INTRODUCTION AND BACKGROUND

This invention relates to a broad band directional antenna and more particularly to a broad band cross polarised antenna.

Broad band cross polarised antennas are of considerable interest due to the large variety of frequencies used in 4G/5G and other communications systems. Broadband type dipole radiators are often arranged above a ground plane reflector surface to achieve a main beam perpendicular to the ground plane surface. This arrangement suffers from frequency limitations, since the ideal spacing for such a radiator is around a quarter wavelength above the reflector surface and which hence causes it to be half a wavelength above the reflector surface for signals having twice such frequency, resulting in destructive interference towards the main beam direction and other pattern irregularities. Metamaterials may be used artificially to delay waves at some frequencies. Hence, positioning a metamaterial ground plane between a radiator and a conductive ground plane may assist in achieving a broader bandwidth. Such assemblies are known, but radiation pattern control (i.e. maintaining the same shape at all frequencies, in other words, maintaining pattern stability) is still problematic over a wide bandwidth. This is due to pseudo surface waves which can exist between the metamaterial ground plane and conductive ground plane and many other undesirable EM interactions, amongst other reasons.

OBJECT OF THE INVENTION Accordingly, it is an object of the present invention to provide a broad band directional antenna with which the applicant believes the aforementioned disadvantages may at least be alleviated or which may provide a useful alternative for the known antennas. SUMMARY OF THE INVENTION

According to the invention there is provided a broad band directional antenna comprising: a conductive ground plane having a main axis extending perpendicularly to the conductive ground plane; - at least one active radiator which is axially spaced from the conductive ground plane in one direction, a metamaterial ground plane assembly comprising: o a metamaterial ground plane having a periphery; o a first conductive wall immediately adjacent the periphery of the metamaterial ground plane, the first conductive wall having a bottom and a top; and o a second wall comprising at least two mutually electrically insulated conductive wall parts located spaced from and outside of the first conductive wall, the metamaterial ground plane assembly being arranged such that the bottom of the first conductive wall is located between the conductive ground plane and the metamaterial ground plane and the top of the conductive first wall is located beyond the at least one active radiator in the one direction; and at least one conductive pillar between the first conductive wall and the conductive ground plane. Shape, dimensions and relative spacing of the conductive ground plane, the at least one active radiator and the metamaterial ground plane assembly are selected to improve antenna bandwidth, pattern consistency or stability and gain. The conductive ground plane and the metamaterial ground plane may have any suitable shape, including a rectangular shape, but preferable a square shape, having four sides.

The first conductive wall preferably is a continuous wall having four sides circumscribing the metamaterial ground plane. The at least one conductive pillar may extend between the bottom of the first conductive wall and a middle of at least one of the sides of the conductive ground plane. In one embodiment, the at least one pillar may comprise at least two pillars extending from a middle of the bottom of at least two sides of the first conductive wall respectively to the middle of two sides of the conductive ground plane. In a preferred embodiment, the at least one conductive pillar comprises four pillars extending respectively from the middle of the bottom of each side of the conductive first wall to the middle of an associated side of the conductive ground plane. The second wall may comprise four electrically insulated conductive wall parts which are respectively located parallel to a corresponding one of the four sides of the first conductive wall.

The at least one active radiator may comprise at least one dipole radiator.

In a preferred embodiment, the at least one active radiator comprises first and second cross polarized dipole radiators, which are driven at respective centre points. The antenna may also comprise at least one passive radiator which is spaced from the at least one active radiator in the one direction.

In the preferred embodiment, the at least one passive radiator is of the same shape and configuration as the at least one active radiator, but smaller in size.

The antenna may also comprise an active patch type radiator having a surface area and which active patch type radiator is axially spaced from the conductive ground plane in a direction opposite the one direction.

