AU610061B2 - Direction-finding antenna system - Google Patents

Description

PHI3.33246 AU.

O RI GI NA L 61005 6Y12/ 4444 4444 4444 ~44I 44 4 4 ~.44f .4's 4 -t t I 41 hiS documIR :i contains h e amendinents made ulkl i ,!Cc [ion 49 ,,In!40 iprinir COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 44 44 4 4 4 I 4 COMPLETE SPECIFICATION FOR THE IITVENTION\ ENTITLED: "Directing-finding antenna system." The following statement is a full description of this invention,inoluding the best method of performing it known to me:- PHB 33246R lA 27.8.90 "DIRECTION-FINDING ANTENNA SYSTEM" The invention relates to a direction-finding antenna system comprising an array of antenna elements mutually spaced in at least one given direction and further comprising feeder means for forming a sum radiation pattern and a difference radiation pattern from the elements. The invention relates particularly but not exclusively to such a system wherein the array is curved in the plane including said one direction and the axes of the radiation patterns.

Antenna systems as set forth in the opening sentence of this specification are well known as exemplified below.

They may be used in so-called monopulse systems for determining the direction of incidence of RF signals, typically microwave signals, in arrangements which compare the amplitudes and/or phases of the signals respectively received with the sum and difference patterns. In such an arrangement, feeding the antenna elements is carried-out in two different ways so that a pencil beam or sum radiation pattern and a wide or difference radiation pattern are obtained. A combination of the two radiation patterns is designed to eliminate from the signals picked up by means of the sum pattern those due to the secondary lobes and it is important that the difference pattern should cover all these lobes to enable a well defined central pencil beam to be obtained. Such a combination may not be especially difficult to achieve with, for example, a large array which extends over many wavelengths, in particular one wherein an amplitude distribution which is not uniform across the array in said one direction can be applied to the contributions of the elements to the sum pattern. However, it may be difficult to achieve in certain conditions, for example with a relatively small array wherein little or no amplitude tapering can be applied to the sum pattern because it is necessary to maximise the gain from the relatively few elements that are available.

US-A-3594811 discloses an antenna system alternately having a directive radiation pattern and a non-directive radiation pattern, the system comprising a horizontal array of vertical dipoles which forms two groups of p elements

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o;/ PHB 33246R 2 27.8.90 regularly spaced on either side ot a central group of K elements, K comprises one or more elements. A radiator is disposed to the rear of the central group of elements. Each group of p elements is divided into inner and outer subgroups. The other sub-groups of elements are interconnected by a first hybrid junction and the inner sub-groups of elements are interconnected by a second hybrid junction.

The sum outputs from the first and second hybrid junctions are combined by a first power splitter/combiner to form an array sum signal which constitutes the directive radiation pattern. The difference output of the first hybrid junction is coupled to a matched load but the difference output of the second hybrid junction together with the signals derived from the central group of elements and the rearwardly ,15 disposed element are combined by a second power 4 9 splitter/combiner to form an array difference signal which constitutes the non-directive radiation pattern. The first and second power splitters/combiners are alternatively connected by a switch to a power source or signal processing 20 device.

If K is odd, then for technical reasons not elaborated upon, the central group of elements do not contribute to the sum signal. The array difference signal is formed by the sum of several elementary patterns, namely: the pattern 4*2.5 produced by feeding in phase opposition the elements of the .64 respective inner groups, the pattern produced by feeding the rearwardly disposed element in phase quadrature with respect to the elements of the respective inner groups and a pattern produced by feeding, if K is odd, the central group in phase with one of the inner groups of elements or, if K is even, the two symmetrical parts of the central group in phase quadrature with the respective inner groups and in phase opposition with respect to each other.

Other known methods for obtaining sum and difference signals from arrays of antenna elements are disclosed in EP- A-0021193, EP-A-0012055, EP-A-0013240, US-A-4283729 and DE- C-2737750. EP-A-0021193 discloses an integrated antenna system comprising a reflector having a double curvature and F microstrip radiators mounted on the reflector. By suitable I n r- -r -i I PHB 33246R 31.1.91 interconnections of the radiators, sum and difference patterns are obtained. EP-A-0013240 discloses the provision of cavity mounted antenna elements in the surface of a reflector having a double curvature. EP-A-0012055 shows the provision of four antenna elements arranged symmetrically about a central antenna element. US-A-4283729 discloses an open planar array antenna having a plurality of columns of radiating elements and a feed network. A typical antenna measures 1.5m by 7.8m. The feednetwork has a sum channel, a difference channel and a sidelobe supression channel formed in common in an input power divider. DE-C-2737750 discloses an airborne IFF system in which sum and difference signals are derived.

