IMPROVEMENTS IN OR RELATING TO MICROSTRIP ANTENNAS

Abstract: The bandwidth of a microstrip circular patch antenna (1) of approximately half-wavelength diameter at the resonant frequency is increased by surrounding it with an annular conducting sheet (6) of approximately quarter-wavelength width separated from the patch (1) by a gap (5) and having its outer edge connected to the ground-plane (3). The resulting structure is particularly suitable for feeding a reflector antenna.

Description: IMPROVEMENTS IN OR RELATING To MICROSTRIP ANTENNAS This invention relates to microstrip antennas comprising a dielectric substrate having a conducting groundplane on one face and a conducting sheet radiator on its other face coupled to a feeding arrangement. The invention has a principal application to such antennas where the radiator is a circular patch or disc approximately half a wavelength in diameter at its resonant frequency, enabling the bandwidth thereof to be substantially increased. The antenna thus formed is particularly suitable for feeding circular reflectors having small focal-length/ diameter (F/D) ratios, eg F/D = 0.3, and which require a low-cost, lightweight, low-profile, simple feed structure, instead of using eg horn feeds. A further advantage in such applications is the low axial ratio obtained, ie the maximum variation in signal amplitude over 3600 polar co-ordinates, which is important where circular polarisation is used. According to the present invention a microstrip antenna comprises: a dielectric substrate having a conducting sheet radiator on a first face thereof and a conducting ground-plane on its second face, said radiator being dimensioned to be resonant at the operating frequency and having feed means connected to the radiator; and a closed annular conducting sheet on said first face surrounding said radiator and having its inner edge spaced by a gap from the edge of said radiator to provide capacitative coupling across the gap between the respective edges; the outer edge of said annular sheet being connected to said ground-plane and said annular sheet being dimensioned to be resonantly energisable at said operating frequency via said capacitative coupling. In one form, where the conducting sheet radiator is dimensioned to function as a half-wave resonator at the operating frequency, the annular sheet is preferably of such width as to function as a quarter-wave resonator. In a preferred embodiment the sheet radiator on the first taco i4 circular and has a diameter of approximately oneth #f the. wavelength in the microstrip at the aFerAtZng frequency, sa¯iQ annular sheet having a width of approximately approximately of said wavelength. The invention also provides a reflector antenna comprising a circular reflector, preferably of parabolic form, having the preferred embodiment as aforesaid located substantially atits focus to provide a feed. To enable the nature of the present invention to be more readily understood, attention is directedtby way of example, to the accompanying drawings, wherein: Fig l is a sectional elevation of an antenna embodying the present invention. Fig 2 is plan view of the antenna of Fig 1. Fig 3 is a graph showing the return loss of a simple circular microstrip antenna. Fig 4 is a graph showing the effect of modifying the antenna of Fig 3 in accordance with the present invention. Fig 5 is a graph showing the co-polar and cross-polar radiation patterns of the embodiment of Fig 4 in the E- and Hplanes. Fig 6 is a graph showing the co-polar and cross-polar radiation patterns of a further example of the embodiment of Figs 1 and 2 in the E- and H- planes. Fig 7 is a graph showing patterns as in Fig 6 but for the two diagonal (450) planes. Figs 1 and 2 show an antenna comprising a circular disc 1 of metallisation located centrally on a disc 2 of dielectric material backed by a conducting ground-plane 3. Separated by a uniform gap 5 from disc 1 is an annular ring 6 of metallisation whose outer edge extends round the edge of disc 2 to join the ground-plane 3. The disc 1 is connected to a coaxial feeder whose inner conductor 