ARRAY ANTENNA

Abstract: A radiating element comprises a groundplane, a dielectric layer and a conductive pattern comprising a folded dipole (20, 21), a feed line (18, 19) for the dipole and a plurality of closely spaced directors (22), all lying in a common plane parallel to the ground plane. An array of such elements with a suitable feed network provides a flat antenna with a squinted beam. The squint angle can be adjusted by adjusting the phase delay between columns of elements. The beam can be steered by selection of the appropriate squint angle and by rotationally adjusting the antenna in its own plane. The antenna is suitable as a less obtrusive alternative to a dish antenna.

Description: ARRAY ANTENNA The present invention relates to an antenna consisting of an array of dipole radiating elements. Although, for convenience, much of the description and explanation of the invention will employ terms appropriate to transmission, it will be appreciated that this is only a matter of convenience. Antennae and radiating elements are reciprocal devices and may be used in transmission mode and in reception mode as desired. It is well known to employ an array of elements which are individually not very directional to create an antenna which is highly directional. If the array is linear, the antenna beam is fan-shaped. If the array is two-dimensional, the beam is a pencil beam. The narrowness of the beam and hence the antenna gain are influenced in particular by the number of elements in the array. Although not limited to any particular application, the invention has been conceived in the context of a particular problem, namely the provision of a receiving antenna for a DBS (direct broadcasting by satellite) receiver. Attention is currently concentrated mainly upon parabolic dish antennae for this purpose. Such antennae are large in all three dimensions and of inelegant appearance: their proliferation In residential areas will seriously degrade the environment. There exists a need for an antenna which does not suffer from these defects and which is also of a more inherently robust construction than a dish antenna with its struts supporting a feed-horn. An array antenna offers the advantage of a robust construction but for DBS usage it is necessary to achieve a very high gain and make suitable provision for aiming the antenna at the desired geostationary satellite. If this were to be done purely by physical positioning (as with a dish antenna), the advantage of a flat, unobtrusive construction is largely lost. What is required is to be able to mount the antenna flat on a suitable wall or possibly roof surface. Moreover, the superficial dimensions of the antenna must be within reasonable bounds if it is to be possible to find suitable mounting areas, say no more than around lm on the side or diameter. Nevertheless, it must be possible to pack in a large number of elements to get adequate gain which demands that the elements themselves be compact. The object of the present invention is to provide an array antenna such as to meet the requirements outlined above. According to the invention, there is provided an array antenna, comprising an array of dipoles formed in a microstrip structure having a dielectric layer sandwiched between a groundplane and a radiating conductive pattern, characterised in that each dipole has a plurality of parasitic elements adjacent thereto, all parasitic , elements lying with the dipoles in a front plane parallel to the groundplane, so as to squint the main beam of the array. Each dipole is preferably a folded dipole because the higher impedance of such a dipole facilitates design of a feed network. The parasitic elements could be reflectors but are preferably directors, for reasons explained below. - It will be appreciated that a radiating element formed by a dioole and adjacent parasitic elements will necessarily have an asymmetrical radiation pattern relative to the normal to the groundplane, because tbe parasitic elements are spaced laterally from the dipole, rather than in the direction of the boresight axis, as is the case with conventional aerials employing parasitic elements. This Is not a disadvantage in the array antenna according to the invention. It is well known that the beam of an array antenna can be steered electrically by adjusting the phases with which the elements of the array are fed - a so-called phased array. Although two- coordinate steering is theoretically possible, only one-coordinate steering is really practicable. In an important development of the invention, the beam of the antenna is aimed in a required look- direction by electrical beam-steering to vary the angle of squint of the beam and rotational adjustment of the antenna in the plane of the array. This makes it possible to mount the antenna flat against a suitable surface, which dictates the plane of the array, but nevertheless aim the beam anywhere within a cone of solid angles symmetrically disposed relative to the normal to the array. The electrical beam-steering may provide only coarse steering, e.g. by 5° increments. In this case the exact angle of the beam relative to the normal to the mounting surface is established by a slight tilt of the antenna relative to this surface. Since this tilt need not exceed 2.5°, the departure from truly flat mounting is insignificant. A particular embodiment of the invention has been developed for use as a DBS antenna operating at 11.9GHz, at which frequency a wavelength is around 2.5 cm. Investigations showed that the pitch of the elements should be one wavelength in the direction of the dipoles but only 0.55 wavelength In the direction perpendicular