Flat plate antenna

Abstract: A dual polarised flat plate antenna (10-38) has a single circular waveguide output (44-48) accommodating probes for both polarisations. A low noise block (58) has a complementary single circular waveguide input (56) and is rotatably attached to the antenna. The low noise block input couples with both probes to feed a common low noise block circuit. Rotation of the low noise block relative to the antenna provides continuously variable polarisation.

Description: This invention relates to a flat plate antenna for receiving polarised r.f.signals. With the advent of direct broadcast satellite (DBS) television services the so-called "flat plate antenna" (FPA) has been developed to provide a low cost, compact, low maintenance, easy to install and relatively unobtrusive antenna for DBS applications. In general such antennas comprise a flat array of receiving elements all connected by a feed network to a common signal output which is usually coupled to a combined down converter and pre-amplifier unit known as a "low noise block" (LNB) affixed to the rear of the FPA. In order to avoid interference between different DBS services, these DBS services transmit signals having similar frequencies but with different polarisations of the r.f. signals, so that a FPA arrayed to receive one DBS transmission will not receive another DBS transmission serving the same (or another) geographical area. This avoids interference between the signals. However, it also means that a single design of antenna cannot be utilised for any required polarisation. It is known to construct a dual polarised FPA having two separate element arrays each having a particular single polarisation, with both arrays being coupled to the LNB. Selection of one of the polarisations is effected by a switchable circuit in the LNB. The present applicants themselves manufacture such a dual polarised antenna with a suitable LNB switchable for reception of DBS signals either from the Astra satellite or the nearby proposed Eutelsat satellite. Such an antenna operates at one or other of two distinct, fixed polarisations, and we have found that this can present a problem because the signal polarisation may not in fact be properly aligned with the element array concerned. The present invention seeks to provide a DBS FPA having a simple construction which overcomes the problem. According to the present invention there is provided a flat plate antenna structure having two separate element arrays in closely spaced parallel relation, the two arrays having respective signal polarisations orthogonal to one another, the antenna structure having a single waveguide output feed, the two arrays each having a coupling into said waveguide output feed, the antenna structure being provided with a low noise block having a waveguide input coupled into the waveguide output feed in such a manner that it is rotatable so that the polarisation angle of the structure can be changed by rotating the block relative to the structure. Embodiments of the invention will now be described with reference to the accompanying drawings, wherein:- Fig. 1 illustrates a sectional side view of a flat plate antenna structure; Fig. 2 illustrates a detail of the FPA of Fig. 1; Fig. 3 illustrates a sectional plan view of the FPA of Figs. 1 and 2; Fig. 4 illustrates a perspective view of the arrangement of coupling probes in the antenna structure of Figs. 1 - 3; Figs. 5a - 5b illustrate the physical arrangement of orthogonal probes in a circular waveguide and their vector relationship respectively, and Fig. 6 illustrates rotation of the low noise block relative to the antenna structure. In the flat plate antenna structure shown in the drawings two thin dielectric films 10, 12, e.g. of polyester, have printed thereon conductor patterns forming probes 14, 16 and respective feed networks 18, 20 for respective first and second element arrays. The films 10, 12 are separated by two foam dielectric sheets 22, 24 sandwiching a metallic middle aperture plate 26. Further foam dielectric sheets 28, 30 space the dielectric films 10, 12 from outer metallic aperture plates 32, 34. The three aperture plates have corresponding arrays of circular apertures which are aligned with one another and with the two intervening arrays of element probes. The probes of the two arrays are orthogonal one to the other and the physical arrangement of the element probes in relation to each other and to the aligned apertures corresponds to the physical arrangement of the probes in the circular waveguide shown in Fig. 5a and to be described later. Suffice it to say that the arrangement of dielectric films, spacing foam dielectric sheets and metallic aperture plates is well known in the art. The front side of the antenna has a comparatively thick, polystyrene spacer sheet 36 laid over the outer aperture plate 32. The rear side of the antenna has a further unapertured metallic reflector plate 38. The antenna is encased in a housing comprising a plastics base 40 and a radome cover 42. The plastics base has formed on the inner surface thereof a number of pillars 54 some of which are detailed to accept self tapping screws and some of which are shouldered at appropriate heights to act as spacing means for the various components in the sandwiched structure. For example, the spacing between the rear aperture plate 34 and the reflector plate 38 is effected by shouldered pillars passing through fixing and locating holes in the plates. The various components are assembled in sandwich form with fixing screws passing through the films and sheets to hold the internal components in secure, aligned and spaced arrangement affixed to the inner side of the base 40. Finally the radome cover 42 is fitted over and secured by adhesive sealing round the edge joint with the base 40. Located at or near the centre of the antenna is a circular waveguide output arrangement consisting of a waveguide skirt 44, two waveguide spacers 46 and a waveguide cover 48. The outer aperture plates 32, 34 have apertures sized to accommodate the waveguide structure, likewise the rear reflector plate 38 and the plastics base 40. The two dielectric films 10, 12 are unapertured and each includes as part of its printed circuit pattern a short length of conductor 50, 52 extending part-way into the circular waveguide space 54. The two lengths of conductor 50, 52 form probes coupling into the waveguide and are arranged orthogonally, as shown in the exploded perspective view of Fig. 4. The waveguide skirt 44 is recessed to allow insertion from outside the housing of a circular waveguide part 56 forming a waveguide input to a low noise block. The waveguide input 56 also has two orthogonally arranged probes 60, 62 (Fig. 4) both of which couple to a common input to a low noise block circuit (not shown). The low noise block 58 is movably attached to the plastics base 40 so that the circular waveguide part 56 when engaged in the recess in the waveguide skirt 44 acts as a pivot, allowing the low noise block to be rotated with respect to the flat plate antenna structure. Fig. 6 shows a rear view of the antenna base 40 with the low noise block 58. The rear surface of the base 40 is contoured to provide a suitable fixing of the low noise block while at the same time allowing the block to be rotated to the base, as indicated by the dotted outlines. Bearing in mind that in general any given antenna structure is useable both to transmit and receive, the variable polarisation coupling between the flat plate antenna and the low noise block is now explained in terms of a transmitting antenna. Fig. 5a shows the E vector in a circular waveguide feed with orthogonal V and H probes. The vector is resolved into V & H components as in figure 5b and each component is conducted in its respective V & H circuit film, being the circuit films 10 and 12 of Fig. 2. At each antenna element the two circuits are brought together in the same aligned configuration as figure 5a and the resultant E vector is as shown in figure 5b. If the electric field vector in the waveguide has a voltage E then the v and h components are:- v = E cos ⊘ h = E sin ⊘ where ⊘ = angle of E from vertical probe. At the aligned antenna elements the resulting transmitted polarisation is given by:- E t = v + h = v² + h² tan⁻¹ h / v (magnitude) (angle) = E(cos²⊘ + sin²⊘) 1/2 at tan⁻¹ E sin⊘ / E cos⊘ = E magnitude in direction of ⊘ i.e. the same as that in the waveguide. Thus, by rotating the LNB, with respect to the FPA the E vector in the waveguide can be rotated and this in turn rotates the element polarisation angle in sympathy. By reciprocity the process works in reverse for a receiving antenna. While the flat plate antenna structure described and shown in the drawings is one having orthogonal probes aligned with simple circular apertures the invention is not restricted to this form of antenna element array. It is equally applicable to other forms of dual polarised antenna element array such as those using radiating patches, non-circular apertures, crossed slots and other well known variants, including elements having capacitive coupled radiating elements. The complete antenna structure with the low noise block 58 attached to the rear plastics base 40 is mounted in conventional manner by means of a bracket 64 secured to the base and adjustably connected to a support member 66 for fixing to a pole or wall bracket (not shown) on a building or other structure.

