Patent Publication Number: US-2004056818-A1

Title: Dual polarised antenna

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a dual polarised antenna.  
       DESCRIPTION OF RELATED ART  
       [0002] U.S. Pat. No. 6,023,244 describes a microstrip antenna with a metal frame arranged on the ground plane in order to control the size and direction of the antenna lobe. The walls forming the metal frame are electrically interconnected at least along one edge, which forms a line along the metal frame.  
       [0003] U.S. Pat No. 6,028,563 describes a dual polarised antenna with an isolation device. Various types of isolation device are described, including an isolation tree or bar, isolation rails, thin isolation rods or wires, or an isolation strip.  
       [0004] WO98/36472 describes a dual polarised antenna with sidewalls in various alternative configurations.  
       [0005] WO02/50953 describes a dual polarised antenna including a pair of side walls running along the length of the antenna, and isolation devices (either rods or walls) between the antenna elements. Rectangular tabs project inwardly towards the antenna elements from the top edges of the side walls.  
       [0006] U.S. Pat. No. 6,072,439 describes a dual polarised antenna with a pair of C-shaped side walls, which are fastened to a backplane by screws.  
       [0007] It is an object of the invention to overcome deficiencies of the prior art.  
       BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0008] The preferred embodiments provide a dual polarised antenna including two or more modules arranged along an antenna axis, each module including a radiating element, and four conductive walls which together form a frame around the radiating element, each wall including a distal edge and a pair of side edges, wherein the side edges of the walls meet, but do not touch, at respective comers of the frame.  
       [0009] In order to minimize intermodulation problems, it is important that there is either complete contact between the edges of the walls, or no contact. It has been recognized that repeatable performance and simple construction results from providing no contact between the edges of the walls.  
       [0010] Typically at least part of the pair of side edges of each wall taper inwardly towards a respective distal edge. In one embodiment the walls taper along their entire length: in another the walls are parallel in a proximal portion of the walls, and taper in a distal portion.  
       [0011] Typically the walls each have a proximal base, which together form an outer border, wherein the distal edges of the walls together form an inner border, and wherein the area within the inner border is smaller than the area within the outer border.  
       [0012] The side edges of the walls may be separated at the corners of the frame by respective air gaps, or insulating fillers may be provided.  
       [0013] The radiating element may be a patch element, or any other element such as a dipole.  
       [0014] The preferred embodiments also provide a dual polarised antenna including two or more radiating elements arranged along an antenna axis; a plurality of conductive side walls running substantially parallel with the antenna axis; and one or more conductive wall assemblies, each wall assembly being arranged between a pair of radiating elements, wherein the or each wall assembly is substantially U-shaped as viewed in a cross-section taken along the antenna axis.  
       [0015] The U-shaped wall assembly can be easily installed on an antenna tray by coupling the base of the “U” to the antenna tray, for instance by way of attachment pins.  
       [0016] The preferred embodiments also provide a dual polarised patch antenna including two or more radiating patch elements arranged along an antenna axis, each radiating patch element having first and second opposite side edges which run substantially parallel with the antenna axis; a plurality of conductive side walls running substantially parallel with the antenna axis; and one or more conductive isolating elements, each isolating element being arranged between a pair of radiating patch elements.  
       [0017] The isolating element(s) may be in any form, such as a conductive wall running substantially transverse to the antenna axis, an axial strip and/or a transverse strip.  
       [0018] A dual polarised antenna including two or more radiating patch elements arranged along an antenna axis; one or more axial isolating elements, each axial isolating element being arranged between a pair of radiating patch elements and including an elongate conductive member arranged with its length substantially parallel with the antenna axis; and one or more transverse isolating elements, each transverse isolating element being arranged between a pair of radiating patch elements and including an elongate conductive member arranged with its length substantially transverse with the antenna axis, wherein the axial and transverse isolating elements do not touch each other.  
       [0019] The preferred embodiments also provide a dual polarised antenna including two or more radiating elements arranged along an antenna axis; and a plurality of planar conductive side walls running substantially parallel with the antenna axis, wherein each side wall has a projection extending from an upper edge of the side wall, each projection being positioned opposite to a centre of a respective radiating element, and wherein each projection lies in the plane of its respective side wall.  
       [0020] The preferred embodiments also provide a dual polarised antenna including a radiating element; a ground plane; and a pair of conductive walls arranged on opposite sides of the radiating element, each wall subtending an acute angle with the ground plane, wherein the ground plane and conductive walls are formed from a single sheet of conductive material. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0021] The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given below, serve to explain the principles of the invention.  
