WO2005091430A2

WO2005091430A2 - Dielectric antenna with metallised walls

Info

Publication number: WO2005091430A2
Application number: PCT/GB2005/000963
Authority: WO
Inventors: Tim John Palmer
Original assignee: Antenova Limited
Priority date: 2004-03-16
Filing date: 2005-03-15
Publication date: 2005-09-29
Also published as: GB2412246A; GB2412246B; WO2005091430A3; GB0405857D0

Abstract

There is disclosed a dielectric antenna comprising a dielectric substrate having first and second opposed surfaces, the second surface being provided with a conductive groundplane and a dielectric pellet being mounted on or adjacent to the first surface. A feed mechanism is formed on or in the substrate for transferring energy to or from the dielectric pellet. The dielectric pellet has a top surface distal from the first surface of the substrate, a bottom surface proximal to the first surface of the substrate, a front surface facing towards a central part of the first surface of the substrate, a rear surface facing away from the central part of the first surface of the substrate and two side surfaces. The top surface and rear surface only of the dielectric pellet are provided with an electrically conductive coating or layer. The antenna is highly compact yet still offers good gain and bandwidth.

Description

DIELECTRIC ANTENNA WITH METALLISED WALLS

The present invention relates to a dielectric antenna that is smaller than previous designs but still has good gain and bandwidth. The electrically small size allows the antenna to be used for low frequency applications such as operations in the 800 and 900MHz GSM bands. Embodiments of the invention include dielectric antennas that are substantially rectangular in shape, but alternative shapes may be employed. Furthermore, embodiments of the present invention relate both to dielectric resonator antennas (DRAs) which comprise a dielectric pellet having a full groundplane or grounded substrate beneath the pellet, but also to high dielectric antennas (HDAs) which have a partial groundplane or no groundplane at all.

Dielectric antennas are antenna devices that radiate or receive radio waves at a chosen frequency of transmission and reception, as used in for example in mobile telecommunications.

The present applicant has conducted wide-ranging research in the field of dielectric antennas, and the following nomenclature will be used in the application:

High Dielectric Antenna (HDA): Any antenna making use of dielectric components either as resonators or in order to modify the response of a conductive radiator.

The class of HDAs is then subdivided into the following:

a) Dielectrically Loaded Antenna (DLA): An antenna in which a traditional, electrically conductive radiating element is encased in or located adjacent to a dielectric material (generally a solid dielectric material) that modifies the resonance characteristics of the conductive radiating element. Generally speaking, encasing a conductive radiating element in a solid dielectric material allows the use of a shorter or smaller radiating element for any given set of operating characteristics. In a DLA, there is only a trivial displacement current generated in the dielectric material, and it is the conductive element that acts as the radiator, not the dielectric material. DLAs generally have a well- defined and narrowband frequency response. b) Dielectric Resonator Antenna (DRA): An antenna in which a dielectric material (generally a solid, but could be a liquid or in some cases a gas) is provided on top of a conductive groundplane, and to which energy is fed by way of a probe feed, an aperture feed or a direct feed (e.g. a microstrip feedline). Since the first systematic study of DRAs in 1983 [LONG, S.A., McALLISTER, M.W., and SHEN, L.C.: "The Resonant Cylindrical Dielectric Cavity Antenna", IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R.K. and BHARTIA, P.: "Dielectric Resonator Antennas - A Review and General Design Relations for Resonant Frequency and Bandwidth", International Journal of Microwave and Millimetre-Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247]. A summary of some more recent developments can be found in PETOSA, A., ITTIPIBOON, A., ANTAR, Y.M.M., ROSCOE, D., and CUHACI, M.: "Recent advances in Dielectric-Resonator Antenna Technology", IEEE Antennas and Propagation Magazine, 1998, 40, (3), pp 35 - 48. DRAs are characterised by a deep, well-defined resonant frequency, although they tend to have broader bandwidth than DLAs. It is possible to broaden the frequency response somewhat by providing an air gap between the dielectric resonator material and the conductive groundplane. In a DRA, it is the dielectric material that acts as the primary radiator, this being due to non- trivial displacement currents generated in the dielectric by the feed.

