GB2421357A

GB2421357A - Dielectric resonator antenna having virtual ground formed on substrate adjacent asymmetric microstrip feed

Info

Publication number: GB2421357A
Application number: GB0602612A
Authority: GB
Inventors: Zhipeng Wu
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-12-07
Filing date: 2002-12-07
Publication date: 2006-06-21
Anticipated expiration: 2022-12-07
Also published as: GB2421357B; GB0602612D0; GB2396746B; GB0228634D0; GB2396746A

Abstract

A dielectric resonator antenna (DRA) comprises a dielectric element 1 of high permittivity material ( e r of 30 or higher) mounted on a substate 4 with a ground plane 5 formed preferably on a back face thereof. A microstrip feed line 3 is formed on the top face of the substrate and is position underneath the dielectric element, but aligned asymmetrically with respect to the dielectric element 1. A virtual ground 6 is formed as a metal patch on the top face adjacent to the microstrip to one side and at one end thereof. The 'virtual ground' patch 6 is similar in size to the dielectric element 1 but is arranged so that the virtual ground extends from underneath the resonator.

Description

2421357

1

BROADBAND MINIATURISED DIELECTRIC RESONATOR ANTENNAS WITH A

VIRTUAL GROUND PLANE

FIELD OF THE INVENTION

This invention relates to miniaturised dielectric resonator antennas of broad bandwidth, and more particularly to a dielectric resonator antenna having a virtual ground.

BACKGROUND OF THE INVENTION

Dielectric resonator antennas (DRAs) are known as antennas made of low loss ceramics or other high permittivity dielectric materials. The dielectric material forms a resonator in rectangular, cylindrical, hemispherical or other geometries. The dielectric resonator has a number of resonant modes with corresponding resonance frequencies. The resonant modes which have electromagnetic fields not entirely confined in the resonator are used for electromagnetic radiation. The mode which has the lowest resonance frequency is the lowest mode of the resonator. The lowest mode is preferred for antenna application as a resonator operating at this lowest mode has the smallest physical dimensions for a given operating frequency. The lowest modes of rectangular and cylindrical dielectric resonators are TEym and HEns modes respectively. These modes can be excited using a microstrip line or other feed structures. In particular, DRAs symmetrically coupled by a microstrip line have been reported in the article "Microstrip transmission line excitation of dielectric resonators" by R A Kraneburg and S A Long in Electronics Letters, Vol.24, No.18, 1988, ppl 156-1157. The dielectric resonator is positioned in the feed structure so that good impedance matching can be obtained at the resonance frequency so as to produce efficient radiation. A DRA operating at the lowest mode has broadside radiation with directivity weakly dependent on the dielectric constant of the material and the resonator dimensions. The resonance frequency of the lowest mode of the DRA is a function of geometrical dimensions of the resonator and it is inversely proportional the square root of the relative dielectric constant of the material, er. The impedance bandwidth of the DRA is inversely proportional to er! 5, and it also depends on the dimensions of the resonator, particularly the length-to-height aspect ratio. The bandwidth at VSWR=2 can vary from over 10% for £r=10 to 3% for £,=40, and further to 1% for £r=90, but it generally increases with the

2

length-to-height aspect ratio. The properties of DRAs have been reviewed in the article "Dielectric Resonator Antennas - A review and general design relations for resonant frequency and bandwidth", by R. K. Mongia and P. Barthia, in International Journal of Microwave and Millimeter-wave Computer-aided Engineering, Vol.4, No.3, 1994, pp230-247.

For a given operating frequency, the dielectric resonator can be miniaturised by using a high permittivity material with er=30 or higher. This leads to a smaller, miniaturised DRA. However, the use of high permittivity material also leads to smaller impedance bandwidth, which can restrict the benefit of size reduction. The reduction in bandwidth of a miniaturised DRA can be compensated by increasing the length-to-height aspect ratio of the DRA. This leads to a low profile miniaturised DRA as described in, for example, the article "Low profile dielectric resonator antennas using a very high permittivity material", by R K Mongia, A Ittipiboon, M Cuhaci, in Electronics Letters, Vol.30, No.17, 1994, pp 1362-1363. However, a miniaturised DRA generally has a narrow impedance bandwidth. A technique for improving the impedance bandwidth is to use stacked DRAs, as described in the article "Broadband stacked dielectric resonator antennas", by A A Kishk, B Ahn and D Kajfez, in Electronics Letters, Vol.25, No. 18, 1989, ppl232-1233. This technique however increases the overall size of the antenna. Other techniques by combining with a microstrip patch antenna, or by introducing a notch or multi-segments have been described in patents US 5,453,754 and EP 0801436A2. These techniques however complicate the fabrication of antenna.

SUMMARY OF THE INVENTION

It is an object of the invention to provide novel methods, which can be easily implemented, for obtaining broadband miniaturised rectangular and cylindrical DRAs for wireless communications at microwave and millimetre wave frequencies.

According to this invention, the bandwidth of a miniaturised rectangular or cylindrical DRA is greatly increased by introducing a virtual ground for the DRA fed by a microstrip line.

The miniaturised rectangular or cylindrical DRA is made of ceramic or other materials with relative dielectric constant, er, of 30 or higher.

