BROADBAND MICROSTRIP ANTENNAS

Abstract: An antenna assembly is provided which comprises a first laminar structure which includes a sheet of dielectric material having on one side a contiguous metal sheet and on the other side a strip transmission line adapted to be coupled with signal feeding means, and a second laminar structure, one side of which is in contact with the transmission line, and having on the side, at least one region but preferable at least two concen­ trically arranged regions of a coated or cladded metal which serves as a radiator, characterised in that the transmission line is non-symetrically disposed with respect to the radiator.

Description: This invention relates to radar antennas and, more particularly, to microstrip antennas for broadband transmission. GB-A-2064877 discloses a log periodic microstrip antenna which consists of a set or series of laminar conductive element regions and an extensive laminar conductive earth plane region which is parallel to the element regions, and spaced from them. The area of each element region varies with its neighbours by some log periodic progression. A conductive feed strip extends alongside at least one of the element regions in electric field coupling relationship therewith and dielectric material is interposed between said earth plane region and said feed strip. The set of element regions is a straight, extended series of regions which occupies a considerable distance and the ground plane has an area equal to more than the total area of the radiative elements. Such known antennas suffer from the disadvantage that they are large and are not readily amendable for use in portable applications such as ground probing radar for locating buried objects such as non-metallic pipework. GB-A-2007919 discloses a microwave terminating structure. This structure can be used in an array antenna ie. an antenna made up of a large number of radiating structures in this case thirty-six arranged in a 6 x 6 matrix. The disclosure is of a very large antenna, not at all suitable for use in ground probing radar. The disclosure states that the thirty-six antenna elements are arranged over a common conductive sheet and each antenna element requires an aperture in the sheet and for each aperture a cup-shaped element is associated with the conductive sheet with the cup-shaped element co-axial with the aperture and forms therewith a ground "plane". Now, in reality, the ground "plane" is not a plane at all but is a bulky structure requiring thirty-six cup-shaped elements distributed over the matrix already referred to. Each radiative element consists of circular concentric slots, for example three slots. Two feed strips are required for each conductive element in order to produce circularly polarised radiation. The feed strips couple to the slots and, in particular, do not overlie the central circular disc arranged at the centre of the arrangement of circular slots. Furthermore, a common conductive layer surrounds each antenna and has circular openings, the edges of which each form the boundary of an antenna element. The teaching of GB-A-2007919 is very different from the teaching herein. We have found that more compact structures can be produced which take the advantages of microstrip antennas ie. the inherent shielding from transmission or reception in the backward direction and yet are portable. According to the invention, an antenna having the features set out above and to be found in GB-A-2064877 is characterised in that said element region as viewed in plan are mutually concentric, are of progressively decreasing external dimensions and lie within the outline of said earth plane region, each said element region except the largest lying within the outline of the largest and the or each element region in said field coupling relationship with said feed strip being divided thereby non-symmetrically as viewed in said plan. Any element region not coupled to the feed strip may be conductively connected to a region that is coupled to the feed strip. The element regions may be coplanar or, alternatively, they may be mutually parallel, spaced apart and decrease in area in the direction away from said earth plane region. The invention will be illustrated by reference to the accompanying drawings, in which: Figure 1 is a vertical section through a first embodiement of antenna; Figure 2 is a plane of two coplanar element regions provided in the antenna shown in Figure 1; Figures 3(a), 3(b) and 3(c) are plans corresponding to Figure 2 showing alternative positions of feed strips; Figures 4(a), 4(b) and 4(c) are scrap views showing alternative forms of feed strips; Figure 5 is a plan corresponding to Figure 2 but showing three coplanar element regions suitable for a modified antenna; and Figure 6 is a vertical section through a further embodiment of antenna showing multilaminate structure. Referring to Figures 1 and 2 of the drawings, a typical antenna was constructed as follows:- All circuits are made in etched copper film mounted on 1.6 mm glass-reinforced plastics (GRP) boards, whose relative permitivity is 4.7. The feed line 2 was a conventional microstrip transmission line of width 2.5 mm, and was mounted in or on a GRP board 1 1, (Fig. 1) approximately 30 x 30 0cm. A continuous metal film 3 was present on the back of the board. On the top of the board 1 is found a conventional microstrip transmission line 2. Its impedance was measured as approximately 75 ohm and the velocity of propagation along it measured as 0.55C, where C is the velocity of light (3 x 10⁸ ms⁻¹). The signal was introduced to the line through a SMA-style microstrip connector (not shown) mounted with its axis perpendicular to the plane of the board. A like connector at the other end of the stripline carried a 50 ohm load. On a metal coated GPR board 4 of dimensions 21 cm x 21 cm a gap 7 of 1.0 mm was etched to define two regions (Figure 2). The inner region 5 was a 10 x 10 cm square and was surrounded by a concentric region 6 whose outer edges were 14.5 cm. There was no metal backing to the board. The two boards 1 and 4 were clamped together with a film of petroleum jelly between them to aid dielectric continuity. Short wires were soldered at A, B and C so as to give electrical continuity. The performance of the antenna varied depending on the positioning of the pattern relative to the stripline below it. Useful configurations are shown in figures 3(a), (b) (c). Two identical antennas were produced, one used as transmitter and one as receiver. Transmission was observed to occur at 550 MHz and 760 MHz. These frequencies corresponded to those at which the overall length (14.7 cm) and the length of the inner rectangle (10 cm) corresponded to a half-wavelength, taking account of the dielectric slowing properties of the substrate. Thus the frequency response of structure 3(c) (550 MHz) could be extended through the addition of a second passband at 760 MHz by the use of structure 3(c). (Structure 3B had a response at 760 MHz with no appreciable transmission at 550 MHz). It was also observed that if the connection at Y was removed then the structure still radiated at two frequencies, but these were now 480 MHz and 870 MHz, with a smaller response at 760 MHz. In addition to all the results described above there were the harmonics (multiples) at higher frequencies. The power of the method of coupling of the input signal by fields rather than by direct connection, as in conventional microstrip 'patch' antennas, is that the feeding transmission line can itself be adjusted in its properties. For example, it need not be straight, it could divide so as to feed several parts of the radiator at once, it could include frequency sensitive components such as filters or directional couplers. Examples are illustrated in figures 4(a), (b) (c). For an extended passband the sections into which the antenna is divided are suitably formed. For example, the width of the transmission peaks observed experimentally was approximately 10% of the centre frequency. Thus, if the ratio of successive sections is approximately 5% the passbands will merge, and the total number of sections will determine the overall bandwidth. In a further example (Fig. 5), the upper GRP board was configurated to provide three regions 8.9,10. Metallic links were soldered at X,X',X'', so that the region 8 was conductively coupled to the region 9, which in turn was coupled to feeding transmission line shown at 21. The antenna was observed to transmit in frequency bands (of width between 50 and 100 MHz) centered on 550 MHz, 700 MHz and 950 MHz, which approximately correspond to the frequencies at which the length of each rectangle is a half-wavelength. Figure 6 illustrates the multilaminate structure arrangement. In this embodiment, the upper GRP board is provided as a stacked layer of boards 14, 15, 16. In alternative interlayers are a plurality of radiators 11, 12, 13, whose sizes conform to a log periodic progression, and the transmission strip 2.