c) Broadband Dielectric Antenna (BDA): Similar to a DRA, but with little or no conductive groundplane. BDAs have a less well-defined frequency response than DRAs, and are therefore excellent for broadband applications since they operate over a wider range of frequencies. Again, in a BDA, it is the dielectric material that acts as the primary radiator, not the feed. Generally speaking, the dielectric material in a BDA can take a wide range of shapes, these not being as restricted as for a DRA. Indeed, any arbitrary dielectric shape can be made to radiate in a BDA, and this can be useful when trying to design the antenna to be conformal to its casing.

d) Dielectrically Excited Antenna (DEA): A new type of antenna developed by the present applicant in which a DRA, BDA or DLA is used to excite an electrically conductive radiator. DEAs are well suited to multi-band operation, since the DRA, BDA or DLA can act as an antenna in one band and the conductive radiator can operate in a different band. DEAs are similar to DLAs in that the primary radiator is a conductive component (such as a copper dipole or patch), but unlike DLAs they have no directly connected feed mechanism. DEAs are parasitic conducting antennas that are excited by a nearby DRA, BDA or DLA having its own feed mechanism. There are advantages to this arrangement, as outlined in UK patent application no 0313890.6 of 16th June 2003.

The dielectric material of a dielectric antenna can be made from several candidate materials including ceramic dielectrics, in particular low-loss ceramic dielectric materials.

It is known to provide a dielectric resonator antenna with two adjacent conductive surfaces [O'KEEFE, S.G., KINGSLEY, S.P. & SAARIO, S: "FDTD simulation of radiation characteristics of half volume HEM and TE mode dielectric resonator antennas", IEEE Transactions on Antennas and Propagation, AP-50, pp. 175-179, February 2002], the full disclosure of which is hereby incorporated into the present application by reference. In this paper, one conductive surface forms the groundplane on the underside and a second conductive surface forms a vertical end wall used to halve the volume of the dielectric.

It is also known (for example from US 6,323,808 (Heinrichs et al.) to provide a dielectric resonator antenna with two adjacent conductive surfaces (in addition to a groundplane, adjacent to both conductive surfaces and contacting at least one of the two conductive surfaces) in order to cause a further halving of the volume of the dielectric. This DRA, therefore, has three adjacent conductive surfaces and causes a quartering of the volume of the antenna for substantially the same resonant frequency.

A similar approach is disclosed in EP 1 396 907, in which a DRA is configured as a generally oblong dielectric resonator having a metallised underside which is mounted directly on a conductive groundplane, and a metallised strip extending up an adjacent surface, the metallised strip having a width less than that of the surface on which it is disposed. The metallised strip may optionally extend onto a top surface of the resonator. The DRA is driven by way of slot feeding through a slot in the conductive groundplane.

According to the present invention, there is provided a dielectric antenna comprising a dielectric substrate having first and second opposed surfaces, the second surface being provided with a conductive groundplane and a dielectric pellet being mounted on or adjacent to the first surface, and a feed mechanism formed on or in the substrate for transferring energy to or from the dielectric pellet, wherein the dielectric pellet has a top surface distal from the first surface of the substrate, a bottom surface proximal to the first surface of the substrate, a front surface facing towards a central part of the first surface of the substrate, a rear surface facing away from the central part of the first surface of the substrate and two side surfaces, and wherein the top surface and rear surface only of the dielectric pellet are provided with an electrically conductive coating or layer.

The dielectric pellet is preferably made from a dielectric ceramics material, preferably with a relative permittivity of at least 10.

The feed mechanism preferably contacts or otherwise drives the dielectric pellet by way of its bottom surface or front surface, but not by way of either the top surface or rear surface.

It is particularly to be noted that the first surface of the substrate is not provided with a conductive layer or coating.

It will be appreciated that this arrangement is different from that of US 6,323,808 because the present invention provides two contiguous conductive surfaces (top and rear), neither of which contacts or is associated with the feed mechanism.

This arrangement also differs from that of EP 1 396 907 in that the bottom surface of the pellet is not metallised.

