An up-to-date literature overview on relevant approaches for controlling circuital characteristics and radiation properties of dielectric resonator antennas (DRAs) is presented. The main advantages of DRAs are discussed in detail, while reviewing the most effective techniques for antenna feeding as well as for size reduction. Furthermore, advanced design solutions for enhancing the realized gain of individual DRAs are investigated. In this way, guidance is provided to radio frequency (RF) front-end designers in the selection of different antenna topologies useful to achieve the required antenna performance in terms of frequency response, gain, and polarization. Particular attention is put in the analysis of the progress which is being made in the application of DRA technology at millimeter-wave frequencies.
The release of the unlicensed 60 GHz band and the development of 5G technologies aimed at increasing data rate on wireless communication network by a factor of 100 [
DRAs rely on radiating resonators that can transform guided waves into unguided waves (RF signals). In the past, these antennas have been mainly realized by making use of ceramic materials characterized by high permittivity and high The size of the DRA is proportional to Due to the absence of conducting material, the DRAs are characterized by high radiation efficiency when a low-loss dielectric material is chosen. This characteristic makes them very suitable for applications at very high frequencies, such as in the range from 30 GHz to 300 GHz. As a matter of fact, at these frequencies, traditional metallic antennas suffer from higher conductor losses. DRAs can be characterized by a large impedance bandwidth if the dimensions of the resonator and the material dielectric constant are chosen properly. DRAs can be excited using different techniques which is helpful in different applications and for array integration. The gain, bandwidth, and polarization characteristics of a DRA can be easily controlled using different design techniques.
The main target of this paper is to present an up-to-date review study summarizing the most relevant techniques to control circuital characteristics and radiation properties of DRAs. In this way, guidance will be provided to RF front-end designers to achieve the required antenna performance in terms of gain, bandwidth, and polarization. Different geometries of radiating resonators will be discussed first, turning then our attention to advantages and disadvantages of different feeding techniques proposed so far in the literature. Various methodologies that have been used to enhance the impedance bandwidth and the antenna gain will be explored. Furthermore, different techniques to achieve circular polarization are summarized. Finally, the most recent implementation of DRAs on chip and off chip will be presented.
By using a suitable excitation technique, any dielectric structure can become a radiator at defined frequencies. It is to be noticed that, for a given resonant frequency, the size of the dielectric resonator is inversely proportional to the relative permittivity of the constitutive material. The lowest dielectric constant material adopted in DRA design is reported in [
The basic principle of operation of dielectric resonators is similar to that of the cavity resonators [
Different radiating structures used for dielectric resonator antennas (DRAs).
Cylindrical DRAs have been studied extensively in literature. Figure
Three-dimensional (a) and cross-sectional view (b) of the probe-fed cylindrical DRA.
The resonant frequency of the modes supported by a cylindrical DRA can be calculated using the following equations [
Roots of the Bessel functions of the first kind,
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2.404 | 5.520 | 8.653 | 11.791 | 14.930 |
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3.831 | 7.015 | 10.173 | 13.323 | 16.470 |
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5.135 | 8.417 | 11.619 | 14.795 | 17.959 |
Roots of the first-order derivative of the Bessel functions of the first kind,
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3.831 | 7.015 | 10.173 | 13.323 | 16.470 |
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1.841 | 5.331 | 8.536 | 11.706 | 14.863 |
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3.054 | 6.706 | 9.969 | 13.170 | 16.347 |
The impact of the geometrical parameters of a cylindrical DRA (radius and height) as well as of the relative dielectric constant (
Resonant frequency as a function of the radius (a) and height (b) of a cylindrical DRA with relative permittivity
Figure
Resonant frequency of a cylindrical DRA with radius
Figure
Three-dimensional view (a) and cross-sectional view (b) of an aperture-fed rectangular DRA.
The main advantage of the rectangular DRA is that it is characterized by three independent geometrical dimensions,
The dielectric waveguide model [
In this section attention is put also on the hemisphere and the cross-shaped and the supershaped DRAs.
