Review Article
Dielectric Resonator Antennas: Basic Concepts,
Design Guidelines, and Recent Developments at
Millimeter-Wave Frequencies
S. Keyrouz
1
and D. Caratelli
1,2
1
e Antenna Company, High Tech Campus 41, Eindhoven, Netherlands
2
Tomsk Polytechnic Uni versity, 84/3 Sovetskaya Street, Tomsk, Russia
Correspondence should be addressed to S. Keyrouz; shady.keyrouz@antennacompany.com
Received July ; Revised Septemb er ; Accepted September
Academic Editor: Ahmed T. Mobashsher
Copyright © S. Keyrouz and D. Caratelli. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
An up-to-date literature overview on relevant approaches for controlling circuital characteristics and radiation properties of
dielectric resonator antennas (DRAs) is presented. e main advantages of DRAs are discuss ed in detail, while reviewing the most
eective 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 dierent antenna topologies us eful to achieve the required antenna per formance in terms of frequency response,
gain, and polarization. Particular attention is put in t he analysis of t he progress which is being made in the application of DRA
technology at millimeter-wave frequencies.
1. Introduction
e release of the unlicensed GHz band and the devel-
opment of G technologies aimed at increasing data rate on
wireless communication network by a factor of [] will
impose stinging specications (large bandwidth, high gain,
small size, and temperature independent performance) on
the design of the radio frequency (RF) electron ics. Various
front-end antenna solutions relying on monopoles, dipoles,
and patch antennas have been proposed for millimeter-wave
applications. ese antennas are characterized by small size,
low weight, and low cost and can be easily integrated on chip.
However, unless advanced design solutions based on the inte-
gration of suitable dielectric superstrates or lensing structures
are adopted, these antennas typically suer from reduced
radiation eciency and nar row impedance bandwidth due
to the eect of lossy silicon substrate materials. On the other
hand, dielectric resonator antennas (DRAs) are promising
candidates to replace traditional radiating elements at high
frequencies, especially for applications at millimeter waves
and beyond. is is mainly attributed to the fact that DRAs
do not suer from conduction losses and are characterized
by high radiation eciency when excited properly.
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 factor (between and ). Currently, DRAs made
from plastic material (PolyVinyl Chloride (PVC)) are being
realized. e main advantages of DRAs are summarized as
follows:
(i) e size of the DRA is proportional to
0
/
𝑟
with
0
=/
0
being the free-space wavelength at the
resonant frequency
0
and where
𝑟
denotes the
relative permittivity of the material forming the radi-
ating structure. As compared to traditional metallic
antennas whose size is proportional to
0
,DRAs
are characterized by a smaller form factor especially
when a material with high dielectric constant (
𝑟
)is
selected for the design.
Hindawi Publishing Corporation
International Journal of Antennas and Propagation
Volume 2016, Article ID 6075680, 20 pages
http://dx.doi.org/10.1155/2016/6075680
International Journal of Antennas and Propagation
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
F : Dierent radiating structures used for dielectric resonator antennas (DRAs).
(ii) Due to the absence of conducting material, the DRAs
are characterized by high radiation eciency when
a low-loss dielectr ic material is chosen. is char-
acteristic makes them very suitable for applications
at very high frequencies, such as in the range from
GHz to GHz. As a matter of fact, at these
frequencies, traditional metallic antennas suer from
higher conductor losses.
(iii) DRAs can be charac terized by a large impedance
bandwidth if the dimensions of the resonator and the
material dielectric constant are chosen properly.
(iv) DRAs can be excited using dierent techniques which
is helpful in dierent applications and for array
integration.
(v) e gain, bandwidth, and polarization characteristics
of a DRA can be easily controlled using dierent
design techniques.
e main target of this paper is to present an up-to-date
review study summarizing the most relevant techniques to
cont rol circuital character istics and radiation properties of
DRAs.Inthisway,guidancewillbeprovidedtoRFfront-
end designers to achieve the required antenna performance
in terms of gain, bandwidth, and polarization. Dierent
geometries of radiating resonators will be discussed rst,
turningthenourattentiontoadvantagesanddisadvantagesof
dierent feeding techniques proposed so far in the literature.
Various methodologies that have been used t o enhance the
impedance bandwidth and the antenna gain will be explored.
Furthermore, dierent techniques to achieve circular polar-
ization are summarized. Finally, the most recent implemen-
tation of DRAs on chip and o chip will be presented.