The feed mechanism used for transferring energy into and out of the dielectric pellet is preferably a direct microstrip feed.

In embodiments of the present invention, it is believed that the conducting walls on the top and rear surfaces of the pellet are acting as mirrors that reduce the size of the pellet by halving it each way. However, the conductive groundplane serves only there to act as a counterpart to the feed mechanism. Removing some of the groundplane does not make a significant difference to the operational characteristics of the antenna, whereas removing some of the conductive coating or layer from the top or rear surface does, since this causes a change of operating frequency. Accordingly, in some embodiments of the present invention the electrically conductive coating or layer on one or other or both of the top and rear surfaces extends over less than a full width of the relevant surface. The conductive coating or layer may be formed as a strip with generally parallel sides. Alternatively, the sides of the strip may be generally convergent or divergent or may be stepped. These arrangements enable the antenna to be tuned to specific operating frequencies and in some embodiments can provide an improvement in bandwidth.

In a first preferred embodiment of the present invention, the conductive coatings or layers on the top and rear surfaces of the pellet are not electrically connected to the groundplane on the second surface of the dielectric substrate.

In a second preferred embodiment of the present invention, the conductive coating or layer on the rear surface is provided with an electrical connection, such as a pin or a shorting strip or the like, to the conductive groundplane on the second surface of the dielectric substrate. The position of the electrical connection in the conductive groundplane has a significant effect on the frequency of operation of the dielectric antenna. Furthermore, the mere presence of the electrical connection can cause a significant drop (in some embodiments around 200MHz) in the resonance frequency of the antenna in comparison to the first embodiment in which the rear surface is not electrically connected to the groundplane. This arrangement is different from that of US 6,323,808, because the conductive groundplane does not contact the bottom surface of the dielectric pellet. Indeed, the arrangement may be considered to be functioning in a similar manner to a dielectrically loaded PILA (planar inverted-L antenna).

In a third preferred embodiment, at least one switched electrical connection is provided between the conductive layer or coating on the top or, preferably, the rear surface of the pellet and the groundplane. This enables the resonance frequency of the antenna to be changed electrically. Switching at more than one point or position between the conductive layer or coating on the top or rear surface and the groundplane enables electrical selection between a plurality of resonance frequencies.

In a fourth preferred embodiment, the bandwidth of the antenna is enhanced by forming the dielectric pellet with a recess, indent or notch in its bottom surface such that only parts of the bottom surface adjacent the front and rear surfaces contact the dielectric substrate, with a central part of the bottom surface being raised above the dielectric substrate. In this embodiment, the pellet has an inverted 'U' shape when viewed from the side. Advantageously, a direct microstrip feed is provided on the first surface of the dielectric substrate as the feed mechanism, the microstrip feed passing under the front end of the bottom surface between the pellet and the dielectric substrate and to the rear end of the bottom surface, and being conformed to the recess, indent or notch in the bottom surface so as to contact the bottom surface along its length. This arrangement is in contrast to US 5,952,972, which shows a similar inverted 'U' pellet but with a microstrip on the second surface of the substrate and feeding through a slot aperture on the first surface. In this embodiment of the present invention, the feed is on the first surface of the dielectric substrate and conformal to the bottom surface of the pellet by being elevated from the substrate in the recessed part of the bottom surface, whereas in US patent 5,952,972 only the pellet is elevated and the feed is located on the second surface of the substrate.

In a fifth preferred embodiment, the bandwidth of the antenna is enhanced by providing a pellet with a recessed, notched or indented bottom surface as described above, and using a very short microstrip feed between only the front end of the bottom surface and the first surface of the dielectric substrate. In this embodiment, the feed does not extend to the recessed part of the bottom surface of the pellet. In US 5,952,972, the microstrip feed is located on the second surface of the substrate and passes beyond a central slot aperture on the first surface.

It will be appreciated that different feeding mechanisms give rise to different resonance mechanisms and so to different electrical characteristics and radiation patterns.