An object of the present invention is to simplify the derivation of the sum and difference radiation patterns when using small arrays of antenna elements.

According to the present invention there is provided a direction-finding antenna system comprising an array of antenna elements mutually spaced in at least one given 20 direction, the array of antenna elements comprising at least two sub-arrays containing equal numbers of antenna elements extending in said given direction on either side of a centre ~of the array, each sub-array having a respective midpoint S• and a feeder network for forming a sum radiation pattern to from signals obtained from the antenna elements of the sub- ~arrays and for forming a difference radiation pattern from signals obtained from at least those antenna elements disposed between the centre of the array and the midpoint of each sub-array, with the exclusion of a possible midpoint coinciding with the centre characterized in that the feeder network is electrically coupled to the antenna elements for simultaneously forming the sum and difference radiation patterns, in that the sum radiation pattern includes signals from all the antenna elements, in that the difference radiation pattern excludes a signal produced by an antenna element located at the centre of the array, if provided, and in that the feeder network is arranged to ensure that the PA4,, contribution of signals from the antenna elements lying Sbeyond the midpoints of the sub-arrays in said given a .1 I PHB 33246R 4 31.1.91 direction is less to the difference radiation pattern than to the sum radiation pattern.

It will be appreciated that whilst the term "contribution" may imply use of the array for reception, the array may additionally or alternatively be used for transmission.

Reducing the contributions of the outer elements to the difference pattern effectively reduces spacing between the midpoints or phase centres of the sub-arrays and thus makes the difference pattern less directional than if the outer elements made the same contribution as to the sum pattern.

One effect of this is to increase the angular spacing between the peaks of the difference pattern and thus tend to raise the magnitude of the difference pattern at angles beyond these peaks. A suitable reduction in the contribution of the outer elements may be determined empirically. (It may be noted that another effect is to broaden the central angular range over which the magnitude of the sum pattern exceeds that of the difference pattern; :i 20 the extent of broadening that is acceptable may vary in different applications. A suitable compromise between this broadening and the desired raising of the difference pattern at larger angles may be achievable empirically).

In a particularly simple embodiment, at least one outer ,,25 element which lies beyond the midpoint of each sub-array 4 1 makes no contribution to the difference pattern.

The invention may be particularly suited for embodiment in an array which is curved in the plane including said one direction and the axes of the radiation patterns; in such an array, it may otherwise be especially difficult to obtain a difference pattern with the desired breadth. Such an array may be one wherein the elements are microstrip patch radiators supported on an antenna reflector which, in 0 association with a feed radiator, operates at a substantially higher frequency than the array: the array may i then have a relatively small extent in said direction which, l as indicated above, may also make it particularly difficult to obtain a difference pattern with sufficient magnitude U relative to the sum pattern. An example of such an 7 arrangement is an IFF (Identification Friend or Foe) array PHB 33246R 5 27.8.90 which may operate in L-band at around i GHz and which is supported on the reflector of a radar antenna operating, for example, in X-band at around 9 GHz; the IFF array may thus be mechanically scanned together with the radar antenna.

Whilst the aperture of the radar antenna may typically amount to many wavelengths at the operating frequency of the radar 30 wavelengths), this dimension (within which the IFF array is constrained to fit) may amount to only a few wavelengths at the IFF frequency.

Embodiments of the invention will now be described, by way of example, with reference to the diagrammatic drawings, in which:- Figure 1 is a schematic front view of a radar antenna

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parabolic reflector supporting a direction-finding array of microstrip patch antenna elements; Figure 2 is schematic sectional view of a row of microstrip patches and the supporting reflector, also depicting the feed horn of the radar antenna; Figure 3 is a schematic diagram of a feeder network for the direction-finding array; Figure 4 shows measured sum and difference radiation patterns for a constructed embodiment, and Figure 5 shows a modified portion of the feeder network.

Figure 1 depicts an array of antenna elements for a direction-finding system embodying the invention. The

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elements of the array are in this instance formed as eight microstrip patch elements 1-8 and supported on a parabolic reflector 9 of a radar antenna. The microstrip patch radiators are arranged in two rows, each of four patches, extending in a horizontal direction; two rows of elements are used (rather than one) to increase the gain. The reflector is curved in a plane including the axes of the sum and difference radiation patterns of the array the normal to the plane of the drawing) and the direction of extent of the rows, that is to say, the reflector is curved in a horizontal plane (as drawn); it is also curved in a vertical plane. The curvature in the horizontal plane is depicted in Figure 2, which is a cross-section on the Line -i ,i ua.aa~~ -1 PHB 33246R 27.8.,90 (2 II-II in Figure 1; for simplicity, only one row of elements is shown. Figure 2 also shows schematically a feed horn radiator 10 operatively associated with the reflector 9.