7 extends through disc 2, and whose outer conductor 8 is connected to the ground-plane 3. It is not essential for the outer edge of ring 6 to make continuous contact with the ground-plane 3 as shown, eg a ring of spaced pins extending through the dielectric material can be used, as will be apparent to those familiar with microstrip antennas. The diameter of the disc 1 is approximately Am/2 at the operating frequency (where Xm is the wavelength in the microstrip structure thus formed) so that the disc functions as a resonant radiator in a known manner, and the position of connection of conductor 7 to disc 1 is adjusted to match the antenna and feeder impedances at this frequency, as likewise known. The width of ring 6 is made approximately Xm/4, this width and the width of gap 5 being adjusted experimentally to give the structure QptiWU bandwith Figs 3-6 show results obtained with an antenna having the following dimensions etc: Disc 1 diameter 19.5mm Ring 6 width 10.0mm Gap 5 width 2.75mm Disc 2 thickness 3.18mm Disc 2 relative permittivity 2.52 Feeder 7,8 impedance 50 ohms Operating frequency N 5 GHz Fig 3 shows the return loss of the antenna in the absence of ring 6, ie ring 1 alone, and Fig 4 shows the effect of adding the ring. The substantial increase in bandwidth (at -10 dB) in the latter case is clearly seen. Fig 5 shows the co-polar radiation pattern in both the E- and H-planes about boresight (00). The antenna is seen to have equal beam-widths in both planes at very wide angles from boresight (eg + 600). The low levels of crosspolarisation obtained ( < -20dB) are also shown. The width of the gap 5 is not critical and the optimum width is readily found by experiment. In the above example it was found that the stated width could be considerably increased without serious deterioration in performance, but could not be much reduced. In a further example of the invention, the foregoing dimensions were unchanged except that the ring 6 width was 9mm and the gap 5 width 2.25mm. The centre frequency was 5.21 GHz. The coaxial feeder 7,8 was offset 0.33 of disc 1 diameter from its centre to obtain a 50 ohm match at resonance as opposed to 0.2 of disc diameter for the disc in isolation, ie without the ring 6. Measurements of the antenna amplitude and phase patterns were made in the principal (E- and H-) and diagonal (450) planes at band-edge and centre frequencies, using improved measuring techniques. As in the earlierdescribed messurements, the antenna was not mounted on a large ground-plane conventionally used for microstrip patch antenna measurements. Figs 6 and 7 show the measured amplitude patterns at band centre, in the principal and diagonal planes respectively, for an antenna suitable for feeding a prime focus fed reflector (ie having its feed located on-axis at its focal point) with FID - 0.3. This corresponds to a bandwidth at the standard -lOdB level of 1600. 6 is again the conventional polar co-ordinate. The patterns show good circular symmetry and cross-polarisation generally below -25 dB within the arc subtended by the reflector, although a maximum cross-polarisation of -22 dB occurs at the edge of the reflector arc. Good circular symmetry is also observed for patterns obtained at the band-edge frequencies with crosspolarisation levels below -21 dB as shown in Table 1, which is a comparison of maximum cross-polarisation levels in both principal and diagonal planes within arc o = + 800. EMI6.1 <tb> <SEP> Cros4 <SEP> X <SEP> <tb> Frequency <SEP> Disc <SEP> 1 <SEP> + <SEP> ring <SEP> 6 <SEP> Isolated <SEP> disc <SEP> 1 <SEP> <tb> <SEP> GHz <SEP> dB <tb> <SEP> 5.00 <SEP> -21 <SEP> -20 <tb> <SEP> 5.21 <SEP> -22 <SEP> -16 <tb> <SEP> 5.44 <SEP> -22 <SEP> -14 <tb> TABLE 1 The minimum variation in phase occurred for a phase centre located on-axis 4mm from the centre of disc 1. The maximum phase error at this position was < 150, with most of the error occurring at the edge of the reflector arc. Table 1 also compares the cross-polarisation level of the present antenna with that of an isolated disc 1 operating at the same frequency and on a ground-plane equal to