In all of the above embodiments, although the top, rear, front and side surfaces of the dielectric pellet may be generally planar, enhancements to bandwidth may be obtained by modifying one or more of the surfaces of the pellet, for example by chamfering or rounding (e.g. by grinding down parts of the surfaces. Furthermore, the aspect ratio of the pellet may be varied to adjust the resonant frequency. Many of the embodiments above may be combined to create a second resonant mode that may be used when dual band operation is required. Dual band operation is often required in radiotelephony such as for the 900/1800 GSM bands or the 2.4/5.8GHz Wireless LAN bands.

Embodiments of the present invention provide an advantage in that the dielectric antenna is much smaller than other dielectric resonator antennas or high dielectric antennas for a given resonant frequency. As an example, antenna dimensions of 15mm x 15mm x 7mm give rise to a resonance at 1110MHz when the relative permittivity of the dielectric material is 77. It will be appreciated that other dimensions, resonant frequencies and relative permittivities may be employed.

A further advantage of the new antenna arrangement is that the antenna is a good match to 50 ohms. This means that by correctly designing the feed, no stub tuning arrangement is needed, thus saving both circuit losses and board area. The antenna also has good gain, efficiency and bandwidth. Figure 4 shows a plot of the Sn return loss bandwidth for the above antenna and a wideband resonance can be seen.

The dielectric pellet may be soldered in place as a surface mount component. This attachment mechanism requires metallised pads on the first surface of the substrate and substantially matching metallised pads on the bottom surface of the pellet. Such metallisation arrangements must be carefully chosen if the performance of the antenna is not to be impaired.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made, by way of example, to the accompanying drawings, in which:

FIGURE 1 shows a perspective view of a general configuration of an antenna of the present invention;

FIGURE 2 shows a side view of the first embodiment of the present invention;

FIGURE 3 shows a side view of the second embodiment of the present invention;

FIGURE 4 is a plot showing S_n1 return loss for an embodiment of the present invention;

FIGURE 5 shows a side view of the fourth embodiment of the present invention; and

FIGURE 6 shows a side view of the fifth embodiment of the present invention.

Figure 1 shows a dielectric substrate 1 in the form of a PCB substrate, having a first, upper surface 2 and an second, lower surface 3. The second surface 3 is provided with a conductive groundplane (see Figures 2 and 3), and the first surface 2 does not have any conductive coating or layer. A generally cuboid or oblong dielectric ceramics pellet 4 is mounted on the first surface 2 of the substrate 1 , and a top surface 5 and a rear surface 6 of the pellet 4 are metallised. The side surfaces 7, front surface 8 and bottom surface of the pellet 4 are not metallised. A feed mechanism 10 in the form of a direct microstrip feedline is provided in the first surface 2 of the substrate 1 , passing between the first surface 2 and the bottom surface of the pellet 4.

Figure 2 shows the first preferred embodiment of the present invention in more detail as a side elevation. The conductive groundplane 11 can be seen on the second surface 3 of the substrate 2, as can the metallised conductive walls on the top 5 and rear 6 surfaces of the pellet 4. The arrangement of the direct microstrip feedline 10 on the first surface 2 of the substrate 1 and under the bottom surface 12 of the pellet is also clearly shown. It is to be noted that the conductive walls on the top 5 and rear 6 surfaces of the pellet are completely isolated from the conductive groundplane 11.

Figure 3 shows the second preferred embodiment of the present invention. The second embodiment is similar to the first, except in that a shorting strip 9 connects the metallised rear wall 6 of the pellet to the conductive groundplane 11. Providing the shorting strip 9 significantly lowers the resonant frequency of the antenna, in some cases by around 200MHz. The length and configuration of the shorting strip 9 and/or its position in relation to the right end 13 of the groundplane 11 also has a significant effect on the impedance and the resonant frequency.

Figure 4 shows a computer simulation of the Sn return loss of the antenna of Figure 2, demonstrating wideband resonance.