to the dipoles. This yields a highly directional array with about 400 elements In the dipole direction and about 700 elements in the orthogonal direction, taken to be the column and row directions respectively. The elements of a column are all co-phased but the phase delay from column to column is adjusted to achieve the desired squint, which is the angle 4> in spherical polar coordinates centered on the normal to the array. The rotational adjustment of the array In Its own plane is the angle θ. Since the pitch along a row is only 0.55 wavelength it is necessary to be able to space the parasitic elements extremely closely to the dipole and to each other. It has been found possible to get five director elements in a space of only 0.1 wavelength. With such a close spacing the array is an array with supergain. With less than five elements the input impedance of an element was found to change too rapidly with frequency. As it is, the element has a bandwidth of only around 4% but this is adequate for its intended purpose. The antenna is linearly polarised. Signals broadcast from a DBS satellite are circularly polarised. In the interests of efficiency and having regard to the fact that the plane of polarisation will be arbitrarily dictated by the θ angle selected for beam-steering purposes, it is desirable to dispose a polarisation converter (circular to linear, parallel to the dipoles) in front of the array of radiating elements. The Invention will be described in more detail by way of example, with reference to the accompanying drawings, in which: Fig 1 illustrates beam-steering with an antenna according to the invention: Fig 2 is a schematic front view of an antenna embodying the invention, illustrating the electrical principles involved; Fig 3 Is a front view of one radiating element of the antenna, and Fig 4 is a view like Fig 2 showing (very diagrammatically) a microwave lens used to determine the column-to-column phase delay. In Fig 1 the rectangle 10 represents a wall with a generally southerly aspect on which is mounted a flat plate antenna 11 shown in full lines in an upright disposition (with the dipoles extending vertically) and defining horizontal and vertical coordinate axes X and Y in the plane of the wall and a horizontal axis Z normal to the plane of the wall. A vector OA is drawn from the centre of the antenna, parallel to the Z axis to the centre of a circle 12 with a horizontal diameter 13. A vector OB is drawn to a point B on this horizontal diameter 13, making an angle ^, with the vector OA. The vector OB represents the squinted boresight axis of the antenna when the columns of dipoles are driven with a given phase shift between columns of elements. By adjusting the phase from column to column of the dipoles it is possible, in well known manner, to modify the look direction of the antenna and vector OC, making a larger angle « with the vector OA, represents an adjusted, more highly squinted look direction for the antenna. By rotating the antenna 11 in its own plane anticlockwise through an angle θ, to the position shown in broken lines, the vector OC is rotated into the vector 0D which represents the desired look direction for the antenna, towards a geostationary satellite. It will be appreciated that, by rotating the antenna in its own plane, any desired look direction intersecting the circle 12 can be chosen. This applies at each possible value of the squint angle 0« so that it is possible to achieve any desired look direction within a substantial cone of solid angles symmetrical about the Z axis. Fig 2 is a highly symbolized representation of the antenna, in the upright position. For simplicity only a 5 by 5 array of dipoles 14 is shown. Each column of dipoles is fed off a vertical feeder 15 and, since the dipoles are spaced vertically by one wavelength, the dipoles in each column are all co-phased. The vertical feeders 15 are fed from a common feed 16 with phase delay devices 17 interposed to adjust the column to column phase delay so as to achieve the desired squint angle 0~. Fig 2 is not intended to indicate the physical form of the feeders or the dipoles and the parasitic elements employed in the present invention are not shown. However, Fig 3 shows one radiating element of the array In detail. The element has been designed by a mixture of modelling and empirical methods to suit a frequency around 11.9 GHz. The element is a microstrip element comprising a dielectric layer sandwiched between a groundplane and a radiating conductive pattern lying in a front plane parallel to the groundplane. It is the said conductive pattern which is shown in Fig 3. In a specific construction the conductive pattern is formed on a Kapton substrate 0.05 mm thick and the dielectric layer is microwave foam 7.2 mm thick, i.e. the conductive pattern is spaced 7.2 mm from the groundplane. Other dielectric materials may be used (e.g. PTFE) but microwave foam has the advantages of low cost and a relatively low loss feed structure. Turning now to the conductive pattern itself, a 200 ohm balanced feed line comprises two tracks 18 approximately 0.4 mm wide. The feed line is coupled to the dipole by a short length (1.9 mm) of 400 ohm line formed by narrower (0.2 mm) tracks 19, used to match out the imaginary component of the input impedance of the element. This technique only works over a narrow bandwidth but is satisfactory in an antenna intended for DBS use where the required bandwidth need be only 4%. The folded dipole Itself consists of back elements 20 0.2 mm wide and a front element 21 0.4 mm wide. The overall length of the dipole Is 10.4 mm. Adjacent the front element 21 are five directors 