EP0378905A1

EP0378905A1 - Slot-coupled patch antenna and phased-array antenna arrangement incorporating such an antenna

Info

Publication number: EP0378905A1
Application number: EP89312573A
Authority: EP
Inventors: Kevin Richard Howard; Gary Patrick Stafford
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-12-16
Filing date: 1989-12-01
Publication date: 1990-07-25
Also published as: GB2226703A; GB8829446D0

Abstract

An antenna having the basic structure of a resonant slot antenna, but with a patch element (6) connected along one edge of the slot (2), and inclined at an angle relative to the ground plane (4), Coupling between a stripline feed (3) and the patch element (6) is achieved by means of the resonant slot (2), the feed element (3)/resonant slot (2)/patch element (6) transitions being impedance matched at the operative frequency. The antenna has a radiation characteristic consistent with conventional flat patch antennas, but provides a higher gain and greater bandwidth. In phased array applications, the inclination of the patch element (6) allows a smaller separation between adjacent antenna elements than is possible with conventional designs. The antenna is suitable for use in a missile-approach radar.

Description

This invention relates to antennas, and in particular to patch antennas of the form of a conductive patch, usually rectangular, overlying a ground plane and connected to it along one edge of the patch to form a section of line short-circuited at one end and providing a radiating open-circuit other end.
The conventional tri-plate patch antenna uses a probe feed, that is, a cable, to connect the patch to a stripline. This type of feed reduces the efficiency of the antenna as the inductive nature of the cable causes energy to be reflected back into the stripline. This feed inductance also contributes to limiting the patch bandwidth to typically 3% at 2:1 VSWR. Although attempts have been made to avoid the need for a probe feed by incorporating the stripline on the top of the patch substrate in a stripline configuration, thereby allowing direct connection to the patch, this configuration does not significantly improve the antenna bandwidth. The performance of patch antennas in this configuration is further degraded by the tendency of the feed network itself to radiate, which causes ripples on the main beam pattern or high sidelobe levels on an array pattern.
It is an object of the present invention to provide an antenna with a radiation pattern substantially consistent with existing patch antennas, but offering a higher gain and greater bandwidth.
It is another object of the invention to provide an antenna which can be built into a phased array with a smaller spacing between the patch elements than has previously been achieved, without significant distortion of the element patterns.
According to the present invention, an antenna comprises a first conductive sheet having a resonant slot, a patch element connected to the first sheet along one side of the slot and extending away from the first sheet at an acute angle, a second conductive sheet substantially parallel to and spaced from the first sheet on the side remote from the patch element, and a conductive feed element positioned between said first and second sheets, the arrangement being such that, in operation, energy is coupled between the feed element and the patch element by means of the resonant slot. The feed element is preferably spaced from said first and second sheets by dielectric material.
The antenna may include means connecting the first and second sheets to form a half-wave resonant cavity centred on said slot.
The patch element of the antenna may be encapsulated in dielectric material to provide an aerodynamic surface substantially parallel to the first sheet.
The antenna may be incorporated into a phased array arrangement comprising a plurality of the antennas arranged in a row with their resonant slots mutually parallel. In this arrangement the first sheets and the second sheets are preferably continuous throughout the array. The first and second sheets may also be curved about an axis parallel to the row of antennas to conform to the curved surface of an aircraft body. The phased array arrangement may be encapsulated in dielectric material so that encapsulation of each patch element provides a continuous aerodynamic surface extending throughout the array. The individual antennas constituting the array may be spaced apart by an amount which would cause a degraded performance in an equivalent array having antenna patches parallel to the second conductive sheet and fed conventionally.
A patch antenna in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 shows a known form of resonant slot antenna;
Figure 2(a) shows in plan view a conventional patch antenna,
and Figure 2(b) shows a section on line AA of Figure 2(a);
Figure 3(a) shows in plan view an antenna in accordance with the invention,
and Figure 3(b) shows a section on line BB of Figure 3(a);
Figure 4 shows the antenna beam characteristic in azimuth;
Figure 5 shows a similar characteristic in elevation;
Figure 6 shows the return loss/frequency characteristic of the antenna transmission coupling;
Figure 7 shows the corresponding reflection phase/frequency characteristic;
Figure 8(a) and 8(b) show schematically a phased array antenna arrangement constructed from antenna elements of the type shown in Figures 3(a) and 3(b);
Figure 9 shows the beam characteristic in azimuth of the array, and
Figure 10 shows the beam characteristic in azimuth of the antenna after encapsulation of the patch element in a dielectric material.

