GB2076225A - Steerable beam antenna - Google Patents

Abstract

A steerable beam antenna comprises a plurality of radiating elements e.g. dipoles arranged in an arc or complete circle. Each element has associated therewith a switchable phase shifter, other elements may also have fixed phase shifters. A microwave source, or detector, is selectively switched to an odd number of adjacent elements with the centre element connected through its associated switchable phase shifter. The combined result of the phase shifters is to form a beam of radiation extending from the central radiating element. Thus the beam may be steered through 360 DEG by appropriate switching of power and phase shifters. A single microwave source may be used or separate phase-locked oscillators associated with each element.

Description

SPECIFICATION Steerable beam antenna This invention relates to a steerable beam antenna.

In many radar applications it is necessary to sweep a beam of microwave radiation to detect e.g. aircraft. This may be done by a rotating antenna system or a fixed antenna having multiple emitters and varying the phase of signals applied to each emitter.

Sometimes this latter system is termed a phase array: it is mechanically simpler than a rotating antenna system but is electronically complicated and expensive because of the many phase changing circuits required.

According to this invention a steerable beam antenna comprises a plurality of radiating elements arranged in an arc, each element having associated therewith, at least one switchable phase shifter for applying an additional phase shift separately to each element and switching logic for selectively switching a microwave source or detector to the required number of elements and applying the additional phase to the required element, the arrangement being such that a beam of radiation may be formed by causing at least three adjacent elements to radiate or receive energy and applying the additional phase to the centre of the radiating elements.

The elements may be arranged in a complete circle to that the beam may be steered through 360 . The elements may be dipoles or slots, or of any other suitable radiating form.

The source of microwave power may be a single oscillator whose output is switched to a desired number of elements. Alternatively a separate source may be associated with each element and means provided for locking each source to a common frequency and phase; such means may include phase locked oscillators. The frequency of radiation may be a single frequency or different frequencies applied in a desired manner. The frequency may be applied continuously (cw) or in a pulsed manner with constant or varying pulse length and spacing.

One form of the invention will now be described, by way of example only, with reference to the accompanying drawings of which:- Figure 1 is a plan view of an antenna with twelve radiating elements; Figure 2 is an elevation view corresponding to Fig. 1; Figure 3 is a circuit diagram of the antenna of Figures 1 and 2; Figure 4 is an enlarged diagrammatic plan view of part of Fig. 1.

As shown in Figs. 1, 2 an antenna comprises twelve dipoles d, to d,2 arranged around the circumference of a cylindrical container forming a reflector 1. The dipoles d comprise two half wave strips separated by a dielectric ring 2.

Each dipole d, Fig. 3 is supplied with signals by an associated locked power oscillator 3, to 312 e.g. an Impatt oscillator which receives d.c. power from a power source 4 via switching logic 5. A master oscillator 6 e.g. a Gunn oscillator, supplies a desired frequency signal to a locking oscillator 7 e.g. an Impatt oscillator which is connected to a power divider 8. Twelve ouputs 9, to 912 are taken from the power divider 8, alternate odd outputs 9,, 93.... pass through fixed 270 phase shifters 10" 103,.... 10" and circulators 111, 113,. .. to the oscillators 3" 33,. . . 3it.

Alternate even outputs 92, 94 912 pass through 90 phase shifters 1 02, 104,.... 10,2 and circulators 112, 1 1d".... 1 1,2 to the oscil- lators 32, 34,. 312. Each circulator has attached thereto a one bit phase shifter 12, to 12,2 which receives control signals from the switch logic 5. These one bit phase shifters 12, on command, add to the signals passing through the circulators 11 a 270 phase shift.

Operation of the aerial and reasons for its particular choice of phase shift values will now be described with reference to Fig. 4.

Assume five contiguous dipoles d", d,2, d1, d2, d3 are to energised. For a beam to be formed in the direction X of a normal to the dipole d1 the separate radiation from all radiating dipoles should have a common phase value in a plane Y-Y normal to X.

Let a, r, a, e1, e2 represent the measurement showing in Fig. 4 and microwave wavelength then a = r cos a/2 e, = a - a cos a e2 = a - a cos 2a For the antenna shown with twelve dipoles a = 30 .

If r = 50 m thene, = 6.47 mm and e2 = 24.15 mm.

