Dual polarisation patch antenna

Abstract: The antenna element comprises a printed circuit board (1) on one side of which is a conductive ground plane (2) and on the other side of which are two symmetrical conductive feed tracks (3,4) which excite orthogonal slot apertures (12,13) etched in the ground plane. Radiation from the slots induces orthogonal currents into a radiating patch (14) supported in a spaced relationship with the slots. Two signals can be radiated from the patch simultaneously with 90° separation in polarisation. The slots radiate to the rear as well as to the front. A symmetrical conductive cavity (17) enclosing the slots contains rear radiation.

Description: This invention relates to electromagnetic radiation antenna structures capable of receiving and transmitting radio signals that may include dual orthogonally polarised components. In a complex urban environment of buildings, structures and obstacles, a radio signal will be reflected and scattered and may not follow a straight line path between a transmitter and a receiver. Polarisation rotation of the radio signal may occur due to reflection and scattering. To overcome the effects of polarisation rotation, polarisation diversity reception is known to be used. Polarisation diversity requires an antenna to be able to receive components of a signal of any polarisation, both horizontally polarised and vertically polarised signals or any polarisation between. A typical cellular mobile radio base station antenna tower will have one transmit antenna and two receive antennas in a "space diversity" configuration for any sector. The receive antennas are spaced apart with the transmit antenna placed between them. One receive antenna will be in a zone of increased signal strength relative to the other receive antenna, should multi-path scattering effects occur. This arrangement typically requires a complex infrastructure, as three antennas are used in each sector, usually nine to a tower. Such known antenna arrangements are relatively large, expensive and visually un-appealing. An aperture-coupled, circularly polarised antenna is known from US-A-5 241 321 having a planar radiating patch, and with a ground plane interposed between the radiating patch and a feed network. This antenna is designed to produce circular polarisation at two different frequencies, from two different inputs. EP-A-0 384 777 describes a cavity-backed dual-slot antenna element for single circular polarisation, and correspondingly with a single feed. The radiation takes place primarily from resonant slots. From EP-A-0 605 338 an arrangement of an aperture-coupled, dual linearly polarized antenna is known, incorporating non-intersecting slot apertures and non-identical feed tracks, with the possibility of a lower ground plane to reduce back radiation. US-A-4 903 033 shows an aperture coupled, dual (linear or circular) polarized antenna with a planar radiating patch and with a ground plane interposed between the radiating patch and a feed network. It is an object of the present invention to provide an easily manufactured antenna element for use in a relatively small, lightweight, visually more appealing dual polarisation antenna array of simple construction having good bandwidth and polarisation isolation. According to a first aspect of the invention there is provided an antenna element for transmitting and/or receiving radio frequency signals that may include dual orthogonally polarised components, said antenna element comprising a planar dielectric element supporting on one side thereof a first conductive ground plane element and on an opposite side thereof two substantially identical conductive feed track arrays disposed at right angles to each other and each being electrically symmetric about a bisecting plane, said first ground plane element having two substantially identical slot aperture arrangements each comprising at least one elongate slot of predetermined length, said aperture arrangements' longitudinal axes being disposed at right angles to each other and cross at their respective mid-points, each slot aperture arrangement being symmetrical about a respective bisecting plane bisecting a feed track array , a symmetrical conductive patch element disposed in a predetermined spaced relationship with said slot aperture arrangement and above said opposite side of said planar dielectric element, and a symmetrical conductive cavity element comprising a bottom wall portion and at least one side wall portion having a rim, disposed on said first ground plane element and electrically coupled thereto, said cavity element enclosing said elongate slot aperture arrangement within the said wall portions and a surface portion of said first ground plane element that is proximate