Referring to the drawings, Figure 1 shows a standard resonant slot antenna, comprising a conductive sheet 1 containing the slot 2, a stripline feed 3 lying beneath the sheet 1 and arranged to feed the slot 2, and a lower ground plane sheet 4. A dielectric substrate 5 separates the sheets 1 and 4, and also serves to support the stripline feed 3. It will be appreciated that, for the purpose of clarity, in this diagram, and the subsequent Figures 2,3, and 8, the thickness of the conductive sheets and dielectric substrate has been exaggerated relative to the overall size of the antennas. The length of the slot S is chosen so that it acts as a dipole antenna fed from the centre by the stripline. Thus, the slot length S usually corresponds to a half-wavelength at the operative frequency of the antenna which may typically be in a range of several gigahertz to several tens of gigahertz, although the invention is not in fact limited to such a range. A good coupling between the slot 2 and the stripline 3 is obtained when the stripline extends beyond the centre of the slot by a distance equal to a quarter-wavelength. A major advantage of this antenna is its thin, conformal shape which makes it suitable, for example, for securing to the surface of an aircraft body. The bandwidth is also good, typically as much as 5%. However, usefulness of the antenna is limited in this application by its narrow beam pattern which typically extends to a maximum of only 60° either side of a broadside normal through the slot.
Figures 2(a) and 2(b) show a conventional patch antenna which has a similar flat construction. A dielectric substrate 5 separates the radiating patch element 6 from its ground plane 4. A short-circuit is formed at one edge 7 of the patch element by connecting it through the substrate 5 to the ground plane 4, by, for example, a row of shorting pins 8. The length L of the patch is chosen so that, at the operative frequency, the edge 9 of the patch, opposite the short-circuit at 7, constitutes an open-circuit, allowing the signal energy to be maximal at this point. The patch length L is commonly somewhat less than a quarter-wavelength for this purpose, the actual length being dependent on the choice of the substrate material 5. The signal is fed to the patch by means of a stripline (not shown) between the patch 6 and the ground plane 4, as in the resonant slot antenna of Figure 1, except that a direct connection is made between the patch and the stripline by a short cable or probe (also not shown). This form of antenna produces a much broader beam than the resonant slot, but has a very narrow bandwidth, typically 2 or 3%, partly due to the inductive nature of the probe.
An improved method of coupling a stripline to a patch element has been proposed by David M. Pozar ("A Microstrip Antenna Aperture Coupled to a Microstrip Line", Electronic Letters, 1985, 21 pp.49-50). Essentially the method involves exchanging the relative positions of the stripline and the ground plane, so that the ground plane lies between the patch element and the feed line. In this way, the ground plane acts as a screen to prevent direct radiation from the feed line. In place of a direct connection between the stripline and the patch element, coupling is achieved by means of a small aperture in the ground plane, maximum coupling occurring between the stripline and the patch when the aperture is centred under the patch. Since the coupling is directly between the stripline and the patch, the aperture merely provides a necessary break in the ground plane to allow energy transfer from the stripline to the patch.
The present invention concerns an antenna which combines desirable features of both the resonant slot antenna and the patch antenna. Referring now to Figures 3(a) and 3(b), which show one embodiment of the invention, it will be seen that the antenna has a standard tri-plate substrate structure comprising a conductive sheet 1, having a resonant slot 2, a stripline feed 3, and a ground plane sheet 4, the two conductive sheets 1 and 4 being separated from the stripline 3 and from each other by two layers of dielectric material 5. The dielectric material may be RT Duroid 5880. The slot length S corresponds to a half-wavelength at the operative frequency of the antenna. The patch element 6 is connected along one edge 10 of the slot and is inclined at an acute angle relative to the sheet 1 so as to be over the slot. The patch may be secured in this position by soldering or welding it to the sheet 1. Two rows of shorting pins 11, spaced a distance D apart, connect the two sheets 1 and 4 together to form a resonant cavity. The separation D is equivalent to a half-wavelength at the operative frequency. The half-wave resonant cavity so formed serves to centre the peak of the electric field in the region of the slot. The sheets 1 and 4 are also connected together at the edges of the antenna (not shown).
The signal from the stripline feed 3 is coupled into the slot 2 in the same way as in the resonant slot antenna. It would be expected that an antenna constructed in this way would have substantially the same radiation pattern as the standard resonant slot antenna, but with the patch element acting to deflect the centre of the narrow beam away from its normal broadside position to some acute angle relative to the plane of the antenna. However, it is found that the slot itself, instead of behaving like the final radiating element of the antenna, directly couples its resonant energy into the patch element. By matching the impedance of the slot to that of the patch element, the slot transfers most of its energy to the field below the patch. The width W of the patch 6 is made approximately 67% greater than the length S of the resonant slot 2 to minimise the effect of energy radiating from the slot interacting with that of the patch, which results in ripples on the radiation patterns. The patch element thus becomes the significant radiating element, and the antenna has a radiation pattern substantially consistent with that of a conventional 'flat' patch antenna, but having a higher efficiency than the probe fed patch. The absence of an inductive feed and the screening effect of the sheet 1 in reducing direct radiation by the stripline 3, lead to an improved gain and a greater bandwidth. The arrangement has been shown to produce a gain of up to 8dB and a 2:1 VSWR bandwidth of 10% has been achieved. Figures 4 and 5 show the antenna beam characteristics in E-plane (azimuth) and H-plane (elevation) respectively. It is seen that the antenna provides the typical broad beam pattern that is characteristic of conventional patch antennas. Figures 6 and 7 show the improved return loss and reflection phase/frequency performance of the antenna transmission coupling. The return loss over a band 5% either side of the operative frequency f_o is better than -10dB.
The angle of the patch relative to the ground plane is chosen to provide the optimum coupling, that is, the best match between the slot and the patch. If the angle is made too small, the radiated energy tends to be coupled into the ground plane, whereas if the angle is made too large the resonant slot tends to become the dominant radiating element, reducing the strength of the much broader beam of the patch. Although in the embodiment just described the slot lies beneath the patch element, the antenna could be constructed with the slot exposed. However, this arrangement would considerably weaken the coupling between the slot and the patch element, and the impedance matching would be more difficult to achieve. The slot would also become a significant source of radiation, reducing the strength of the patch element beam.
Figures 8(a) and 8(b) show a phased array antenna built up of individual antenna elements of the type shown in Figures 3(a) and 3(b). Although, for clarity only four such elements are shown, it will be appreciated that a practical array is likely to have ten or more elements per row. The shorting pins (11 in Figure 3(a)) have also been omitted for clarity only. The ground plane 4 and the conductive sheet 1 containing the resonant slots 2 are continuous throughout the array. The stripline feeds 3 to the individual patches 6 include a right-angled bend so that each feed emerges essentially parallel to its associated resonant slot.
The feature whereby the patch is inclined relative to the ground plane means that in phased array applications the minimum inter-element spacing can be less than that needed in an array of flat patches. Experiment has shown that individual element patterns are not significantly distorted in arrays with an element separation of one third of the operative wavelength. Figure 9 shows the beam characteristic in azimuth of an array using this amount of inter-element separation. The smaller separation of the patch elements has the benefit of reducing the size and the weight of the array.
The antenna, whether as a single element or an array, offers a further advantage in its ease of construction over existing patch antennas in that the patch element needs only to be soldered or welded along the short-circuit edge. Conventional patch elements require the insertion of shorting pins or the formation of plated-through holes through the dielectric substrate to the ground plane. The shorting pins or plated-through holes have an associated shunt susceptance whose effect is to degrade the quality of the short-circuit at the patch edge (7 in Figure 2) which necessitates an adjustment to the design length of the patch.
In order to facilitate incorporation of the antenna, or an array of the antennas, into an aircraft or missile body, the antenna may be constructed so that the sheets 1 and 4 (and any intervening dielectric) are curved about an axis in the plane of, but transverse to, the resonant slot 2. This form of construction may require that the patch element 6 be similarly curved in order to maintain contact with the sheet 1 at the short-circuit edge 10 of the patch (Figure 3).
Further, the exterior face of the antenna, including the space between the patch element 6 and the sheet 1 may be encased in a dielectric material of relatively low dielectric constant. A suitable material is P10 foam which has a dielectric constant of 1.5. Figure 10 shows the radiation pattern of an antenna encased in this foam. The material serves three distinct functions : it provides a smooth aerodynamic external surface suitable for inclusion in an aircraft body or missile radome panel; it acts as a supporting medium for the patch; and it decreases the effective resonant wavelength of the patch, allowing the size of the antenna to be reduced, a significant advantage in any airborne application.

