Microstrip antenna

Abstract: The invention relates to a microstrip antenna (2) for radiating or receiving circularly polarised radiation. The antenna (2) includes an array of radiation elements (4), a first feed means (12) coupled to the elements (4) in a manner for effecting circular polarisation in a first sense of signals afforded to the elements (4), and further feed means (14) arranged in a non-overlapping relationship with respect to the first feed means (12) and coupled to the elements (4) in a manner for effecting circular polarisation in the opposite sense of signals afforded to the elements (4).

Description: This invention relates to a microstrip antenna comprising a substantially planar rectangular array of microstrip radiation elements each of which is capable of radiating radiation circularly polarised in one sense when energised at a first feed point and radiation circularly polarised in the opposite sense when energised at a second feed point, a first signal port for the array, first feed means coupling the first port to the first feed points of at least some of the elements of the array, a second signal port for the array, and second feed means coupling the second port to the second feed points of at least some of the elements of the array. A known such antenna is disclosed in GB Patent 2189080 and in Conference Proceedings Military Microwaves '86, June 1986, Brighton, England, pages 323 - 328. In this known antenna, which can be used for both senses of polarisation simultaneously and independently thus enabling simultaneous transmission and reception, the array is a 3 x 2 array of square conducting elements each with two opposite corners bevelled. Each element has two feed points appropriate for left-hand circular polarization (LHCP) and two feed points appropriate for right-hand circular polarization (RHCP). The array in fact is made up from two 2 x 2 element sub-arrays arranged side-by-side, with the centre two elements common to both sub-arrays. The first feed means couples the first port to first feed points of all the elements of one sub-array and the second feed means couples the second port to second feed points of all the elements of the other sub-array. The two feed means, which therefore provide the circular polarization in the one sense and in the opposite sense respectively, are arranged side by side. The relative orientation of elements in the group and the mutual phasing in the feed means conforms to the requirements of the broadbanding technique now commonly known in this art as "sequential rotation". This term will be assumed to be understood by persons familiar with microstrip antenna techniques and will not, therefore, be described further in the context of the present application. A plurality of such antennae may be arranged to form an antenna array. Haneishi and Takazawa have proposed, in Electronics Letters, Vol. 21 no. 10, May 9th, 1985, at pages 437 - 8, a 4 x 4 antenna array formed by four 2 x 2 element antennae each of which corresponds to the same one of the two 2 x 2 sub-arrays, and its associated feed means, employed in the known antenna referred to above. This antenna array has the disadvantage that its use is limited to one sense of circular polarisation determined by the feed points used at the elements. One form of microstrip antenna array suitable for use with orthogonal linear polarisations is described in US Patent No 4,464,663. It comprises a linear array of pairs of square microstrip antenna elements, each connected to two feeds so that one feed renders each element responsive to one of two orthogonal linear polarisations and the other feed renders each element responsive to the other of the two orthogonal linear polarisations. In this way, the connections between the two input/output ports and the four associated elements render each of these four elements responsive to respective orthogonal linear polarisations. The element pairs are operated back-to-back by a feed and the necessary 180° phase correction is provided by the asymmetrical connection. In this way, the isolation between the two polarisation feeds is enhanced. This antenna array is narrow-band and both orthogonal linear polarisations operate at the same frequency. EP-A-0360692, which was published after the priority date of the present application and which forms prior art only according to Article 54(3) and (4) EPC, discloses an antenna comprising at least two radiation elements each of which is capable of radiating radiation linearly polarized in orthogonal directions when energised at first and second feed points respectively. The present invention seeks to provide an improved compact and relatively low profile microstrip antenna with a simplified feed arrangement and for which a number can be readily combined to form a composite array of arbitrary shape. According to the invention an antenna as defined in the first paragraph is