said slot aperture arrangement. According to a second aspect of the invention, there is provided an antenna element for transmitting and/or receiving radio frequency signals that may include dual orthogonally polarised components, said antenna element comprising a planar dielectric element supporting on one side thereof a first conductive ground plane element and on an opposite side thereof two substantially identical conductive feed track arrays disposed at right angles to each other and each being electrically symmetric about a bisecting plane, said first conductive ground plane element having two substantially identical slot aperture arrangements of predetermined length, said aperture arrangements' longitudinal axes being disposed at right angles to each other and cross at their respective mid-points , each slot aperture arrangement being symmetrical about a respective bisecting plane bisecting a feed track array , a symmetrical conductive patch element disposed in a predetermined spaced relationship with said slot aperture arrangement and above said opposite side of said planar dielectric element, a second conductive ground plane supported in a predetermined spaced relationship with said first conductive ground plane , and a symmetrical conductive cavity element comprising a bottom wall portion and at least one side wall portion having a rim, said cavity element being interposed between said first conductive ground plane and said second conductive ground plane , said bottom wall portion electrically contacting said second conductive ground plane and said rim being capacitively coupled to said first conductive ground plane, said cavity element enclosing said elongate slot aperture arrangement within the said wall portions and a surface portion of said ground plane element that is proximate said slot aperture arrangement. According to a third aspect of the invention, there is provided an antenna array comprising a plurality of antenna elements of the present invention operatively coupled together. In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 shows a side view of a first embodiment of the antenna element. Figure 2 shows a top view of the element shown in Figure 1 without the radiating patch. Figure 3 shows a top view of the element shown in Figure 1 with the radiating patch. Figure 4 shows a bottom view of the antenna element shown in Figure 1. Figure 5 shows a top view of an alternative radiating patch arrangement. Figure 6 shows a side view of the radiating patch shown in Figure 6. Figure 7 shows an alternative slot-aperture arrangement. Figure 8 shows a side view of a second embodiment of the antenna element. Figure 9 shows a top view of a dish-shaped conductive cavity supported on a second ground plane. Figure 10 shows an antenna array comprising a plurality of antenna elements of the present invention. Referring to Figures 1-4, the antenna element comprises a printed circuit board 1, on one side of which is a first conductive ground plane 2, and on the other side of which are two symmetrical U-shaped conducting feed track arrays 3 and 4 disposed at right angles to each other, each being electrically symmetric about a bisecting plane. An air bridge 5 is provided where feed track 3 crosses feed track 4. Each feed track includes an input means 6 and 7, and preferably an open circuit stubs 8 and 9, and optional matching tabs 10 and 11. Each electrically symmetric feed track array 3,4 is also physically symmetric except for the air bridge 5 and the bends in the open circuit stubs 8,9. Two orthogonal slot apertures 12 and 13, intersecting at their mid-points are etched in the first conducting ground plane 2. A conductive radiating patch 14 is fixedly spaced from aperture slots 12 and 13 by pillars 15 and 16. A symmetrical conductive cavity 17 is attached to and electrically connected to the first conductive ground plane 2, such that it encloses slot apertures 12 and 13. Alternatively, the symmetrical conductive cavity 17 can be attached in a non-contacting manner to the first conductive ground plane 2 by means of adhesive tape, preferably of the kind that comprises a mounting tape with adhesive material on two opposite sides, such as, for example, Normount (Reg. Trademark) V2830 high performance mounting tape. One side is adhered to an outwardly extending flange (not shown) provided on the rim of the conductive cavity 17, and then the conductive cavity 17 is pressed onto the first conductive ground plane 2 to which it becomes attached by virtue of the adhesive material on the opposite side of the tape. There is sufficient capacitance through the tape to achieve an equivalent of an electrical connection. Signals are fed via transmission lines (not shown) to the input means 6 and 7 of the feed