3

The virtual ground is an isolated metal patch printed on the same surface as the microstrip line used to feed the DRA. It is positioned on one side of the microstrip line, leaving a small gap to the microstrip line. The DRA is placed on top of the virtual ground, the microstrip line and the substrate, covering a major part of the virtual ground and a part of the microstrip line. The virtual ground therefore extends beyond the bottom surface of the DRA. The area on the bottom surface of the DRA overlapping the virtual ground can be metallised so that the DRA can be soldered onto the virtual ground to gain mechanical support. The feed structure causes the lowest mode to degenerate, producing multiple resonances. The input impedances of these degenerated modes are well matched to the characteristic impedance of the microstrip line This gives rise to an increased, broad impedance bandwidth.

These and other aspects of the invention, preferred embodiments and variants thereof, and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.

BRIEF DECSRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of examples only, with reference to the accompanying drawings, in which:

FIG. la is a top view of a microstrip line coupled rectangular DRA with a virtual ground which provides broad impedance bandwidth in accordance with the invention;

FIG. lb is a side view of a microstrip line coupled rectangular DRA with a virtual ground shown in FIG.la;

FIG.2a is a top view of a microstrip line coupled cylindrical DRA with a virtual ground which provides broad impedance bandwidth in accordance with the invention;

FIG.2b is a side view of a microstrip line coupled cylindrical DRA with a virtual ground shown in FIG.2a;

FIG.3 is a plot of the measured reflection coefficient, iSni in dB against frequency, of a microstrip coupled cylindrical DRA of radius 7.5mm, height 2.5mm and er =37 on

4

a virtual ground plane (see FIG2.a) where the microstrip line is printed on a substrate of £,• =2.2 and thickness of 0.79mm, and has a width of 2.4mm.

DETAILED DESCRIPTION OF THE INVENTION

FIG la shows a top view of a microstrip coupled rectangular DRA 1 with a virtual ground 6. The microstrip 3 is printed on a substrate 4 with a ground plane 5. The virtual ground 6 is a metal patch in rectangular, circular or other geometries printed on the same substrate 4 on the side and toward the end of the microstrip line with a small gap to the microstrip line. The area of the virtual ground 6 is comparable to the cross-sectional area of the rectangular DRA 1. The rectangular DRA 1 is positioned on top of the virtual ground 6, the microstrip line 3 and the substrate 4 with an offset to the centre of the virtual ground 6. This asymmetrical feed structure causes the lowest mode, TEym, to degenerate. The input impedances of these degenerated modes are well matched to characteristic impedance of the microstrip line. This gives rise to an increased, broad impedance bandwidth. The presence of the virtual ground 6 also results in the resonance frequencies to decrease, compared with that without the virtual ground. The reduction in resonance frequency implies the reduction in resonator size for a given operating frequency. A side view of the microstrip side coupled rectangular DRA 1 is shown in FIG. 1 b.

FIG.2a shows a top view of a microstrip coupled cylindrical DRA 2 with a virtual ground 6. The cylindrical DRA 2 is positioned on top of the virtual ground 6, the microstrip line 3 and the substrate 4 with an offset to the centre of the virtual ground 6. This asymmetrical feed structure causes the lowest mode, HEng, to degenerate. The input impedances of these degenerated modes are well matched to characteristic impedance of the microstrip line. This gives rise to an increased or broad impedance bandwidth. A side view of the microstrip side coupled cylindrical DRA 2 is shown in FIG.2b.

The single substrate structures shown in FIGs.1-2 can be equivalently replaced by multi-layered structures having multiple substrates and conductor layers.

FIG.3 shows an example of the measured reflection coefficient, IS n I in dB against frequency, of a microstrip coupled cylindrical DRA with a virtual ground in FIG.5a. The cylindrical DRA has a radius 7.5mm, height 2.5mm and £r =37. The microstrip feed line is printed on a substrate of £, =2 2 and thickness of 0.79mm, and has a width of 2.4mm. The virtual ground is printed on the

5

substrate in line with the end of the microstrip line, and has a size of 12mm x 12mm. The gap between the microstrip line and the virtual ground is 0.5mm. The rectangular DRA with a virtual ground has a central frequency of 5.8GHz, and -lOdB bandwidth of 10.5%. In accordance with this invention, by introducing the virtual ground with the microstrip feed, the bandwidth is increased by 3.5 times compared with the same DRA fed by a probe. Two degenerated modes with good impedance matching can be observed from the response in FIG.3.

6

Claims

1. A broadband miniaturised dielectric resonator antenna comprising:

(a) a dielectric resonator made of a high permittivity material;

(b) a microstrip line for transferring energy into or from the said dielectric resonator as an antenna;

(c) a dielectric substrate on which the said microstrip line is fabricated, and

(d) a virtual ground on which the said dielectric resonator is placed with the area of the virtual ground plane extending beyond the bottom surface of the said dielectric resonator.

2. The broadband miniaturised dielectric resonator antenna of claim 1 wherein the dielectric resonator is in rectangular or cylindrical geometry.

3. The broadband miniaturised dielectric resonator antenna of claim 1 wherein the microstrip line has an open end.

4. The broadband miniaturised dielectric resonator antenna of claim 1 wherein the virtual ground is a metal patch printed on the said dielectric substrate.

5. The broadband miniaturised dielectric resonator antenna of claim 1 wherein the virtual ground is in rectangular, square or circular shape.

6. The broadband miniaturised dielectric resonator antenna of claim 1 wherein the virtual ground is positioned on one side and towards the end of the said microstrip line with a gap to the said microstrip line.

7. The broadband miniaturised dielectric resonator antenna of claim 1 wherein the dielectric resonator is positioned on the virtual ground, the microstrip line and the dielectric substrate.

8. The miniaturised dielectric resonator antenna with increased impedance bandwidth of claims 1 wherein the microstrip line includes a microstrip line fabricated on multi-layered structures.