2. Dielectric Resonators
Byusingasuitableexcitationtechnique,anydielectricstruc-
ture can become a radiator at dened 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. e lowest dielectric
constant material adopted in DRA design is reported in [–
], where commodity plastics with relative dielectric constant
smaller than have been utilized for the realization of
supershaped DRAs.
e basic principle of operation of dielectric resonators is
similartothatofthecavityresonators[]andisthoroughly
discussed in literature. e most two popular radiating
dielectric resonators are the cylindrical and the rectangu-
lar ones. ey will be reviewed in this section. Design
equations to calculate the relevant resonant frequencies
are given. More complex dielectric resonators, such as the
spherical/hemispherical, cross-shaped, and supershaped (see
Figure ) ones, will be also discussed in this section.
2.1. Cylindrical DRA. Cylindrical DRAs have been stud-
ied extensively in literature. Figure shows the three-
dimensional view (a) and the cross-sectional view (b) of
a probe-fed cylindrical DRA. e antenna consists of a
cylindrical dielectr ic resonator (DR) with height , radius ,
and dielectric constant
𝑟
.eDRisplacedontopofaground
plane and fed by a coaxial connector. e main advantages of
the cylindrical DRA consist in the ease of fabrication and the
ability to excite dierent modes.
e resonant frequency of the modes supported b y
a cylindrical DRA can be calculated using the fol lowing
equations []:
TE
𝑛𝑝𝑚
=
2
𝑟
𝑟
𝑛𝑝
2
+
(
2+1
)
2
2
,
()
TM
𝑛𝑝𝑚
=
2
𝑟
𝑟
𝑛𝑝
2
+
(
2+1
)
2
2
,
()
International Journal of Antennas and Propagation
(a) (b)
Feed point
Ground plane
Coaxial feed
h
2a
F : ree-dimensional (a) and cross-sectional view (b) of the probe-fed cylindrical DRA.
109876532
1
4
Radius (cm)
Resonant frequency (GHz)
2
2.5
3
1
1.5
0.5
(a)
109876532
1
4
Height (cm)
Resonant frequency (GHz)
2
2.5
1
1.5
(b)
F : Resonant frequency as a function of the radius (a) and height (b) of a cylindrical DRA with relative permittivity
𝑟
=10.
T : Roots of the Bessel functions of the rst kind,
𝑛𝑝
.
=1 =2 =3 =4 =5
=0 . . . . .
=1 . . . . .
=2 . . . . .
where
𝑛𝑝
and
𝑛𝑝
denote the roots of the Bessel functions
of the rst kind and of the relevant rst-order derivatives,
respectively (see Tables and ).
e impact of the geometrical parameters of a cylindrical
DRA (radius and height) as well as of the relative dielectric
constant (
𝑟
)ontheresonantfrequencyisinvestigated.
Equation () is used to calculate the resonant frequency of the
fundamental model TM
110
of the cylindrical DRA. Figure
showstheresonantfrequencyasafunctionoftheDRA’s
radius (s ee Figure (a)) and as a function of the DRA’s height
(see Figure (b)). e dielectric constant used to calculate the
T : Roots of the rst-order derivative of the Bessel functions
of the rst kind,
𝑛𝑝
.
=1 =2 =3 =4 =5
=0 . . . . .
=1 . . . . .
=2 . . . . .
resonant frequencies is set to
𝑟
=10. It is indicated in the
gures that the resonant frequency decreases by increasing
either the radius or the height or both of them simultaneously.
Figureshowstheeectoftherelativedielectricconstant
(
𝑟
) on the resonant frequency. It can be noticed that the
resonant fr equency of the fundamen tal mode decreases by
increasing the dielectric constant of the DRA. is behavior is
the most important characteristic of the DRA since it allows
decreasing the size of the DRA by increasing its dielectric
constant. It is to be noted that the impedance bandwidth is
inversely proportional to the relative permittivity of the DR.
International Journal of Antennas and Propagation
100
80604020
0
0.5
1
1.5
2
2.5
0
Resonant frequency (GHz)
Relative dielectric constant (
r
)
F : Resonant frequency of a cylindrical DRA with radius = . cm and height = cm as a function of the relative dielectric constant.
x
y
d
b
b
a
Substrate
Ground
plane
Feed line
x
z
(a) (b)
F : ree-dimensional view (a) and cross-sectional view (b) of an aperture-fed rectangular DRA.
erefore, the use of materials with high dielectric constant
can result in a narrowband antenna b ehavior.