tracks 3 and 4. Optional matching tabs 10 and 11 provide impedance compensation. The input means 6,7 is connected to two transmission lines consisting of parallel arms of the U-shaped feed tracks 3 and 4. The transmission lines extend symmetrically over respective slot apertures 12 and 13. By having feed tracks 3 and 4 on the same side of the printed circuit board 1 as the radiating patch, and opposite the conductive cavity 17 side, the feed tracks 3 and 4 are advantageously accessible for adjustment, and do not require cut-outs in the conductive cavity 17 as with some prior art arrangements in which the feed elements of the antenna are located within the conductive cavity. For maximum coupling of the signal to the radiating aperture slot, maximum signal current should be present in the vicinity of the slot. The open circuit stubs 8 and 9, approximately λ/4 long, ensure a current maximum occurs on the transmission lines at the point where they cross over the aperture slots 12 and 13. The orthogonal aperture slots 12 and 13 are excited by the transmission lines. The radiation from the slots then induces orthogonal currents in the patch 14, which induces orthogonal radiation. Two signals can be radiated from the patch 14 simultaneously with 90° separation in polarisation. The cross-coupling between the signals is less than -25 dB. The aperture slots 12 and 13 radiate to the rear as well as the front of the printed circuit board 1. In an array of antenna elements, the radiation from the rear can couple into another array element, degrading the impedance matching characteristics and the radiation pattern. The conducting cavity 17 contains the rear radiation by enclosing the aperture slots 12 and 13 on the ground plane side of the printed circuit board. The conductive cavity 17 is preferably symmetric in order to maintain good isolation between the two signals. Referring to Figures 5 and 6, an alternative radiating patch arrangement comprises a square-shaped conductive plate 18 having two rectangular troughs 19 and 20, whose respective longitudinal axis are mutually perpendicular and intersect at mutual mid-points. The troughs 19 are interrupted by a central square aperture 21. The troughs 19 could be V-shaped, hemicycle, or any other symmetrical shape. The troughs 19 preferably face towards the aperture slots 12 and 13. The conductive plate 18 and the aperture 21 can be any symmetrical shape. The aperture 21 is optional but can have manufacturing or electrical benefits. The conducting patch 14, 18 can be implemented by attaching it to a radome, thereby removing the need for pillars 15 and 16. The shapes of the aperture slots 12 and 13, conductive cavity 17, feedlines and patch 14,18 could be varied to achieve desired results. Referring to Figure 7, an alternative slot aperture arrangement comprises two pairs of end-loaded slots 22, 22a and 23, 23a, the common longitudinal axes of each pair of slots being mutually perpendicular and intersecting at mutual mid-points. This slot aperture arrangement is preferably used with the radiating patch described in relation to Figures 5 and 6. Referring to Figures 8 and 9, a further embodiment of the antenna element comprises a printed circuit board 1, a first conductive ground plane 2, feed tracks 3 and 4, aperture slots and radiating patch 14 arranged in the same manner as shown in Figure 1, except for the conductive cavity. In this embodiment a second conductive ground plane 24 is supported in a spaced relationship with the first conductive ground plane 2. Interposed between the two conductive ground planes 2 and 24 is a circular dish shaped conductive cavity 25 whose rim 26 is spaced from the first conductive ground plane 2 and capacitively coupled thereto, and whose base is in electrical contact with the second conductive ground plane 24. A conductive frame could substitute the dish-shaped conductor cavity 25. Referring to Figure 10, a typical array of these aforementioned elements is shown on a single printed circuit board. The respective sides of each element of the array are fed separate signals. A signal X is fed to the left hand side of each antenna element, similarly a separate signal Y is fed to the right hand side of each antenna element. Therefore the signals are kept at orthogonal polarisations. The antenna element of the present invention, although primarily used for electronic communications applications, is suitable for use in medical diathermy and microwave heating. A metallic patch of appropriate dimensions could be applied to material to be heated. The patch could be excited by the feed arrangement of the present invention with no physical contact between the patch and the feed arrangement. Such a method may be applied to heating parts of the human body.