     [0022]FIGS. 1 a  and  1   b  together show an exploded isometric view of a panel antenna;  
     [0023]FIG. 2 is a side view of the antenna;  
     [0024]FIG. 3 is a plan view of the antenna;  
     [0025]FIG. 4 is an enlarged side view of the part of the antenna encircled in FIG. 2;  
     [0026]FIG. 5 a  is a top view of a PCB;  
     [0027]FIG. 5 b  is a underside view of a PCB;  
     [0028]FIG. 6 is a perspective view of an alternative panel antenna;  
     [0029]FIG. 7 is a plan view of the antenna of FIG. 6; and  
     [0030]FIG. 8 is a side view of the antenna of FIG. 6. 
    
    
     [0031] A panel antenna  1  (dual slant, ±45° polarisation) is shown in FIGS.  1  to  3 . Certain elements of the finished article (for instance the radome) are omitted from the figures for clarity. In addition, the antenna is shown in FIGS. 1 a  and  1   b  in a horizontal orientation, although in use the antenna is generally oriented vertically with the patches directed outwardly. The terms “upper”, “lower” etc will be used with reference to the orientation of FIG. 1. FIG. 1 a  shows the upper items of the antenna in exploded isometric form, and FIG. 1 b  shows the remaining lower items of the antenna in exploded isometric form. Patch tray  10  is shown in both FIGS. 1 a  and  1   b  to provide a link between the two figures.  
     [0032] Four upper patch elements  2  are mounted on lower patch elements  3  by insulating spacers  4 . The upper patch elements  2  are formed from square sheets with cut-off corners to provide the eight-sided shape shown. The lower patch elements  3  are provided with circular central apertures  5 , and are mounted on patch tray  10  via insulating spacers  11 . The base of the patch tray  10  acts as a ground plane.  
     [0033] A pair of end walls  12  are mounted at opposite ends of the patch tray  10 , and three double wall assemblies  13  are provided, each positioned between a respective pair of patch assemblies. The double wall assemblies  13  are generally U-shaped in cross-section and have a base  14 , and a pair of outwardly angled walls  16  which lie transverse to the antenna axis. The walls  12 , 16  have angled side edges (one of which is labelled  25  in FIGS. 3 and 4) and taper from a relatively broad base to a relatively narrow distal upper edge  26 .  
     [0034] The patch tray  10  is formed from a single planar sheet of aluminium, which is folded at the edge to form side walls  30  with tabs  32 . Four sets of apertures  31  are punched out of the base of the patch tray  10 , aligned with each patch assembly. Off-cuts are provided between the side walls  30  so as to provide angled side edges (one of which is labelled  27  in FIGS. 3 and 4). Thus the side walls  30  taper from a relatively broad base to a relatively narrow distal upper edge  28 . The rectangular tabs  32  project from the upper edges  28  and lie in the same plane as the side walls  30 .  
     [0035] Thus the side walls  30  and transverse walls  12 , 16  substantially enclose the patch assemblies in a tapered box-like frame, as shown most clearly in the plan view of FIG. 3. However, the walls do not completely enclose the patch assemblies. In particular, the side edges  25 , 27  of the walls at the corner of the square frame do not touch. The air gap  42  between the edges  25 , 27  tapers slightly as shown in FIGS. 3 and 4. Alternatively the edges  25 , 27  may be parallel. However, it is important that there is no contact between the edges  25 , 27 . Insulating spacers may be mounted in the air gap  42  if desired, although preferably the air gap  42  is left open.  
     [0036] The sheet is not completely cut-away between the side walls  30 . Instead, a low side wall  29  is left between the side walls  30 . The side wall  29  provides mechanical stiffening of the patch tray  10  to minimise flexing.  
     [0037] The apertures  31  are arranged in a cross-configuration with the apertures oriented at± 45 °to the antenna axis. This causes the patches to emit and receive radiation with the orientation of the dominant components of the electric fields being at ±45° to the antenna axis.  
     [0038] Four printed circuit boards (PCB&#39;s)  40  are mounted on the rear of the patch tray  10 . Four shields  50  are mounted to the underside of PCB&#39;s  40 . The shields direct radiation from apertures  31  towards the patch assemblies.  
     [0039] A phase shifter support  60  is mounted to a back tray  70  by clamp members  71 . A phase shifter assembly  61  is mounted to the rear of the phase shifter support  60 .  
     [0040] Isolating assemblies are positioned between each patch assembly, each isolating assembly comprising a transverse strip  20  and an axial strip  21 . The isolating assemblies are mounted by insulating studs  33 ,  34  shown in detail in FIG. 4. The studs are omitted from FIGS.  1 - 3  for clarity. The axial strips  21  have downwardly bent arms  22 , which are mounted on walls  16  by insulating studs  33 , which pass through apertures in the arms  22  and walls  16 . The transverse strips  20  have arms  23  (shown in the plan view of FIG. 3) which are mounted to the base  14  of the double wall assembly  13 . An additional insulating stud  34  mounts the longitudinal strip  21  on the transverse strip  23 . The stud  34  passes through apertures in the strips  21 , 23 . The isolating assemblies improve the isolation between the +45° and the −45° antenna ports.  