22 0.2 mm wide and spaced from each other and from the front element 21 by 0.3 mm. The director elements 22 have a length of 8.8 mm. The feed network for the antenna can utilise a 50 ohm unbalanced coaxial line connected to a 50 ohm unbalanced microstrip line which is coupled to the balanced 200 ohm line by means of a balun introducing a 4:1 impedance transformation. Such a balun can consist of a half wavelength of microstrip line. The unbalanced microstrip line has an upper groundplane spaced 1.6 mm above the feed line by a second layer of microwave foam. The upper groundplane does not extend near the radiating elements themselves. A radiating element utilising the conductive pattern of Fig 3 has been extensively tested and exhibited a satisfactory input impedance, an absolute gain of between 8 d3i and 9 dBi and satisfactory co- and cross-polar radiation patterns. The co-polar radiation patterns exhibited the required element shaping in the H plane and a dipole pattern in the E plane. The cross-polar radiation level in the E plane was fairly high off-broadside but this would not be important in an array antenna because broadside, is the wanted direction of the main beam in this plane. The phase delay devices may comprise a microwave lens 25 (Fig 4) mounted at the back of the array and distributing energy to the different columns via array ports 26, with different path-length phase delays so as to establish the required squint angle. The lens has a plurality of beam ports 27, each corresponding to a different squint angle and the common feed 16 is coupled to that port 27 which gives the required squint angle. Since this arrangement will only allow coarse adjustment of the squint angle, fine adjustment is completed by slight tilting of the antenna 11 (Fig 1) relative to the mounting surface 10.

Claims: CLAIMS 1. An array antenna, comprising an array of dipoles formed in a microstrip structure having a dielectric layer sandwiched between a groundplane and a radiating conductive pattern, characterised in that each dipole has a plurality of parasitic elements adjacent thereto, all parasitic elements lying with the dipoles in a front plane parallel to the groundplane, so as to squint the main beam of the array. 2. An array antenna according to claim 1, wherein each dipole is a folded dipole. 3. An array antenna according to claim 1 or 2, wherein the parasitic elements are director elements. 4. An array antenna according to claim 3, wherein the director elements are so closely spaced that the array operates with supergain. 5. An array antenna according to claim 4, wherein each dipole has five adjacent director elements within a space not exceeding one tenth of a wavelength at the operating frequency of the dipole. 6. An array antenna according to any of claims 1 to 5, wherein the dipoles and parasitic elements are formed by conductive deposits on an insulating film supported on the dielectric layer. 7. An array antenna according to any of claims 1 to 6, wherein the dielectric layer Is a microwave foam layer. 8. An array antenna according to any of claims 1 to 7, wherein the dipoles are fed by a microstrip balanced line feeder. 9. An array antenna according to claim 8, wherein the microstrip balanced line feeder is coupled to each dipole by a short length of a higher impedance balanced line. 10. An array antenna according to any preceding claim, wherein the elements are aligned in columns and comprising a feed network which includes phase delay means establishing delays from column to column such as to adjust the squint angle of the antenna. 11. An array antenna according to claim 10, wherein the phase delay means comprises a microwave lens having array ports coupled to respective columns of elements, beam ports corresponding to different squint angles of the antenna, and means for coupling a common feed to a selected one of the beam ports. 12. A method of using an array antenna according to claim 10, wherein the antenna is mounted flat against a supporting surface and is aimed at a signal source by selecting the squint angle and orientation of the antenna within its own plane. 13. A method of using an array antenna according to claim 11, wherein the antenna is mounted substantially flat against a supporting surface and is aimed at a signal source by selecting one of the beam ports, thereby to effect coarse selection of the squint angle, by adjusting the tilt of the antenna relative to the supporting surface to effect fine adjustment of the squint angle relative to the normal to the supporting surface, and by adjusting the orientation of the antenna within its own phase. 14. A method of aiming an array antenna at a predetermined point In space, the antenna comprising a plurality of radiating elements aligned in adjacent linear arrays and a feed network between the linear arrays including means for establishing an adjustable phase delay from linear array to linear array, so as to determine a squint angle of the antenna, the method comprising the steps of selecting the squint angle and rotating the antenna within its own plane. 15. A method according to claim 14, wherein the means for establishing an adjustable phase delay is a microwave lens. 16. A method according to claim 14 or 15, wherein the means for establishing an adjustable phase delay effect coarse selection only of the squint angle and fine selection is effected by adjusting the tilt of the antenna.

Inventors: MADDOCKS MARK CHARLES DOBELL, MADDOCKS, MARK, CHARLES, DOBELL
Applicants: BRITISH BROADCASTING CORP, BRITISH BROADCASTING CORPORATION
Application year: 1987
Main classification: H01Q21
IPC codes: H01Q9, H01Q19, H01Q21, H01Q25