The surface area of the active patch type radiator is preferably larger than the surface area of the metamaterial ground plane assembly. An optional passive patch type radiator may be provided between the active patch type radiator and the conductive ground plane.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein: figure 1 is an exploded perspective view of an example embodiment of a broad band directional antenna; figure 2 is a perspective view from below of a conductive ground plane and a metamaterial ground plane assembly of the antenna; figure 3 is a view similar to that of figure 2, partially exploded; figure 4 is a perspective view of cross polarized active radiators of the antenna provided on a top and bottom surface respectively of a substrate; figure 5 is another perspective view of the cross polarized active radiators with the substrate in phantom, for better clarity; figure 6 is perspective view of the antenna in assembled form; figure 7 is a diagrammatic side view of the antenna in the direction C shown in figure 1 ; figure 8 is a section on line VIII in figure 7; figure 9 is a plot of antenna gain against frequency for the example embodiment of the antenna compared to a similar antenna, but adapted to lack conductive pillars between the bottom of a first wall of the metamaterial ground plane assembly and the conductive ground plane of the antenna; figures 10(a) and (b) are polar plots for the antennas in figure 9 for a lower range of frequencies; figures 11 (a) and (b) are also polar plots for the antennas in figure 9 for a higher range of frequencies; figure 12 is a plot of antenna gain against frequency for the example embodiment of the antenna compared to a similar antenna adapted to lack a passive radiator of the example embodiment; and figures 13(a) and (b) are polar diagrams for the antennas in figure 12.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An example embodiment of a broad band directional antenna is generally designated by the reference numeral 10 in figures 1 , 6, 7 and 8.

Referring to figure 1 , the antenna comprises a conductive ground plane 12 having a main axis 14 extending perpendicularly to the conductive ground plane 12. At least one active radiator 13 is axially spaced from the conductive ground plane in one direction A. A metamaterial ground plane assembly 16 has a surface area. The metamaterial ground plane assembly comprises a metamaterial ground plane 17 having a periphery 18. A first conductive wall 20 is located immediately adjacent the periphery 18, such that the first conductive wall 20 abuts the periphery of the metamaterial ground plane 17. The first conductive wall has a bottom 22 and a top 24. A second wall 26 comprising at least two mutually electrically insulated conductive wall parts 26.1 and 26.2 is located spaced from and outside of the first conductive wall 20 relative to the metamaterial ground plane 17. The metamaterial ground plane assembly 16 is arranged such that the bottom 22 of the first conductive wall 20 is located between the conductive ground plane 12 and the metamaterial ground plane 17 and the top 24 of the conductive first wall 20 is located beyond the at least one active radiator 13 in the one direction A. There is provided at least one conductive pillar 28.1 (see figures 2 and 3) between the bottom 22 of the first conductive wall 20 and the conductive ground plane 12.

In the example embodiment, the metamaterial ground plane 17 comprises an electrically insulating substrate 31 and a plurality of mutually spaced rectangular or square conductive pads 33 printed on the substrate in a matrix pattern. Each pad defines a matrix of four holes exposing the underlying substrate. It has been found that a thickness t of the substrate should preferably be as small as possible, without compromising a mechanical strength of the substrate that may be required. A conventional printed circuit board with copper pads may be used.

As will become clearer below, the conductive ground plane 12 and the metamaterial ground plane assembly 16 may have any suitable shape and/or dimensions. However, shape, dimensions and relative spacing of the conductive ground plane 12, the at least one active radiator 13 and the metamaterial ground plane assembly 16 and its constituent parts are selected to improve antenna bandwidth, pattern consistency or stability and gain. In the example embodiment shown, the conductive ground plane 12 is square having four equi-dimensioned sides 12.1 , 12.2, 12.3 and 12.4.

As best shown in figures 2 and 3, the first conductive wall 20 is a continuous wall having four first wall parts 20.1 , 20.2, 20.3 and 20.4 circumscribing the metamaterial ground plane 17. Also as shown in these figures, there is provided a conductive pillar 28.1 between first wall part 20.1 of wall 20 and the middle of corresponding side 12.1 of the conductive ground plane 12. Similarly, there are provided conductive pillars 28.2 to 28.4 between first wall parts 20.2 to 20.4 of wall 20 and the middle of corresponding sides 12.2 to 12.4 of the conductive ground plane

12.

