift between adjacent antenna elements, the phase distribution of said further and said yet further mode signal being defined by the relationship: 2rrM N where M is an integer, and N is the number of antenna elements.

8. A direction finding system for sensing the angle of reception of a radio signal, said system comprising (i) at least three antenna elements disposed equidistantly about the periphery of a circle centred on a detecting axis.

(ii) beam forming means for dividing the signal output by each antenna with at least two component signals and for recombining respective single component signals of said antennae to form at least two mode signals each of which having a circular amplitude response pattern in azimuth about said detecting axis and each having a predetermined phase distribution about said axis, and (iii) mode signal processing means for deducing the relative phase shift between said mode signals, thereby to determine the azimuthal angle of reception with respect to said axis.

9. A phased array antenna system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.