characterized in that the rectangular array is a 2 x 2 array, in that the first feed means couples the first port to the first feed points of all the elements of the 2 x 2 array, and in that the second feed means couples the second port to the second feed points of all the elements of the 2 x 2 array in such manner that it does not overlap the first feed means when the feed means are viewed in a direction normal to the plane of the array. As a modification the 2 x 2 rectangular array of microstrip radiation elements may be replaced by an array of three microstrip radiation elements in a triangular disposition. The present invention will not be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows schematic plan and side views of a microstrip antenna having coplanar radiation elements and feed network; Figure 2 shows schematic plan and side view of a microstrip antenna having a triplate feed network; and Figure 3 shows a schematic plan view of a microstrip antenna incorporating a preferred form of feed network. Referring to the drawings, in which like reference numerals have been used to designate like parts of the invention, a microstrip antenna 2 comprises, in the embodiment shown, four radiation elements 4 arranged as a 2 x 2 array in the form of a microstrip layer on a substrate 6. The substrate 6 may comprise any suitable dielectric such as RT/duroid material. The elements 4 are coupled to ports 8, 10 by means of a feed network which includes a first or outer feed 12 and a further or Inner feed 14. In the embodiment shown the outer feed 12 is arranged so as to provide left hand circular polarisation (LHCP) and the inner feed 14 is arranged so as to provide right hand circular polarisation(RHCP) of signals fed via the ports 8, 10. The ports 8, 10 serve, respectively, as LHCP and RHCP ports. The spacing of the elements 4 is, preferably, chosen to be about 85% of the signal wavelength. The advantages of this mutual spacing are that it is large enough to enable the feed networks 8, 10 to be accommodated within the area between the patches, as shown in Figure 3, and it also ensures that the mutual coupling between the patches 4 is relatively weak. Furthermore, the spacing is not sufficiently large to allow grating lobes to appear in the antenna radiation pattern and thereby reduce the directivity of the antenna. Moreover, the axial ratio of the elements 4 may be optimised at the required centre frequency of radiation by suitable choice of the length of the side of the square elements 4 and the degree of chamfering at the corners of the elements. Each element 4 is capable of simultaneously radiating both LHCP and RHCP radiation from independent orthogonally positioned feed points 16, 18, as shown in Figure 3. A sequentially rotated feed system is also used for the outer and inner feeds 12, 14. To achieve sequential rotation each element 4, for the four element array shown in the drawings, is rotated by 90° with respect to neighbouring patches and the outer and inner feeds 12, 14 are arranged to increase in length between consecutive elements by one-quarter the wavelength of the signal to be radiated. One form of feed network providing sequential rotation is shown in Figure 3, where it can be seen that the increase in feed path length between the ports and consecutive elements occurs in an anti-clockwise direction for the outer (LHCR) feed 12 and a clockwise direction for the inner (RHCP) feed 14. It can be seen from Figures 1 to 3 that, the feeds 12, 14 are arranged in a non-overlapping relationship. This is particularly advantageous as the feeds 12, 14 can, therefore, be arranged coplanar with the elements 4 in a purely microstrip configuration, as shown schematically in Figure 1, with the elements 4 coupled to the feeds 12,14 by either direct coupling or gap coupling techniques, as is known in this art. Alternatively, the feeds 12, 14 may be incorporated in a separate triplate layer 20, as shown schematically in Figure 2. As is known in this art the triplate layer 20 comprises a feed layer 20a, in which the feeds 12, 14 are defined, arranged between dielectric layers 20b and 20c. The feeds 12, 14 of the feed layer 20a are coupled to the elements 4 using either direct coupling, such as pins or plated through holes extending through the substrate between the triplate feed layer 20a and the elements 4, or aperture coupling through small areas in a shared ground plane 22 which have been cleared of metallisation used to form the ground plane. The use of a triplate layer 20 for the feeds 12, 14 ensures that the circularly polarised radiation from the elements 4 is not adversely affected by spurious radiation from the feeds 12,14. Furthermore, the layout of the feeds 12, 14 can also be optimised so as to be substantially independent of the layout of the elements 4, such as laterally displacing the layout of the feeds relative to the elements. This flexibility