As best shown in figures 1 to 3, the second wall comprises mutually insulated wall parts 26.1 to 26.4. In the example embodiment shown, wall part 26.1 extends parallel to first wall part 20.1 of first wall 20. Similarly, parts 26.2 to 26.4 extend parallel to first wall parts 20.2 to 20.4 respectively. Each of the wall parts 26.1 to 26.4 are secured to the metamaterial ground plane 17 by insulating arms 30.

Referring to figures 1 , 4 and 5, the at least one active radiator 13 comprises first and second cross polarized dipole radiators 13.1 and 13.2 which are driven at respective centre points 32.1 and 32.2. One conductive element of each of the dipoles is provided on a top surface of substrate 34, whereas the other element is provided on a bottom surface of the substrate. Referring to figures 1 and 6, the example embodiment of the antenna 10 comprises at least one passive radiator 36 which is spaced from the at least one active radiator 30 in the one direction A. In a preferred embodiment, the at least one passive radiator is of the same shape and configuration as the at least one active radiator, but smaller in size.

Referring to figures 1, 6, 7 and 8, the example embodiment of antenna 10 comprises an active low frequency patch type radiator 38 having a surface area and which patch type radiator 38 is axially spaced from the conductive ground plane 12 in a direction B opposite the one direction A. The surface area of the patch type radiator 38 is preferably larger than the surface area of the metamaterial ground plane assembly 16. Known feeds for the patch type radiator are shown at 40.

Still referring to figures 1 , 6, 7 and 8, the example embodiment of the antenna 10 may comprise an optional passive patch type radiator 42 which may be provided between the active patch type radiator 38 and the conductive ground plane 12. The example embodiment of the antenna 10 further comprises a known support structure 44 with diplexer 46, which structure is spaced from the patch type radiator 38 in the other or opposite direction B. The example embodiment of the antenna 10 is designed to operate in the frequency band 1 ,7GHz to 3,7GHz.

In figure 9, there is shown a plot of antenna gain against frequency (shown by the solid line) for the example embodiment of the antenna 10 with the conductive pillars 28.1 to 28.4 in position as shown in figures 2 and 3 compared to that (shown in broken lines) of an adapted antenna without such pillars, but with bottom 22 of the first wall 20 in conductive contact with conductive ground plane 12, thereby effectively cavity backing the metamaterial ground plane. The graph clearly indicates a large increase in gain of about 5dB for frequencies below 3,2GHz for the example embodiment of the antenna. The polar diagrams in figures 10(a), 10(b), 11 (a) and 11 (b) also clearly illustrate far more stable radiation patterns for the case in figures 10(a) and fig 11 (a) with the conductive pillars, as opposed to the case in figures 10(b) and 11 (b) with the bottom 22 of wall 20 in direct contact with the conductive ground plane 12. It is believed that the pillars 28.1 to 28.4 serve to suppress pseudo surface waves that propagate on the conductive ground plane 12 and which cause unwanted radiation and thereby negatively affects the radiation pattern. In figure 12, there is shown a plot of antenna gain against frequency

(shown by the solid line in figure 12 for the example embodiment of the antenna 10 compared to that (shown in broken lines) of a similar antenna, but adapted to lack the passive radiator 36. The plot clearly indicates an increase in bandwidth for the antenna with the passive radiator 36. The polar diagrams in figure 13(a) (for the example embodiment of the antenna) and figure 13(b) (for the adapted antenna) also illustrate more stable radiation patterns for the case in figure 13(a) with the radiator 36, as opposed to the case without the radiator in figure 13(b). It has also been found that the parasitic dipole 36 increases the gain by 4 -

5dB in the frequency band 3,4GHz - 3,8GHz.