Figure 5 shows the fourth preferred embodiment of the present invention, in which a notch or recess 14 is formed in the bottom surface 12 of the pellet 4 so as to leave the pellet 4 supported by legs 15, 16. The top 5 and rear 6 surfaces of the pellet 4 are metallised as before, and an optional shorting strip 9 may be provided between the rear surface 6 and the conductive groundplane 11. The microstrip feedline 10 passes between the first surface 2 of the substrate 1 and the leg 15, and is then conformed to the recess 14 before passing between the first surface 2 and the leg 16, where it terminates. This arrangement provides enhanced bandwidth. Figure 6 shows the fifth preferred embodiment of the present invention, which is similar to that of Figure 5, except that the microstrip feedline 10 is relatively short, and only passes a short way between the leg 15 and the first surface 2 of the substrate 1 without continuing further into the recess 14. This arrangement also provides for enhanced bandwidth.

In all of the above embodiments, small metallised pads may be provided between corner or edge portions of the bottom surface 12 of the pellet and the first surface 2 of the substrate 1 so as to provide structural stability by compensating for the thickness of the microstrip feedline 10.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS:

1. A dielectric antenna comprising a dielectric substrate having first and second opposed surfaces, the second surface being provided with a conductive groundplane and a dielectric pellet being mounted on or adjacent to the first surface, and a feed mechanism formed on or in the substrate for transferring energy to or from the dielectric pellet, wherein the dielectric pellet has a top surface distal from the first surface of the substrate, a bottom surface proximal to the first surface of the substrate, a front surface facing towards a central part of the first surface of the substrate, a rear surface facing away from the central part of the first surface of the substrate and two side surfaces, and wherein the top surface and rear surface only of the dielectric pellet are provided with an electrically conductive coating or layer.

2. An antenna as claimed in claim 1, wherein the feed mechanism is a direct microstrip feed mechanism.

3. An antenna as claimed in claim 2, wherein the direct microstrip feed mechanism directly contacts the pellet on its bottom or rear surface.

4. An antenna as claimed in any preceding claim, wherein the conductive coating or layer on the top or rear surface is not electrically connected to the conductive groundplane.

5. An antenna as claimed in any one of claims 1 to 3, wherein the conductive coating or layer on the top or rear surface is electrically connected to the conductive groundplane.

6. An antenna as claimed in claim 5, wherein the conductive coating or layer is connected to the conductive groundplane by way of at least one switched connection.

7. An antenna as claimed in any preceding claim, wherein the dielectric pellet is formed with a recess, indent or notch in its bottom surface such that only parts of the bottom surface adjacent the front and rear surfaces contact the first surface of the dielectric substrate.

8. An antenna as claimed in claim 7 depending from claim 3, wherein the direct microstrip feed mechanism passes between the bottom surface of the dielectric pellet and the first surface of the dielectric substrate and is conformal to the recess, indent or notch in the bottom surface.

9. An antenna as claimed in claim 7 depending from claim 3, wherein the direct microstrip feed mechanism extends between the part of the bottom surface adjacent the front surface of the pellet and the first surface of the dielectric substrate but does not extend into the recess, indent or notch, nor to the part of the bottom surface adjacent the rear surface of the pellet.

10. An antenna as claimed in any preceding claim, wherein the conductive coating or layer covers all of the top and rear surfaces of the dielectric pellet.

11. An antenna as claimed in any one of claims 1 to 9, wherein the conductive coating or layer covers only part of the top or the rear surfaces of the dielectric pellet.

12. An antenna as claimed in claim 11, wherein the conductive coating or layer on the top or the rear surfaces has parallel sides.

13. An antenna as claimed in claim 11, wherein the conductive coating or layer on the top or the rear surfaces has non-parallel sides.

14. An antenna as claimed in claim 13, wherein the conductive coating or layer has divergent or convergent.sides on the top or the rear surfaces.

15. An antenna as claimed in claim 13, wherein the conductive coating or layer has stepped.sides on the top or the rear surfaces.

16. An antenna as claimed in any preceding claim, wherein the surfaces of the dielectric pellet are substantially planar.

17. An antenna as claimed in any one of claims 1 to 15, wherein at least one of the top, front, rear or side surfaces or edges of the dielectric pellet is curved or chamfered or rounded.

8. A dielectric antenna substantially as hereinbefore described with reference to ors shown in the accompanying drawings.