Miniature Planar Antenna Design for Ultra-Wideband Systems

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Chapter 2
Miniature Planar Antenna Design for Ultra-Wideband
Systems
Mohammad Alibakhshikenari,
Mohammad Naser‐Moghadasi,
Ramazan Ali Sadeghzadeh, Bal Singh Virdee and
Ernesto Limiti
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/intechopen.68612
Provisional chapter
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
DOI: 10.5772/intechopen.68612
Miniature Planar Antenna Design for Ultra-Wideband
Systems
Mohammad Alibakhshikenari,
MohammadNaser-Moghadasi,
RamazanAliSadeghzadeh, Bal Singh Virdee
and Ernesto Limiti
Additional information is available at the end of the chapter
Abstract
Demand for antennas that are compact and operate over an ultra‐wideband (UWB) fre‐
quency range is growing rapidly as UWB systems oer high resolution imaging capa‐
bility and high data rate transmission in the order of Gb/s that is required by the next
generation of wireless communication systems. Hence, over the recent years the research
and development of UWB antennas has been widely reported in literature. The main
performance requirements sought from such antennas include: (1) low VSWR of <2; (2)
operation over 7.6 GHz from 3 to 10.6 GHz; and (3) good overall radiation characteristics.
Signicant size reduction and low manufacturing cost are also important criteria in order
to realize a cost‐eective and miniature system. Other desirable requirements include
compatibility and ease of integration with RF electronics.
Keywords: ultra‐wideband antenna, metamaterial, composite right/left‐handed
transmission line, antenna miniaturization, slit antennas
1. Introduction
Ever since the Federal Communications Commission (FCC) released a bandwidth of 7.5 GHz
(from 3.1 to 10.6 GHz) for ultra‐wideband (UWB) wireless communications, UWB technology
has rapidly developed for high data rate wireless communications [1–5]. As is the case in con‐
ventional narrowband wireless communication systems, an antenna plays a very crucial role
in UWB systems. However, there are greater challenges in designing a UWB antenna than a
narrow band one. A suitable UWB antenna should be capable of operating over an ultra‐wide
© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.

bandwidth as dened by the FCC. At the same time, satisfactory radiation properties over the
UWB frequency range are also necessary, including nondispersive nature in order to minimize
distortion in the transmied signal. The antenna needs to have a low voltage standing wave
ratio (VSWR) (<2) over 3.1–10.6 GHz band and omnidirectional radiation characteristics [6].
Nowadays, in most applications signicant size reduction of antennas is paramount in order
to achieve minimization of communication systems. Other desirable features include being a
planar structure that is cost‐eective to fabricate in large volumes.
Design of a UWB antennas is challenging for systems operating at the lower part of the micro‐
wave spectrum. This is due to the fact that the wavelength is very large at these frequen‐
cies and the resulting physically large antennas are not desirable due to space limitations of
modern systems. This problem is particularly severe in systems that operate at HF, VHF, and
UHF bands. This is because in such applications having antennas with low visual signatures
is of paramount importance. The current antennas of choice for these applications tend to be
monopole whip antennas. These antennas, however, suer from two major drawbacks. First,
the relatively large heights of a conventional monopole whip antenna, when mounted on a
vehicle, such an antenna signicantly protrudes from the top surface of the vehicle drastically
increasing the visual signature of the vehicle. The second issue with monopole whip anten‐
nas is their narrow bandwidths, which limits the types of waveforms that they can receive
or transmit. Therefore, development of compact, low‐prole, and ultra‐wideband antennas
is of particular interest in many communications systems that operate at HF, VHF, and UHF
frequencies. To increase the bandwidth of monopole‐type radiators, a number of dierent
techniques have been examined. A variety of printed monopole antennas that provide UWB
operation in a planar form factor are examined in the 3.1–10.6 GHz band [7–9]. However, at its
lowest frequency of operation, a printed monopole tends to have relatively large dimensions.
Numerous techniques have been investigated and reported in recent years to reduce the size
of microstrip antennas. These techniques are mainly based on loading a patch antenna with
reactive components realized with suitably designed slots, shorting posts, and lumped ele‐
ments. The eectiveness of these techniques is, however, limited in the reduction of the foot‐
print of planar antennas which is required by modern wireless systems [10–12]. The size of
patch antennas can also be reduced by fabricating the antenna on dielectric substrates with a
high permiivity. This well‐established technique, however, results in increased surface wave
excitation over the antenna that degrades its impedance bandwidth, radiation eciency, and
its radiation characteristics [13].
Techniques mentioned above fail to meet the challenges for miniaturization of antennas. In
this chapter, the exploitation of articially engineered materials that are based on metamate‐
rial transmission lines is shown to provide the solution for miniaturization of planar anten‐
nas [14–22]. In fact metamaterials, which are also accurately referred to as composite right‐/
left‐handed (CRLH) transmission lines, are a novel paradigm in electromagnetic engineering
as such materials exhibit electrical characteristics, that is, negative permiivity and perme‐
ability, not possible with naturally occurring materials [11]. These properties are exploited
in this chapter to design planar antennas with a small footprint using standard manufactur‐
ing photolithography techniques [11–18]. In particular, in this chapter, a unique CRLH‐TL
structure is employed in the design of an antenna using conventional microwave integrated
Trends in Research on Microstrip Antennas40