For a frequency of 1 OGHz = 30 mm and the phase change over distances e, and e2 are

These two phase angles are to approximated to 90 and 270 . Obviously the approximatations may be made more accurate by chang ing the number of dipoles and hence a, and changing r and A.

At the plane YY radiation from dipoles d" and d3 will have a phase of 270 (from the fixed phase shifters) + 270 (through travelling distance e2) = 180 . Radiation from dipoles d,2 and d2 will 90 (fixed phase shifters) 10,2, 102 + 90 (distance e,) = 180 .

A signal from the control logic 5 switches a 90 phase shift into the feed to dipole d,. The phase at YY for dipole d, is thus 270 (through the fixed phase shifter 1 O,) + 270 (by the 1 bit phase shifter 12,) = 180 .

Thus all radiation from dipoles d11, d12, d1, d2, d3 is in phase resulting in a beam along X.

If now it is required to radiate a beam normal to dipole d2 the 1 bit phase shifter 12, associated with dipole d1 is switched off and that for dipole d2 switched on. Also d.c.

power is supplied to the power oscillators 3 of dipole d,2, d1, d2, d3, d4 only.

The phases of radiation in a plane incluåing dipole d2 are then as follows: dipoles d,2 and d4 90 (fixed phase shift) + 270 (through distance e2) = 360 dipoles d, and d3 270 (fixed phase shift) + 90 (through distance e1) = 360 dipole d2 90 (fixed phase) + 270" (through the 1 bit phase shifter 1 22) = 360 .

Thus all radiation is in phase.

From symmetry it is thus seen that the radiation beam can be caused to be propagated in each of twelve directions by radiating from five adjacent dipoles and switching in an extra 270 phase to the centre of these five dipoles.

As noted above the phase values of e1, e2 were approximated. More accurate beams could be obtained if instead of switching in a 270 phase to the central dipole only, a compensating phase were also supplied to the two dipoles either side the central dipole.

However this induces more complexity.

In a modification the fixed phases are reduced by 90 phase. Thus the 270 phases become 180 phase shifters and the 90 phase shifters are no longer required.

An advantage of the present invention is the simplicity which allows a steerable beam and the combining of power from a plurality of smaller radiating sources.

One use of the antenna of Figs. 1 to 4 is in small maritime radar sets. A purely electrical sweeping of the radar beam is achieved by the antenna of Figs. 1 to 4. A similar antenna may be used to receive the reflected radar signal. The various oscillators of Fig. 3 are of course not used. Alternatively, the antenna of Figs. 1 to 4 may be used to both transmit and receive in which case switches are needed to pass received signals into detectors whilst preventing the relatively high transmitted power being channelled direct into the detectors. The various fixed and switched phase shifters of Fig. 3 could be use with advantage to provide antenna gain.

Claims (11)

1. A steerable beam antenna comprising a plurality of radiating elements arranged in an arc, each element having associated therewith at least one switchable phase shifter for applt- ing an additional phase shift separately to each element and switching logic for selectively switching a microwave source or detec-' tor to the required number of elements and applying the additional phase to the required element, the arrangement being such that a beam of radiation may be formed by causing at least three adjacent elements to radiate or receive energy and applying the additional phase to the centre of the radiating elements.

2. An antenna as claimed in claim 1 wherein the elements are arranged so that the beam may be steered through 360 .

3. An antenna as claimed in claim 1 or 2 wherein the microwave source is a single oscillator.

4. An antenna as claimed in claim 1 or 2 wherein the micro wave source comprises a plurality of microwave oscillators one associated with each element and means for locking each oscillator to a common frequency.

5. An antenna as claimed in any one of claims 1 to 4 wherein a plurality of elements have associated therewith fixed phase shifters.

6. An antenna as claimed in any one of claims 1 to 5 wherein five adjacent elements are caused to radiate power and a single phase shift is switched between the control element and its associated power source.

7. An antenna as claimed in any one of claims 2 to 5 comprising detector of microwave radiation and a switch for switching the antenna between transmitting and receiving microwave radiation.

8. An antenna as claimed in claim 4 wherein the microwave source includes phase lock oscillators.

9. An antenna as claimed in any one of claims 1 to 6 wherein the microwave source is a cw source.

10. An antenna as claimed in any one of claims 1 to 6 wherein the microwave source is a pulsed source.

11. An antenna as claimed in claim 1 constructed, arranged, and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.