the ring 6 outer diameter. The radiation patterns for the isolated disc showed good circular symmetry for small ground-plane sizes, but with H-plane cross-polarisation > - 20 dB for angles > 25 from boresight (0 ) which arises from diffraction from the edges of the ground-plane and overmoding in the disc. Table 1 indicates that the addition of ring 6 exerts considerable control of the sources of cross-polarisation, giving reduced levels within the arc subtended by the reflector. EMI7.1 <tb> Gap <SEP> 5 <SEP> -lOdB <SEP> return <SEP> loss <SEP> Gain <SEP> at <SEP> band <SEP> edges <SEP> with <tb> width <SEP> bandwidth <SEP> respect <SEP> to <SEP> peak <SEP> gain# <SEP> dB <tb> <SEP> Lower- <SEP> Upper <tb> <SEP> irim <SEP> <SEP> edge <SEP> edge <tb> <SEP> frequency <SEP> <SEP> frequency <tb> <SEP> 2.5 <SEP> 6 <SEP> -0.5 <SEP> -0.5 <tb> <SEP> 2 <SEP> 25 <SEP> 8-9 <SEP> -1 <SEP> <SEP> -l <SEP> <tb> <SEP> 2.0 <SEP> 10 <SEP> -1 <SEP> <SEP> -3 <tb> TABLE 2 Table 2 shows the results of bandwidth and approximate gain fall-off for different values of gap 5 width. For convenience the gap 5 widths were achieved by changing the disc 1 diameter which resulted in a 10% variation in frequency, but the latter was not considered to affect significantly the bandwidth and gain results. The accuracy of gain measurement was approximately + 0.5 dB. Bandwidths up to and greater than 108 were obtainable, but with some reduction in input return loss (not shown in Table 2) and a significant fall-off in gain at the upper band-edge frequency. The input return loss could not be greatly improved by repositioning the coaxial feeder. The increase in bandwidth is due to an additional resonance mode close to the fundamental mode, and it is considered that losses in this mode account for the reduction in gain at the higher frequency. These results obtained with the further example confirm the improved performance over that of an isolated disc and its particular suitability, as stated, for feeding reflectors, with small F/D, which require a low-cost, lightweight, lowprofile simple feed structure. The X m/4 ring can also be applied to circularly polarised circular resonant radiators, eg energised with a 900 phase difference at points on two orthogonal radii, where, as stated, the low axial ratio obtained is particularly valuable. The invention may also be applicable to other than circular half-wave resonant sheet radiators, eg to those of elliptical shape.

Claims: CLAIMS 1. A microstrip antenna comprising: a dielectric substrate having a conducting sheet radiator on a first face thereof and a conducting groundplane on its second face, said radiator being dimensioned to be resonant at the operating frequency and having feed means connected to the radiator; and a closed annular conducting sheet on said first face surrounding said radiator and having its inner edge spaced by a gap from the edge of said radiator to provide capacitative coupling across the gap between the respective edges; the outer edge of said annular sheet being dimensioned to be resonantly energisable at said operating frequency via said capacitative coupling. 2. An antenna as claimed in claim 1 wherein the conducting sheet radiator is dimensioned to function as a half-wave resonator at the operating frequency and the annular sheet is of such width as to function as a quarter-wave resonator. 3. An antenna as claimed in claim 2 wherein the conducting sheet radiator is circular and has a diameter of approximately one-half wavelength at the operating frequency, said annular sheet having a width of approximately one-quarter said wavelength. 4. A reflector antenna comprising a circular reflector having an antenna as claimed in claim 3 located substantially at its focus to provide a feed. 5. A reflector antenna as claimed in claim 4 wherein said reflector is parabolic.

Inventors: HALL, PETER SCOTT, PRIOR, CHRISTOPHER JOHN
Applicants: THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNIC MAJESTY'S GOVERNMENT OF THE UNITED KINGDOM OF GREAT BRITAIN AND
Application year: 1985
Main classification: H01Q9
IPC codes: H01Q13, H01Q9, H01Q19