A probe-fed hemispherical DRA with an air gap of hemispherical shape between the dielectric structure and the ground plane is presented in [
Probe-fed hemispherical DRA with an air gap for bandwidth enhancement as suggested in [
The authors in [
The cross-shaped dielectric resonator is mainly suggested to design circularly polarized antenna. A cross-shaped DRA as suggested in [
Cross-shaped DRA (a) and a sequentially fed cross-shaped DRA array (b) as suggested in [
In order to achieve higher impedance bandwidth and larger circular polarization bandwidth, a sequentially fed array of cross DRA is proposed in the same paper as shown in Figure
Recently, a very similar concept has been adopted in [
A plastic-based supershaped DRA has been presented in [
Probe-fed supershaped DRA [
The main advantage of the presented supershaped DRA is that it uses plastic (PolyVinyl Chloride (PVC)) as dielectric material which makes it very cost effective and easily manufacturable.
In addition to the basic dielectric resonators that have been discussed in this section, numerous attempts have been performed to combine different dielectric resonators together in order to optimize antenna performance. Figures
One of the main advantages of DRA technology is that various feeding techniques can be used to excite the radiating modes of a dielectric resonator.
The probe-fed DRA is among the first reported DRAs [
Probe-fed rectangular DRA.
By optimizing length and position of the feeding probe, the input impedance of the DRA can be tuned [
A reduced manufacturing complexity is achieved by placing the excitation probe adjacent to the DR (see Figure
For high-frequency applications where the antenna is manufactured on a Printed Circuit Board (PCB), or is directly integrated on chip, probe-fed DRAs are not practical.
Another way to feed the DRA is by using printed transmission lines. Figure
Three-dimensional view of conventional (a) and conformal (b) microstrip transmission line feeding networks.
In a conformal transmission line-fed DRA, the resonator is placed directly on the PCB substrate and the feeding microstrip is bent over the resonator as shown in Figure
The coplanar waveguide excitation was first introduced in [
Rectangular DRA fed by a CPW circular-loop network (a). Inductive slot feed (b). Capacitive slot feed (c).
The most popular feeding technique for DRAs is via a slot in the ground plane. This excitation method is known as aperture coupling. The guided wave propagating along the transmission line is coupled, through the slot, to the resonant modes of the DR.
Figure
Top view of an aperture-coupled hemispherical DRA as presented in [
Cross-sectional view of a two-layer rectangular DRA for size reduction as suggested in [
The two most common techniques to minimize the size of a DRA are either to use material with high dielectric constant or to insert a metal plate in the symmetry plane of the DR. These techniques are reviewed in this section.
In general, the size of the DRA is inversely proportional to the dielectric constant (
To overcome the limitation related to reduced bandwidth a multilayer DRA topology can be adopted. By optimizing the dielectric constant and height of each layer, the DRA parameters (size, impedance bandwidth, and gain) can be controlled and optimized. Figure
The second design technique to minimize the size of a DRA consists in the integration of metal plates along the relevant symmetry planes, as it easily follows from the application of image theory [
Cross-sectional view of a probe-fed rectangular DRA (a) and its miniaturized version (b) by placing a conducting plate at
As indicated in the figure, the size of the DRA can be halved by placing a conducting plate along the central cross-section of the dielectric resonator at
Effect of the miniaturization of a rectangular DRA [
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|
Volume |
|
Conducting |
|
Impedance |
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88 | 40 | 70.4 | 12 | No | 1.22 | 16% |
44 | 40 | 35.2 | 12 | Yes | 0.918 | 5.4% |
Similarly, the size of a cylindrical DRA can be reduced by 75% by placing a Perfect Electric Conductor (PEC) and a Perfect Magnetic Conductor (PMC) at the symmetry planes of cylinder, as shown in [
The most straightforward approach to increase the gain of a DRA is by arraying individual DRAs. The gain of a DRA array increases by increasing the number of DR elements in the array. However this approach is not the main goal of this section. Attention is put here on two alternative techniques to enhance the gain of a single DRA element.