Claims: A flat plate antenna structure (10-38) having two separate element arrays (14-20) in closely spaced parallel relation, the two arrays having respective signal polarisations orthogonal to one another, the antenna structure having a single waveguide output feed (44-48), the two arrays each having a respective coupling (50-52) into said waveguide output feed, the antenna structure being provided with a low noise block (58) having a waveguide input (56) coupled into the waveguide output feed characterised in that the low noise block is rotatable relative to the antenna structure about a common axis of the waveguide when coupled to the antenna structure so that the polarisation angle of the structure can be adjusted by rotating the block relative to the structure. A flat plate antenna structure according to claim 1 wherein the antenna structure has a single circular waveguide output feed (44-48), the two arrays each have a respective probe (50-52) for coupling into said circular waveguide, the two probes being in orthogonal relationship. An antenna according to claim 2 characterised in that the two element arrays provide respective pairs of orthogonally arranged probes aligned with corresponding apertured metallic plates, the two element arrays and apertured plates being spaced with intervening dielectric. An antenna according to claim 3 characterised in that the intervening dielectric is foamed dielectric sheet which acts as a spacing means. An antenna according to claim 4 characterised in that the two element arrays are in alternating spaced relationship with three apertured plates in a sandwiched construction. An antenna according to claim 3 or claim 4 further including rear reflector metallic plate spaced from the aperture plate or element array nearest the low noise block, the reflector plate being unapertured except to allow access to the antenna waveguide and to make provision for fixing screws and support pillars.

Inventors: TWELVES ALAN, TWELVES, ALAN
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1992
Main classification: H01Q21
IPC codes: H01Q21