With respect to the horizontal direction normal to the axes of the radiation patterns, there is an element at each of four locations mutually spaced by the same distance D.

As shown in Figure 2, each of the patches comprises a thin conductive layer 11 which forms the radiator. This is disposed on a dielectric layer 12 which in turn is supported on the reflector 9. To provide the ground plane of the microstrip, the reflector may be of conductive material, or may be of dielectric material with a conductive coating on at least the surface facing the feed radiator 10. In this embodiment, the front surface of the dielectric layer 12 supporting the conductive layer 11 of each patch element is planar whilst the rear surface conforms to the curved surface of the reflector 9. As a result, the thickness of the patch element, i.e. the spacing between the conductive layer 11 and the ground plane provided by the reflector 9, varies across the patch: this increases the bandwidth of the patch antenna element, which is advantageous for transmission and reception at different respective frequencies.

To form a sum radiation pattern, the whole array of eight elements is used. The eight elements may be considered as forming two sub-arrays, comprising elements i, 2, 5, 6 and 3, 4, 7 and 8 respectively, on opposite side of the phase centre 13 of the whole array. As will be mentioned again with reference to Figure 3, the feeder network of this embodiment comprises phase delays for the outermost elements of the array so that, with respect to transmission or reception at an angle close to the axes of the sum difference patterns (boresight), the elements are effectively substantially collinear in the horizontal plane.

The phase centres, 14 and 15 respectively, of each of the two sub-arrays are thus mid-way between the horizontal locations of the elements of the respective sub-array.

RFigure 3 depicts schematically a feeder network for Sforming the sum and difference radiation patterns from the i t.

i s, PHB 33246R 7 27,8.90 array of elements 1-8. The network comprises four 3 dB inphase power dividers/combiners 17-20 which respectively combine the signals from each two adjacent elements, respectively in the two rows, at the same location in the horizontal direction.

As mentioned above, the feeders for the outermost elements (combiners 17 and 20) include phase delays, denoted schematically by 0 at 21, 22 respectively, so that the four elements in a row are effectively substantially collinear with respect to transmission or reception on boresight.

tit Following these phase delays, the feeders are denoted a and t b respectively, and the feeders from combiners 18 and 19 are denoted c and d respectively. The signals on lines a and b i are added in a further 3 dB in-phase power combiner 23 to '5 form the sum of the signals from the outer four elements, o. 04. The signals from the inner four elements on lines c and d are respectively supplied to two ports of a hybrid junction 24 which at two further ports produces the sum and the difference of these signals, 14 and A 14 00..20 respectively. Z 14 is added to Z 04 in yet another 3 dB ino phase power combiner, 25, to produce the sum of the signals from all eight elements, Z 8 Since the difference signal a 14 is derived from only the inner elements which are spaced a distance D apart in the horizontal direction while the sum signal is It l derived from the two sub-arrays whole phase centres are a S' distance 2D apart, the difference pattern is less directional than if it had been derived from the two subarrays from which the sum pattern is derived. Consequently, i 30 whereas if the difference pattern were derived from said two sub-arrays, its magnitude would tend to fall below that of the sum pattern at angles fairly close to boresight (apart from the central null of the difference pattern), this need not be the case for the embodiment of the invention. Figure 4 shows, in dB against the angle 0 in degrees with respect to mechanical boresight, a sum pattern (continuous line) and a difference pattern (dashed line) measured on a constructed embodiment similar to that described above with reference to i Figures 1 to 3. This embodiment had to satisfy the U r IV dej PHB 33246R 8 27,8.90 criterion that, apart from the central null, the magnitude of the the difference pattern should exceed the magnitude of the sum pattern if the magnitude of the sum pattern was within 20 dB of its peak value. As can be seen, this criterion was satisfied by a substantial margin.

As an alternative to using two phase delays (21,22) between combiner 23 and combiners 17 and 20 respectively, a single phase delay i may be used between combiners 23 and As an alternative to outer elements of the array making no contribution at all to the difference pattern, their contributions (relative to those of the inner elements) may merely be somewhat redL 'ed, This has the possible advantages of increasing the gain of difference pattern and of reducing the central angular range over which the magnitude of the sum pattern exceeds that of the difference pattern, but the disadvantage of being somewhat more complex. Figure 5 shows schematically a modified portion of the feeder network of Figure 3 in which the one hybrid junction 24 and the two combiners 23 and 25 of Figure 3 are replaced by at least two hybrid junctions and two combiners.