2.2. Rectangular DRA. Figure shows the three-dimensional
and cross-sectional views of a rectangular DRA fed by a
slot aperture. e rectangular DRA consists of a rectan-
gular dielectric resonator with relative dielectric constant
𝑟
.edimensionsoftherectangularDRare××
(width ×length ×height).
e main advantage of the rectangular DRA is that it is
characterized by three independent geometrical dimensions,
, ,and(see Figure (a)); this oers more design exibility
as compared to the cylindrical DRA. Furthermore, t he
rectangular DRA is characterized b y low cross-polarization
level as compared to the cylindrical DRA [].
e dielectr ic waveguide model [] is used to analyze the
rectangular DRA. When the DRA is mounted on a ground
plane, TE modes are excited. e resonant frequency of the
fundamental mode, TE
111
,iscalculatedbymeansofthe
following equations []:
0
=
2
𝑟
2
𝑥
+
2
𝑦
+
2
𝑧
,
𝑥
=
,
𝑧
=
2
,
Air gap
Ground plane
Probe feed
Hemisphere
F : Probe-fed hemispherical DRA with an air gap for
bandwidth enhancement as suggested in [].
=
2
𝑦
tanh
𝑦0
𝑦
,
𝑦0
=
2
𝑥
+
2
𝑧
,
()
where
𝑟
istherelativedielectricconstantofthematerial
forming the resonator.
2.3. Hemisphere and Cross-Shaped and Supershaped DRAs. 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 [] and is shown in Figure .
It has been found that, by incor porating this gap, the DRA
achieves an impedance bandwidth which is nearly twice that
International Journal of Antennas and Propagation
Feed line
Slot
(a) (b)
90
∘
270
∘
180
∘
0
∘
F:Cross-shapedDRA(a)andasequentiallyfedcross-shapedDRAarray(b)assuggestedin[].
without an air gap. A dedicated section summarizing the most
relevant techniques to improve the impedance bandwidth
will be presented later in the paper.
e authors in [] demonstrated for the rst time the
possibility of using glass dielectric resonators as light covers.
A dual-band hollow and solid hemispherical glass DRAs are
presented in []. e hollow hemisphere is excited by a slot
while the solid one is a probe-fed DRA. By taking advantage
ofthetransparencyoftheglassanLEDwasinsertedintothe
air gap through the ground plane resulting in a DRA that
can be used as a light cover. It has been demonstrated in the
paper that the insertion of an LED inside the DRA (for both
solid and hollow DRA) has a negligible eect on the antenna
performance.
e cross-shaped dielectric resonator is mainly suggested
to design circularly polarized antenna. A cross-shaped DRA
as suggested in [] is shown in Figure (a). To achieve a wide-
band circular polarization bandwidth the cross DRA should
be rotated
∘
with respect to the slot and the length of the
arms should be optimized []. e individual cross-shaped
DRA supports a circular polarization bandwidth of % with
an axial ratio less than dB. In addition to the circular
polarization (CP), the measured impedance bandwidth of the
cross DRA reaches % at dB return-loss level [].
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 (b). In this way, the impedance bandwidth and the
CPbandwidthareincreasedto%and%,respectively.
Recently, a very similar concept has been adopted in []
to design an mm-wave DRA. e presented design consists of
a four-by-four array of sequentially fed rectangular dielectric
resonators excited by a cross-shaped slot in order to achieve
circular polarization. e simulated impedance bandwidth
extends from . GHz to . GHz, where as the simulated
axial rat io bandwidth at dB level extends from . GHz to
. GHz.
A plastic-based supershaped DRA has b een presented in
[–] and is shown in Figure . e reported impedance
bandwidth is extended to % by using a supershaped
dielectric resonator.
Feed point
Ground plane
F : Probe-fed supershaped DRA [].
e main advantage of the presented supershaped DRA
is that it uses plastic (PolyVinyl Chloride (PVC)) as dielectric
material which makes it ver y cost eective and easily manu-
facturable.
In addition to the basic dielectric resonators tha t have
been discussed in this section, numerous attempts have been
performed to combine dierent dielectric resonators together
in order to optimize antenna performance. Figures (d), (e),
and (f) show dierent congurations of combined DRAs.
e advantages of combining dierent DRs will be presented
laterinthepaperbutrstdierentfeedingstructuresofDRAs
will be discussed in the following section.
3. Feeding Structures
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.
3.1. Probe-Fed DRA. e probe-fed D RA is among the rst
reportedDRAs[,]andisshowninFigure.Inthiscon-
guration,theDRisdirectlydisposedonthegroundplane
and is excited by a coaxial feed through the substrate. e
coaxial probe can either penetrate the DR (see Figure (a))
orcanbeplacedadjacenttotheDRasshowninFigure(b).