An efficient way to increase the gain of a DRA relies on the integration of additional structures useful to focus the antenna radiation in one direction and increase, in this way, its gain. A straightforward example of such design approach is the surface mounted short horn (SMSH) DRA presented in [
A surface mounted short horn (SMSH) configuration as suggested in [
The suggested DRA consists of a slot-fed rectangular dielectric resonator with four additional inclined metallic plates used to form a horn-like structure. The effect of the SMSH structure on the DRA gain is quantified in Table
Measured gain of a DRA with and without SMSH [
Frequency (GHz) | 5.8 | 5.9 | 6.0 | 6.1 | 6.2 |
Gain without SMSH (dBi) | 1.4 | 2.8 | 3.8 | 4.2 | 2.7 |
Gain with SMSH (dBi) | 7.9 | 8.8 | 8.2 | 8.2 | 6.9 |
Recently a high-gain mushroom-shaped DRA has been presented in [
High-gain mushroom-shaped DRA structure as presented in [
The second method to increase the gain of an individual DRA is by exciting the relevant higher-order modes, thus making the DRA electrically larger with respect to its fundamental resonant frequency. It has been shown in [
The design of a pattern-reconfigurable cylindrical DRA with high gain resulting from the excitation of higher-order
Recently, a cylindrical DRA with a measured peak gain of 11.6 dBi has been reported in [
One of the advantages of DRAs is that their polarization can be easily controlled. In this section different techniques to design circularly polarized DRAs are reviewed.
Exciting a DRA by using two ports with a 90° phase shift is a simple but effective means for achieving circular polarization of the radiated electromagnetic field. Figure
Circularly polarized dual-fed cylindrical DRA as suggested in [
The same design strategy has been applied in [
Circularly polarized DRAs can be designed also by using a single feed placed at a location where two orthogonal modes can be excited. This technique has been applied in [
In order to achieve circular polarization in slot-fed DRAs, the dielectric resonator and the feeding slot are to be properly rotated relative to each other, so that two orthogonal modes can be excited with the desired phase difference. Figure
Cross-sectional view (a) and top view (b) of a rotated rectangular DRA as suggested in [
A novel circularly polarized three-layer stacked DRA has been presented in [
Instead of rotating the DR itself, location and geometry of the feeding slot can be optimized in order to achieve circular polarization. This technique has been used in [
Another method for exciting two orthogonal modes and enforcing a circular polarization of the radiated electromagnetic field relies on the use of parasitic metal strips/patches attached to the surface of the dielectric resonator. The parasitic metallic element introduces an asymmetry in the DRA structure which eventually leads to the excitation of two orthogonal modes with the desired phase difference [
Three-dimensional view of a cylindrical DRA with a grounded parasitic patch (a) as introduced in [
A circularly polarized cylindrical DRA with grounded parasitic strip has been presented in [
In this section, different techniques useful to broaden the impedance bandwidth of a DRA are summarized.
Dielectric resonators can be combined together to broaden the impedance bandwidth of the resulting DRA structure. Figures
An alternative design approach, useful to achieve large impedance bandwidths, consists in the tapering of the permittivity distribution within the DR. This can be done either by combining multiple layers with different dielectric constants or by drilling holes of different diameters into the DR in order to taper the effective permittivity of the structure. The geometry of a perforated DRA is shown in Figure
Top (a) and cross-sectional view (b) of a perforated DRA as suggested in [
Dual-band or wide-band characteristics can be easily synthesized by exciting different resonant modes in multilayered DRAs consisting of different dielectric laminates with properly selected permittivity. A cross-sectional view of a two-layer DRA is shown in Figure
An alternative antenna topology that can feature wide impedance bandwidth or dual-band operation is the one consisting of two dielectric resonators next to each other and separated by a gap, as shown in Figure
When shaping the dielectric resonator, different modes can be excited and, in this way, a wide impedance bandwidth achieved. A U-shaped DRA (
A CPW-fed fractal DRA topology has been proposed in [
Numerically Simulated Characteristics of fractal-based DRAs as Reported in [
DRA profile | Resonant frequency (GHz) | Impedance bandwidth (MHz) | Gain |
---|---|---|---|
Minkowski | 6.59 | 2870 | 4.12 |
Koch | 6.8 | 2540 | 3.51 |
Sierpinski curve | 8.0 | 1610 | 1.05 |
Different fractal boundary structures proposed and simulated in [
An alternative antenna topology that provides a large impedance bandwidth is the supershaped DRA shown in Figure
Finally, the cup-shaped inverted hemispherical DRA design presented in [
Cup-shaped inverted hemispherical DRA as suggested in [
In order to increase the impedance bandwidth of an aperture-fed DRA, an annular slot can be used as proposed in [
Top (a) and cross-sectional view (b) of a rectangular DRA excited by an annular slot.