Claims

1. An antenna comprising a first conductive sheet (1) having a resonant slot (2), a second conductive sheet (4) substantially parallel to and spaced from said first sheet (1) and a conductive feed element (3) positioned between said first and second sheets (1,4), characterised by having a patch element (6) connected to said first sheet (1) along one side of said slot 2 on the face of the first sheet (1) remote from said second sheet (4) and extending away from the first sheet (1) at an acute angle, the arrangement being such that, in operation, energy is coupled between said feed element (3) and said patch element (6) by means of said resonant slot (2).

2. An antenna according to Claim 1, wherein said feed element (3) is spaced from said first and second sheets (1,4) by dielectric material (5).

3. An antenna according to Claim 1 or Claim 2, including means (11) connecting said first and second sheets (1,4) to form a half-wave resonant cavity centred on said slot (2).

4. An antenna according to any preceding claim, wherein said patch element (6) is encapsulated in dielectric material providing an aerodynamic surface substantially parallel to said first sheet (1).

5. A phased array antenna arrangement comprising a plurality of antennas according to any preceding claim, the antennas being arranged in a row with the resonant slots (2) of the antennas mutually parallel.

6. A phased array antenna arrangement according to Claim 5, wherein said first sheets (1) and said second sheets (4) are respectively continuous throughout the array.

7. A phased array antenna arrangement according to Claim 5 or Claim 6, wherein said first (1) and second sheets (4) are curved about an axis parallel to said row of antennas to conform to the curved surface of an aircraft body.

8. A phased array antenna arrangement according to any of Claims 5 to 7, as appendent to Claim 4, wherein the encapsulation of each patch element (6) and said aerodynamic surface extend continuously throughout the array.