Claims: An antenna comprising a laminar conductive earth plane region (3), laminar conductive element regions (5, 6; 8, 9, 10; 11, 12, 13) parallel to said earth plane region (3) and spaced from it, a conductive feed strip (2;21) extending alongside at least one of said element regions (5, 6; 8, 9, 10; 11, 12, 13) in electric field coupling relationship therewith and dielectric material (1) interposed between said earth plane region (3) and said feed strip (2, 21) and dielectric material (4; 14, 15, 16, 17, 18) interposed between said feed strip (2, 21) and said element regions (5, 6; 8, 9, 10; 11, 12, 13) characterised in that said element regions (5, 6; 8, 9, 10; 11, 12, 13) as viewed in plan are mutually concentric, are of progressively decreasing external dimensions and lie within the outline of said earth plane region (3), each said element region (5, 6; 8, 9, 10; 11, 12, 13) except the largest lying within the outline of the largest and the or each element region (5, 6; 8, 9, 10; 11, 12, 13) in said field coupling relationship with said feed strip (2; 21) being divided thereby non-symmetrically as viewed in said plan. An antenna according to claim 1, any element region (5; 8) not coupled to the feed strip (2; 21) being conductively connected to a region (6; 9) that is coupled to the feed strip (2). An antenna according to claim 1 or claim 2, said element regions (5, 6; 8, 9, 10) being coplanar. An antenna according to claim 1 or claim 2, said element regions (11, 12, 13) being mutually parallel, spaced apart and decreasing in area in the direction away from said earth plane region(3). An antenna according to claim 3, said element regions (5, 6; 8, 9, 10) comprising a square region (5; 8) and one or more encircling regions (6; 9, 10) each bounded by an outer square boundary, the inner boundary of each encircling region (6; 9, 10) being spaced from the outer boundary of the adjacent inner region (5; 8, 9) by a gap (7) which is uniform throughout itself. An antenna according to claim 4, each element region (11, 12, 13) being square. An antenna according to claim 6, the areas of said element regions (11, 12, 13) conforming to a log periodic progression. An antenna according to claim 4, or claim 6 or claim 7, said feed strip (2) extending in zig-zag manner and having runs interleaved between said element regions (11, 12, 13). An antenna according to any claim of claims 1 to 8, said feed strip (2) comprising a branch or branches.

Inventors: GUNTON, DAVID JOHN
Applicants: BRITISH GAS PLC
Application year: 1986
Main classification: H01Q9
IPC codes: H01Q9