WO1999066593A1

WO1999066593A1 - Antenna device

Info

Publication number: WO1999066593A1
Application number: PCT/AU1999/000487
Authority: WO
Inventors: Rodney Bruce Waterhouse; Stephen Donald Targonski
Original assignee: Royal Melbourne Institute Of Technology
Priority date: 1998-06-17
Filing date: 1999-06-17
Publication date: 1999-12-23
Also published as: AUPP415698A0

Abstract

An antenna device (10) having reflector elements (35) disposed behind the ground plane (20) of a printed slot or aperture coupled microstrip antenna (12).

Description

ANTENNA DEVICE

This invention relates to an antenna device.

Two methods commonly used to improve the radiation characteristics of printed slot antennas and aperture coupled microstrip antennas are to place a shielding plane behind the antenna or to enclose the slot or the aperture in a cavity behind the ground plane. Both approaches are for the purpose of eliminating unwanted radiation in the back region. However, the incorporation of a shielding plane allows the propagation of parallel plate waveguide modes. Power transferred into these modes can seriously degrade the efficiency of the antenna, thereby diminishing the benefits gained by incorporation of the shielding plane. Also, the propagation of these parallel plate modes can result in serious degradation of performance in an array environment. Propagation of parallel plate modes can be suppressed by soldering shorting pins between the two planes, however, this is a tedious and expensive manufacturing task. Use of a cavity also eliminates the problem of propagating parallel plate modes, but again at the expense of much greater manufacturing complexity, especially at millimetre wave frequencies.

In one aspect, the invention provides an antenna device comprising a reflective antenna element located behind the ground plane of a printed slot antenna or aperture coupled microstrip antenna. The antenna element may comprise one of a plurality of antenna elements located behind the ground plane.

The reflector device formed by the antenna element or the plurality of antenna elements may be arranged to reduce the radiation levels in the half-space located behind the ground plane. In such case, a more directional radiation pattern may result.

The or each antenna element may be a printed antenna element, being for example printed on a planar concave or convex eg part spherical surface. Other geometries may however be employed. The reflector device may be composed of one or more said elements located on the same or multiple substrates.

The invention is further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is an exploded diagrammatic perspective view of an antenna device constructed in accordance with the invention;

Figures 2(a) and 2(b) are side views of further respective different forms of antenna device constructed in accordance with the invention;

Figure 3 is a graph of relative magnitude and phase of reflector and aperture far fields for an experimental antenna device constructed in accordance with the invention; and

Figure 4 illustrates radiation patterns of a microstripline fed aperture antenna device constructed in accordance with the invention.

The antenna device 10 shown in Figure 1 has an aperture coupled stacked patch microstrip antenna 12 which may, for example, be of conventional construction. Here it is shown as being formed of two substrates 14, 16 with attached director elements 25, and a substrate 18 which has a ground plane 20 and a central slot-like aperture 22. A conventional antenna feed line may be attached to the ground plane 20.

Located behind the antenna 10 is a reflector device 24 formed of three reflector element substrates 26, 28, 30.

Substrates 28, 30 have reflector elements 35 thereon.

All of the directive devices 25, reflector elements 35 and the ground plane may be formed as printed conductive layers on the respective supporting substrates.

In use, the antenna 12 functions generally in the usual fashion. However, parameters of the reflector device 24, including for example the size and shape of the elements thereof, the thickness, shape and relative permittivity of the substrates, are tuned so that the total radiated field of the reflector elements effectively cancels out the radiated field of the aperture 22 ove r a selected area of the back half space of the antenna. This in turns yields antennae with increased directivity and efficiency. The tuning of the elements may be accomplished by the design procedure described herein.

Parasitic printed antenna elements such as described above are typically operated around resonance so that they radiate strongly. The action of the elements in the reflecting system of the invention are normally such that they are intentionally operated well above the first resonance of the element, and this produces a radiated field which is nearly 180 degrees out of phase with the radiated field of the aperture. Then, by properly choosing the parameters of the substrates in the reflecting system, the magnitude of the radiated field of the reflector system can be made approximately equal to that of the aperture over a selected region of the back half-space. This results in a cancellation of the total radiated field in that direction.

Figure 2(a) and 2(b) show exemplary types of geometries that are possible for the reflector device 24. Planar, concave, convex, or spherical substrates and reflector devices 24 may be used, and these substrates may consist of air, foam, or dielectric material. The elements can be of virtually any geometry inasmuch as their relative dimensions are chosen so that they operate in the behaviour described in the previous paragraph. Figure 2(a) shows a convex, part spherical, reflector device 24, and Figure 2(b) a concave, part spherical, reflector device 24.

The analysis of the structure has been performed using a spectral domain moment method model incorporating a multilayer Green's function. Use of this Green's function allows for generality and freedom in the design. The radiated far fields were computed from the stationary phase evaluation of the Green's function for the field components of the elements.