Claims: An antenna element for transmitting and/or receiving radio frequency signals that includes dual orthogonally polarised components, which comprises a planar dielectric element supporting on one side thereof a first conductive ground plane element (2) and on an opposite side thereof two substantially identical conductive feed track arrays (3,4) disposed at right angles to each other and each being electrically symmetric about a bisecting plane, characterised in that the first conductive ground plane element (2) has two substantially identical slot aperture arrangements (12,13) each comprising at least one elongate slot of predetermined length, the slot aperture arrangements' (12,13) longitudinal axes are disposed at right angles to each other and cross at their respective mid-points, each slot aperture arrangement (12,13) being symmetrical about a respective bisecting plane bisecting a feed track array (3,4), a symmetrical conductive patch element (14) is disposed in a predetermined spaced relationship with the slot aperture arrangement (12,13) and above the opposite side of the planar dielectric element, and a symmetrical conductive cavity element (17) comprising a bottom wall portion and at least one side wall portion having a rim, is disposed on the first conductive ground plane element (2) and electrically coupled thereto, the cavity element (17) enclosing the elongate slot aperture arrangement (12,13) within the wall portions and a surface portion of the first conductive ground plane element (2) that is proximate the slot aperture arrangement (12,13). An antenna element as claimed in claim 1, characterised in that a second conductive ground plane element (24) is supported in a predetermined spaced relationship with said first conductive ground plane element (2), and wherein a cavity element (25) is interposed between the first conductive ground plane element (2) and the second conductive ground plane element (24), the bottom wall of the cavity element (25) electrically contacting the second conductive ground plane element (24), and a rim (26) of the cavity element's (25) wall portion being capacitively coupled to the first conductive ground plane element (2). An antenna element as claimed in claim 1 or claim 2, characterised in that each said slot aperture arrangement (12,13) comprises a single elongate slot. An antenna element as claimed in claim 1 or claim 2, characterised in that each said slot aperture arrangement (12,13) comprises two collinear end-loaded slot arrangements. An antenna element as claimed in claim 1, characterised in that said rim of said cavity element is attached to said ground plane such that it is in electrical contact therewith. An antenna element as claimed in claim 1, characterised in that said rim of said cavity element is attached to said ground plane by adhesive means such that it is capacitively coupled thereto. An antenna as claimed in claim 1 or claim 2, characterised in that each said feed track array (3,4) comprises U-shaped array including two limbs joined by a base , said limbs crossing an associated slot aperture at right angles, and an input means extending from said base. An antenna element as claimed in claim 7, characterised in that each said limb includes open circuit stub means of a predetermined length, that is located proximate said associated slot aperture. An antenng element as claimed in any one of the preceding claims, characterised in that said patch element (14) comprises a symmetrical conductive plate having two symmetrical shaped troughs whose longitudinal axes are mutually perpendicular and intersect at mutual mid-points. An antenna element as claimed in Claim 9, characterised in that said troughs face said opposite side of said planar dielectric element. An antenna element as claimed in Claim 9 or 10, characterised in that said troughs are interrupted by a central symmetric aperture. An antenna element as claimed in claim 10 or 11, characterised in that said troughs are rectangular- shaped, V-shaped or hemicyclic- shaped. An antenna element as claimed in claim 9 - 12, characterised in that said conductive plate is square-shaped or circular. An antenna element as claimed in Claim 9-13, characterised in that said central symmetric aperture is square shaped or circular. An antenna element as claimed in claimed in claim 2, characterised in that said cavity element (17) is circular. An antenna element as claimed in any one of the preceding claims, characterised in that said planar dielectric element (1) is part of a printed circuit board, said first conductive ground plane and said conductive feed track arrays being conductive layers thereon. An antenna element as claimed in any one of the preceding claims, wherein said patch element (14) forms part of an associated radome element. An antenna array including a plurality of antenna elements as claimed in any one of the preceding claims, operatively coupled together, and including signal input/output means. An antenna array as claimed in claim 18, characterised in that said input/output means are located on said opposite side of the planar dielectric element. An antenna element as claimed in any one of Claims 1-16, characterised in that it forms a heater element in a diathermy machine.

Inventors: HAMILTON S, MCDONALD NOEL, HAMILTON, S., MCDONALD, NOEL
Applicants: CIT ALCATEL, ALCATEL
Application year: 1998
Main classification: H01Q9
IPC codes: H01Q9, H01Q13