of design can be particularly advantageous when a number of antennas are configured into a larger array. The layered structure of the antenna incorporating a triplate feed layer can be seen from Figure 2. The ports 8,10 may comprise any suitable microwave connector, such as SMA surface mount type connectors, protruding from a ground plane 24 of the array and the elements 4 may be fed by probes coupling, by way of the means as described above, the elements to the triplate layer 20. It can be seen from the above that the present invention provides an antenna of low profile with considerable design flexibility. Furthermore, the antenna can be used as a common building block of simple construction that can be used to construct larger arrays of arbitrary gain and shape which may also be made substantially conformal to curved surfaces. The antenna lends itself readily to mass construction using conventional manufacturing techniques, providing a consistency of performance required for many practical applications for this type of antenna. Although the present invention has been described with respect to a particular embodiment modifications may be effected whilst remaining within the scope of the invention. For example, the antenna need not necessarily comprise a 2x2 array of elements. Three elements in a triangular disposition can also be used with appropriate modifications of the feed system to meet the requirements of sequential rotation. Furthermore, other shapes of element may be used such as square shape elements having a diagonally cut rectangular slot or circular patches having diametrically opposed lugs or indentations.

Claims: A microstrip antenna (2) comprising a substantially planar rectangular array of microstrip radiation elements (4) each of which is capable of radiating radiation circularly polarised in one sense when energised at a first feed point (16) and radiation circularly polarised in the opposite sense when energised at a second feed point (18), a first signal port (8) for the array, first feed means (12) coupling the first port (8) to the first feed points (16) of at least some of the elements (4) of the array, a second signal port (10) for the array, and second feed means (14) coupling the second port (10) to the second feed points (18) of at least some of the elements (4) of the array, characterized in that the rectangular array is a 2 x 2 array, in that the first feed means (12) couples the first port (8) to the first feed points (16) of all the elements (4) of the 2 x 2 array, and in that the second feed means (14) couples the second port (10) to the second feed points (18) of all the elements (4) of the 2 x 2 array in such manner that it does not overlap the first feed means (12) when the feed means (12, 14) are viewed in a direction normal to the plane of the array. An antenna as claimed in claim 1 wherein the first (12) and second (14) feed means are coplanar with the radiation elements (4). An antenna as claimed in Claim 1 wherein the first (12) and second (14) feed means are incorporated in a triplate layer (20) underlying the radiation elements (4). An antenna as claimed in any preceding claim, wherein the radiation elements (4) are of substantially identical shape and are oriented relative to each other in such manner that, on going from element to element around the array, there is an effective rotation through a further 90° in the plane of the array from each to the next, wherein the first feed means (12) is such that, starting from a given radiation element (4) and going from element to element around the array, there are progressive changes of λ/4 in the lengths of the signal paths provided thereby from the first port (8) to the successive elements (4) and wherein the second feed means (14) is such that, starting from a given radiation element (4) and going from element to element around the array, there are progressive changes of λ/4 in the lengths of the signal paths provided thereby from the second port (10) to the successive elements (4), where λ is the wavelength of the radiation. An antenna as claimed in any preceding claim, wherein the radiation elements (4) are of square shape having two chamfered oppositely-disposed corners. An antenna array comprising a plurality of microstrip antennae as claimed in any preceding claim. An antenna as claimed in Claim 1, wherein the 2 x 2 rectangular array of microstrip radiation elements (4) is replaced by an array of three microstrip radiation elements in a triangular disposition.

Inventors: OWENS ROGER PHILIP, SMITH ALAN CHRISTOPHER, OWENS, ROGER PHILIP, SMITH, ALAN CHRISTOPHER
Applicants: SECR DEFENCE BRIT, THORN EMI ELECTRONICS LTD, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNIC MAJESTY'S GOVERNMENT OF THE UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN, THORN EMI ELECTRONICS LIMITED
Application year: 1990
Main classification: H01Q9
IPC codes: H01Q21, H01Q9