The design process involves examining the relative magnitude and phase of the radiated far fields due to the aperture and reflector. A typical plot of these quantities at backfire (θ = 180°) versus reflector length is shown in Figure 3. The resonance of the reflector is readily seen at a length of 1 1.6 mm, with its radiated field being much stronger than that of the aperture. However, as with a Yagi-Uda array, the reflector is operated above resonance to achieve the desired effect. At a length of 16mm the fields are of approximately equal magnitude and opposite phase, resulting in a cancellation of the total field and a marked improvement in front-to-back ratio. It will be noted from Figure 3 that, as the element length is increased past resonance, the relative magnitude and phase of the far fields remain fairly constant. These results were obtained for a case where the aperture length was 12.5 mm, aperture width 1 mm, reflector width 1 mm and reflector spacing 5.635 mm, and for which the design operating frequency was 9.5 GHz.

The design procedure for specific adaptations of the invention is also relatively straightforward, in that the relative phase is controlled primarily by the resonant length, while the relative magnitude of the fields can be controlled by adjusting the reflector spacing and width. Increasing the reflector spacing will decrease the magnitude of its radiated far field. The same effect can be achieved by decreasing the reflector width, thereby providing an extra degree of freedom in the design.

In order to validate the theory, an example design was fabricated and tested. Referring to Figure 1, directive patches 25 and supporting substrates 14, 16 were left out of this example to show more clearly the effect of the reflector on the radiation pattern. This antenna was therefore of the printed slot type. The design included a single reflector element 35 of length 15.5 mm and width 1 mm. There were also two substrates 26, 28 in the reflecting system. The first substrate 26 had a relative permittivity of 1.05 and thickness of 5 mm, and the second substrate 28 had a relative permittivity of 2.2 and thickness of 0.635 mm. The feed substrate 18 also had a relative permittivity of 2.2 and thickness of 0.635 mm. The aperture 22 had length and width of 12.5 and 1 mm, respectively. Compared to a typical microstrip line fed printed slot antenna of the same dimensions, the addition of the reflective element in this design provided only a reactive shift to the input impedance of the antenna, which was easily tuned out by adjusting the microstrip line stub length.

Computed and measured radiation patterns for this design are shown in Figure 4. In Figure 4, (a) depicts the radiation pattern at the E-plane and (b) depicts the radiation pattern at the H-plane. The reflector was designed to provide a 20dB decrease in the radiation level at backfire. It can be seen that the radiation level increases away from backfire, particularly in the E-plane. This is due to the relative magnitude of the slot and reflector radiation patterns, which differ quite greatly near +90 degrees. However, a distinct change is seen from the bidirectional aperture pattern, with a corresponding increase in front-to-back ratio. Defining front-to-back ratio as the ratio of total radiated power in the two half-spaces, an increase of 7.4dB was computed with the addition of the reflector element. This in turn resulted in a 2.2dB increase in directivity. A similar improvement in front-to-back ratio can be achieved by using a reflector with aperture coupled patch antennas.

Another result of the freedom and simplicity inherent in the design process is that the reflector can be designed for maximum reduction in the radiation level at any angle in the back half-space. This can be achieved by simply adjusting its length, width, and spacing from the aperture. This has implications for use in arrays scanned off broadside, where the maximum reflection can be designed for the desired scan angle.

The use of the proposed reflecting system provides an alternative solution to the problem of reducing unwanted radiation in the back half-space of printed slot antenna and aperture coupled microstrip antennas. The system is easy to manufacture, as it utilises the same well established printed circuit technology that is used in fabrication of microstrip antennas. It also avoids the problem of propagating parallel plate modes and is a low-cost solution relative to the present technology, since inexpensive microwave substrates may be used and manufacturing costs are reduced. These advantages may become more evident at frequencies in excess of 2 Ghz, and it is in this range where the reflecting system may have its prime application.

The principles of the invention are applicable to microstrip antennae and printed slot antennae. A microstrip consists of one or more conductors bonded to or otherwise carried by one or more dielectric substrates. The conductors are usually thin and metallic, and one of the dielectric substrates is in normal practice grounded in use. The conductors typically have a regular shape, for example, rectangular, circular, or elliptical. Signal feeding is often by means of a coaxial probe or a microstrip transmission line, the latter consisting of a conductor spaced from, such as above, an extended conducting surface. Printed slot antennae consist of a radiating element formed by a slot in a grounded conducting surface. Typically this conducting surface is attached to a dielectric substrate. Similarly, a microstrip antenna may be formed as an aperture coupled antenna, wherein an aperture in the grounded conducting surface is used to couple power from the microstrip feed line to one or more microstrip antenna elements.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The described embodiments of the invention have been advanced merely by way of explanation, and many modifications and variations may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

CLAIMS :-

1. An antenna device comprising a reflective antenna element located behind the ground plane of a printed slot antenna or aperture coupled microstrip antenna.

2. An antenna device as claimed in claim 1, wherein the antenna element comprises one of a plurality of antenna elements located behind the ground plane.

3. An antenna device as claimed in claim 1 or claim 2, wherein a reflector device formed by the antenna element or the plurality of antenna elements is arranged to reduce the radiatio n levels in the half-space located behind the ground plane.

4. An antenna device as claimed in any preceding claim, wherein the or each antenna element is a printed antenna element.

5. An antenna device as claimed in claim 4, wherein the or each element is printed on a planar concave or convex surface.

6. An antenna device as claimed in claim 5, wherein said concave or convex surface is part-spherical.

7. An antenna device as claimed in claim 3 or any one of claims 4 to 6 as appended thereto, wherein the reflector device is composed of one or more said elements located on the same or multiple substrates.