The arrangement of Figure 5 uses the name four feeder lines a-d as in Figure 3. Lines c and d are the inputs for a first hybrid junction 24 producing outputsj 14 and j 14 (as in Figure while lines a and b are the inputs to a second junction 27 producing output A 04 (the difference between the signals from the two outer pairs of elements) and 7 04" The sum signals 14 and 04 are applied to a 3 dB in-phase combiner 28 which produces the signal 08. The difference singal L04 is supplied to an attenuator 29 whose output is added to the difference signal A 14 in a 3 dB in-phase combiner 30 which produces the signal zA 14 k 4 04) where 0 k 1. As an alternative to the attenuator 29 and equal power combiner 30, a combiner/divider giving unequal combination/division may be used.

As an alternative to reducing the absolute magnitudes of the contributions of the outer elements to the difference R7 pattern, the absolute magnitude of the contributions of the t-f inner elements to the difference pattern (but not the sum PHB 33246R 9 27.8.90 pattern) may be increased by amplification, so that the contributions of the outer elements relative to the contributions of the inner elements is less to the difference pattern than the sum pattern.

Where the invention is embodied in an array comprising, for example, eight or more elements uniformly spaced in the relevant direction, the outer elements whose relative contributions are less to the difference pattern than to the sum pattern need not be the outermost elements with respect to said direction, but should nevertheless lie beyond the phase centres of the sub-arrays from which the sum pattern is formed (rather than between or at the phase centres) so as to achieve the desired broadening of the difference pattern.

The array may comprise elements at an odd number (rather than an even number) of locations spaced in the relevant direction. For example, there may be an element at the phase centre of the whole array. In that case, the central element would not be used in forming the difference pattern but could be used in forming the sum pattern, and the contribution of the central element to the sum pattern could then notionally be divided equally between the two sub-arrays on opposite sides of the phase centre of the whole array.

1,25 In the above-mentioned constructed embodiment, the S patches were 9 cm wide horizontally and 11 cm wide vertically. Their spacing was 17 cm horizontally (distance D) and 20 cm vertically. The value of the phase delay p was of the order of 50-60 degrees; the power dividers/combiners were of the Wilkinson type, and the hybrid junction was a 180 degrees hybrid ring with a circumference of one and a half wavelengths. These components were formed on alumina substrates of 0.635 mm thickness. The thickness of the dielectric of the patches varied from 1.5 cm at the edges to about 2.0 cm at the centre of the patches; the dielectric material was P10 polyurethane foam available from the Plessey Co. The radar antenna reflector was of conductive i carbon fibre material; it had apertures of approximately 107 Y cm horizontally and 41 cm vertically, and had a focal length PHB 33246R 9A 27.8.90 of 43 cm. The patch array operated at 1.03 and 1.09 GHz, and the radar antenna operated in the range 9.0-9.5 GHz.

Claims (5)

1. 2. A direction-finding antenna system as claimed in Claim i, characterised in that the feeder network comprises means for deriving an outer element difference signal from signal obtained from those antenna elements of the respective sub- arrays disposed outwards of their respective midpoints with the exclusion of signals from antenna elements of a possible sub-array with a midpoint coincidence with the centre, means for deriving an inner element difference signal from signals obtained from the antenna elements disposed between the '3 5 centre of the array and the midpoint of each sub-array with the exclusion of a possible midpoint coinciding with the cRA4 centre, signal combining means for combining the outer and SLinner element difference signals to form the difference A radiation pattern, and a magnitude-affecting means coupled 7e to an input of the signal combining means for affecting the PHB 33246R 11 31.1.91 magnitude of either the outer element or inner element difference signal such that the contribution of the outer element difference signal to the difference radiation pattern is smaller than the contribution of the inner element difference signal.
2. 3. A direction-finding antenna system as claimed in Claim 2, characterised in that the magnitude-affecting means comprises an attenuator for attenuating the outer element difference signal being applied to the signal combining means.
3. 4. A direction-finding antenna system as claimed in Claim 1, 2 or 3, characterized in that the antenna elements comprise microstrip patch radiators arranged on a curved antenna reflector, and in that each patch radiator comprises a planar conductive layer on a front surface of a dielectric layer, the rear surface of the dielectric layer conforming to the curvature of the antenna reflector. A direction-finding antenna system as claimed in Claim 4, characterised in that delay means are provided in the signal paths of those signals obtained from those antenna elements of the respective sub-arrays disposed outwards of their respective midpoints, said signals contributing to the sum radiation pattern.
4. 6. A direction-finding antenna system as claimed in any one of claims 1 to 5, characterised in that each sub-array comprises at least two columns, each column comprising at least two of said elements.
5. 7. A direction-finding antenna system substantially as described with reference to the accompanying drawing. DATED THIS THIRTY-FIRST DAY OF JANUARY 1991 N. V. PHILIPS' GLOEILAMPENFABRIEKEN