     [0041] The angle between the inner faces of transverse walls  12  and  16 , and the base of tray  10 , is approximately 56°. The angle between the side walls  30 , and the base of tray  10 , is approximately 58°. The upper edges  26  of transverse walls  16  are higher above ground plane  10  than the upper edges  28  of side walls  30 .  
     [0042] It can be seen from the plan view of FIG. 3 that the distal edges  25 , 28  of the walls form a rectangular inner border around the patch assembly. The bases of the walls (where the walls join the ground plane) form a rectangular outer border. The area within the inner border is smaller than the area within the outer border. By reducing the area within the inner border (for instance by changing the angle or size of the walls) and increasing the size of the aperture  5 , the 3 dB beam width can be increased, possibly up to 120°.  
     [0043] A PCB  40  is shown in detail in FIGS. 5 a  and  5   b . FIG. 5 b  shows the lower side of the PCB (that is, the side opposite to the patches) and FIG. 5 a  shows the upper side of the PCB (that is, the same side as the patches). The PCB comprises a Taconic® substrate with copper layers on both faces. The copper on the lower face is etched away to leave the microstrip feedline network shown in FIG. 5 b . Feedline  45  is coupled to feedlines  46 , 47  via junction  48 . Feedline  52  is coupled to feedlines  53 , 54  via junction  55 . The copper on the upper face is etched away in four regions to form “dumb-bell” shaped slots  56  shown in FIG. 5 a . The upper face carries a thin layer of adhesive dielectric, which adheres the PCB  40  to the base of the patch tray  10 , providing a capacitive connection between the copper on the upper face of the PCB, and the aluminium patch tray  10 . The slots  56  are positioned in line with the apertures  31  in the base of the patch tray  10 . The feedlines  46 , 47 , 53 , 54  pass across the central stem portion of the “dumb-bell” shaped slots  56 . Thus the feedlines couple energy into slots  56 , which in turn couple the energy into apertures  31 , which in turn couple the energy to the upper and lower patches  2 , 3 . The slots  56  (and thus the patches  2 , 3 ) radiate energy in two polarisations at 45 degrees to the antenna axis.  
     [0044] All parts of the antenna are formed from aluminium except for the PCB.  
     [0045] The off-cuts between the side walls  30  improve the isolation. In particular, the off-cuts enable energy to radiate outwardly, thus preventing radiation from one set of apertures  31  from coupling with a neighbouring patch assembly. The off-cuts also enable the U-shaped assemblies  13  to be inserted easily.  
     [0046] The walls  16  between the patch assemblies improve the isolation.  
     [0047] It will be noted that the patches  2 , 3  have sides, which lie parallel with the side walls  30 . This provides two advantages in comparison with conventional antennas in which the square patches are oriented with their diagonals aligned with the antenna axis (see for instance WO02/50953). Firstly, the antenna has a greater beamwidth because the dimension of the radiating element in the horizontal plane (perpendicular to the antenna axis) is lower. Secondly, the relatively constant gap between the sides of the patches and the side walls results in improved repeatability of the electrical performance of the antenna.  
     [0048] The antenna is designed to operate in the 800 to 960 MHz frequency band, although it could be designed to work in other frequency bands. Isolation between polarisations is more than 30 dB for electrical downtilt angles from 1° to 10°. The height of the antenna above the base of the patch tray 10 is 60 mm. This is a lower profile than for any other existing radiator with a 90° 3 dB beamwidth.  
     [0049] FIGS.  6  to  8  show an alternative antenna. Reference numerals are repeated for identical elements shown in FIGS.  1  to  4 . The side walls have upright proximal portions  80  folded at right angles to the ground plane with parallel side edges  81 . The side walls also have distal portions  82  folded parallel to the ground plane, with side edges  83 , which taper toward distal edges  84 . The transverse walls are similar in construction, with upright proximal portions  90  folded at right angles to the ground plane with parallel side edges  91 , and distal portions  92  folded parallel to the ground plane with side edges  93  which taper toward distal edges  94 . A small parallel-sided air gap is left between edges  81 , 91  and edges  83 , 93 .  
     [0050] The walls of FIGS.  6  to  8  are easier to construct than the angled walls of FIGS.  1  to  6 . However a disadvantage is that more material is required for a given beamwidth.  
     [0051] The axial strips  21  of FIGS.  1  to  6  are replaced by axial strips  100  with downwardly bent arms  101 . The arms  101  are mounted on proximal portions  90  of the transverse walls by insulating studs (not shown), which pass through apertures in the arms  101  and wall portion  90 .  
     [0052] In an alternative embodiment (not shown) the arms  101  may be omitted and the axial strips  100  mounted on the distal wall portions  92 .  
     [0053] The drawings refer to an antenna with  4  radiating elements. However, any number of elements may be used, for instance  8 .  
     [0054] While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant&#39;s general inventive concept.