The annular slot-based approach has been extended in [
Circular aperture slot (a) as introduced in [
An alternative feeding aperture geometry which allows broadening the impedance bandwidth of a DRA is the U-shaped slot shown in Figure
Hybrid design approaches rely on the combination of DRAs and radiating patch/slot antennas. By tuning the resonant frequency of the radiating patch/slot antenna in order to be close to that of the combined DRA, a wide impedance bandwidth can be achieved. A hybrid DRA resulting from the combination of a ring-shaped dielectric resonator and a circular microstrip patch, fed by a resonant slot, is presented in [
Cross-sectional view of a hybrid DRA consisting of a monopole and a ring DR (a) as suggested in [
Top view (a) and cross-sectional view (b) of a hybrid ring/patch DRA as introduced in [
A hybrid DRA combining a rectangular DR and a CPW inductive slot (see Figure
A similar design approach is adopted in [
One of the intrinsic characteristics of DRAs is the absence of conduction losses. This property makes DRAs very suitable for high-frequency applications. In this section, major and more recent advances of DRA technology at mm-wave frequencies are summarized.
At millimeter-wave frequencies and beyond, the size of dielectric resonators become very small, which makes the manufacturing of the DRA challenging without resorting to expensive fabrication techniques. In order to overcome this limitation and, at the same time, improve antenna gain, electrically large DRAs exploiting higher-order modes are to be adopted.
A hybrid approach combining the excitation of higher-order modes with the radiation from the antenna feed has been presented in [
Most Recent Advances in dielectric resonator antennas for millimeter-wave Applications.
Reference/Year | DRA |
Relative |
Feeding |
Technology | Frequency |
Impedance |
Gain |
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[ |
Ring with microstrip Patch | 10 | Coupling slot | PCB | 60 | 15 | 12 |
[ |
Cylindrical | 10 | Coupling slot | PCB | 60 | 24 | 5.5 |
[ |
Rectangular | 10 | Coupling slot | PCB | 24 | 3.8 | 6.3 |
[ |
Rectangular | 10.2 | Capacitive slot/CPW | PCB | 60 | 17.4 | 10.2 |
[ |
Cylindrical | 10.2 | Coupling slot | PCB | 60 | 14.3 | 15.7 |
[ |
Cylindrical | 10.2 | Coupling slot | PCB | 60 | 22.9 |
8.6 |
[ |
Cylindrical | 10.2 | Coupling slot | PCB | 60 | 10 |
— |
[ |
Ring with microstrip Patch | 10.2 | Coupling slot | PCB | 60 | 12 | 16.5 |
[ |
Rectangular | 48 | CPW | Integrated on chip | 60 | 2.8 | 3.2 |
[ |
Cylindrical | 42 | Folded dipole/CPW | Integrated on chip | 27.7 | 9.8 | 1 |
[ |
Rectangular | 38 | H-slot/CPW | Integrated on chip | 35 | 12 | 0.5 |
[ |
Rectangular | 10 | Meander slot/CPW | Integrated on chip | 130 | 12 | 2.7 |
[ |
Two-layer rectangular | 10 | Meander slot/CPW | Integrated on chip | 130 | 11 | 4.7 |
[ |
Rectangular | 12.6 | Coupling slot | Integrated on chip | 60.5 | 6.1 | 6 |
[ |
Rectangular | 9.8 | E-shaped/CPW | Integrated on chip | 340 | 12 | 10 |
[ |
Circular | 12.6 | Dipole/CPW | Integrated on chip | 60 | 3.78 | 7 |
A class of linearly and circularly polarized cylindrical DRAs fed by substrate integrated waveguides is presented in [
The effect of the fabrication tolerances on the frequency shift of higher-order modes is investigated in [
Simulated frequency shift of higher order modes of a rectangular DRA due to fabrication tolerances [
Fabrication error |
TE111 mode |
TE115 mode |
TE119 mode |
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0.01 | 0.38 | 0.21 | 0.12 |
0.05 | 2.04 | 1.00 | 0.74 |
0.1 | 3.99 | 1.83 | 1.41 |
The reduced sensitivity of higher-order modes to fabrication errors can be readily understood by observing that a DRA driven on a higher-order mode is physically much larger as compared to the same DRA when operating on the relevant fundamental mode. In order to highlight this point, the volume of rectangular DRAs excited at the higher-order modes TE115 and TE119 is compared, in Table
Comparison between the volume of DRAs excited at 24 GHz by TE111, TE115, and TE119 modes.
Resonant mode | Dimensions |
DR volume |
---|---|---|
TE111 | 2.5 × 2.5 × 2.1 | 13.13 |
TE115 | 4.0 × 4.0 × 6.1 | 97.6 |
TE119 | 4.2 × 4.2 × 10.7 | 188.75 |
The gain of a single DRA element can be further increased by placing a superstrate on the top of the DR. This approach has been suggested in [
Rectangular DRA with superstrate for gain improvement as suggested in [
An extensive study on the effect of dielectric superstrates in combination with millimeter-wave DRAs has been reported in [
Performance comparison between the DRAs shown in Figure
Antenna 1 | Antenna 2 | Antenna 3 | Antenna 4 | |
---|---|---|---|---|
Impedance bandwidth (%) | 36.5 | 14.36 | 13.5 | 14.36 |
Antenna gain |
6.9 | 7.8 | 14.36 | 15.7 |
Millimeter-wave DRA topologies presented in [
Antenna 1
Antenna 2
Antenna 3
Antenna 4
A novel cylindrical DRA for 60 GHz applications has been presented in [
The integration of a suitable FSS wall has been suggested in [
Front side (a) and back side (b) of the FSS unit cell useful to reduce the mutual coupling between DRAs [
In this section, different DRA topologies for SoC integration are reviewed. The direct integration of the antenna on chip eliminates the need of bond-wiring with a beneficial impact on manufacturing costs and miniaturization.
A 60 GHz rectangular DRA and a 27 GHz circular DRA integrated on silicon substrate are reported in [
A cylindrical DRA printed on silicon substrate with 45% efficiency and 1 dBi gain is reported in [
A rectangular DRA excited by a H-slot in CPW technology is presented in [
Single- and two-layer DRAs realized in CMOS technology for applications at 130 GHz are presented in [
An on-chip DRA operating at 340 GHz is presented in [
Side view of the suggested on-chip antenna as presented in [
In the aforementioned on-chip DRA designs, the two main challenges are associated with the alignment of the DR to the feed, as well as the manufacturing of the DR with the prescribed shape. Recently, a cylindrical DRA realized starting from a single silicon wafer has been presented in [
Manufacturing process of the DRA as suggested in [
Recent developments in millimeter-wave DRA technology have been presented and discussed in detail in this survey. Furthermore, useful design guidelines have been provided to RF front-end designers in order to control circuital characteristics and radiation properties of this class of antennas.
Different feeding techniques for DRAs have been first introduced, while outlining the relevant advantages and disadvantages. Furthermore, design approaches useful to achieve size reduction of DRAs have been discussed in detail. By using high permittivity materials or by placing conducting plates along specific symmetry planes of the resonator body, one can make DRAs considerably smaller. On the other hand, the gain of a DRA can be increased either by exciting the relevant higher-order modes (electrically large DRAs), or by integrating horn-like structures.
Particular attention has been put on hybrid design techniques which rely on the combination of DRAs and radiating patch/slot antennas. In this way, the antenna impedance bandwidth can be easily tuned in such a way as to synthesize a dual-band rather than wide-band frequency response. Circular polarization of the electromagnetic field radiated by DRAs can be achieved by using various design methodologies. In this respect, the cross-shaped feeding slot-based approach provides different benefits in terms of high coupling to the DR and additional degrees of freedom to control the polarization purity (axial ratio) of the DRA, in combination with ease of manufacturing and integration.
Finally, advances in the application of DRA technology at millimeter-wave frequencies have been presented, and the most recent implementation of on-chip DRAs and off-chip DRAs has been reviewed. It has been shown that DRAs realized on silicon substrates with standard CMOS process can be characterized by good efficiency and gain, thus proving the good potential of dielectric resonator antennas for said applications.
The authors declare that they have no competing interests.
This study has been carried out in the framework of the research and development program running at