circuit (MIC) manufacturing techniques. The unit cells of the CRLH‐TL structure was realized
by engraving L‐ and T‐shaped slits on a rectangular conductor that is grounded through a
high impedance microstrip stub that are spiraled. In the antenna the L‐ and T‐shaped slits
exhibit capacitive property, and the stub acts like an inductor. The antenna structure, which
was created using these unit cells, was modeled and optimized for UWB performance while
maintaining its radiation characteristics in terms of gain and radiation eciency.
2. Composite right‐/left‐handed antenna design
A left‐handed transmission line structure can be created from an arrangement consisting of a
series capacitor and shunt inductor. This LC circuit conguration can be achieved for antenna
applications by simply etching a dielectric slot in a metallic‐radiating patch, where the patch is
grounded using a high impedance microstrip line. The slot created in the patch acts like a series
capacitance CL and the high impedance microstrip line acts like a shunt inductance LL. The
inductive element in the structure can be coiled to reduce the overall footprint of the antenna
structure [23, 24]. This approach is used here to implement a compact antenna by cascading
together a number of CRLH transmission line unit cells comprising left‐handed LC unit cells.
The equivalent circuit model for a lossless CRLH transmission line unit cell is shown in
Figure 1. It consists of a per‐unit length impedance Z (W/m) constituted by a right‐handed
per‐unit‐length inductance LR (H/m) in series with a left‐handed per‐unit‐length capacitance
CL (F/m), and a per‐unit‐length admiance Y (S/m) constituted by a right‐handed per‐unit‐
length capacitance CR (F/m) in parallel with a left‐handed per‐unit‐length inductance LL
(H/m). The complex propagation constant γ and the propagation constant β of the CRLH
transmission line unit cell are given by
Figure 1. Equivalent circuit model for an ideal CRLH transmission line.
Miniature Planar Antenna Design for Ultra-Wideband Systems
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41

γ = α + jβ =
√
___
ZY (1)
Where
β
(
ω
)
= s
(
ω
)
√
______________________
ω
2 L
R C
R + 1
______
ω
2 L
L C
L
−
(
L
R
_
L
L
+
C
R
_
C
L
)
(2)
s
(
ω
)
=
⎧
⎪
⎨
⎪
⎩
− 1 ifω < ω
se = min
(
1
_
√
_
L
R C
L
, 1
_
√
_
L
L C
R
)
0 if ω
se < ω < ω
sh
+1 ifω > ω
sh = max
(
1
_
√
_
L
R C
L
, 1
_
√
_
L
L C
R
)
⎫
⎪
⎬
⎪
⎭
(3)
and
Z
(
ω
)
= j
(
ω L
R − 1
_
ω C
L
)
(4)
Y
(
ω
)
= j
(
ω C
R − 1
_
ω L
L
)
(5)
where β
(
ω
)
, s
(
ω
)
, Z
(
ω
)
, and Y
(
ω
)
represent the dispersion relation, sign function, impedance, and
admiance of the antenna structure, respectively. Series and shunt resonance frequencies,
respectively, are given by
ω
se = 1
_____
√
_____
L
R C
L
(6)
ω
sh = 1
_____
√
_____
L
L C
R
(7)
The unit cell’s permiivity and permeability are given by
μ = Z
__
jω = L
R − 1
____
ω
2 C
L
(8)
ε = Y
__
jω = C
R − 1
____
ω
2 L
L
(9)
The prototype antenna based on CRLH transmission line unit cells, shown in Figure 2, con‐
sists of L‐ and T‐shaped slits etched on a rectangular radiation patch. The patch is short‐
circuited to ground through high impedance microstrip lines that are coiled to reduce the
antennas’ size. Appropriate number of CRLH unit cells is cascaded together in the antenna
that is terminated in a matched load to achieve the required bandwidth and radiation charac‐
teristics. The antenna was fabricated on glass‐reinforced epoxy FR‐4 substrate with a dielec‐
tric constant of 4.6, thickness of 0.8 mm, and loss tangent of 1 × 10−3. Standard manufacturing
technique was used to realize the antenna.
The antenna design was rst analyzed and optimized using ANSYS high frequency structure
simulator (HFSS™). Two waveguide ports were dened to represent the input and output
of the antenna structure, as shown in Figure 2(a), in order to evaluate its performance. The
antenna was excited at port 1 through an SMA connector, and terminated to a matched 50 Ω
load at port 2 using surface mount technology (SMD1206) of dimensions 3.5 × 1.8 mm2.
Trends in Research on Microstrip Antennas42

(a)
(b)
(c)
Figure 2. The proposed antenna based on four CRLH transmission line unit cells. (a) Conguartion of the proposed
CRLH transmission line antenna. (b) Isometric view of the proposed CRLH transmission line antenna. (c) Fabricated
antenna prototype.
Miniature Planar Antenna Design for Ultra-Wideband Systems
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A more accurate model of the proposed CRLH transmission line unit cell that is employed
in the antenna is shown in Figure 3. The model includes loss components in the unit cell
structure that are represented by right‐handed resistance (RR) and conductance (GR), and left‐
handed resistance (RL) and conductance (GL). These parameters account for the radiation emit‐
ted by the antenna. Optimized magnitudes of these parameters in the unit cell were obtained
from ANSYS HFSS™, that is, LL = 6 nH, CL = 2.4 pF, LR = 2 nH, CR = 1 pF, RL = 5 Ω, RR = 3 Ω,
GL = 4.5 ℧, and GR = 2 ℧.
The actual antenna’s dimensions are 22.6 × 5.8 × 0.8 mm3 or 0.037 λ0 × 0.009 λ0 × 0.001 λ0, where
λ0 is free space wavelength at 0.5 GHz. The simulated and measured impedance bandwidth
of the antenna are 11.1 GHz (0.35–11.45 GHz) and 10.8 GHz (0.5–11.3 GHz), respectively, for
voltage standing wave ratio (VSWR) < 2. This corresponds to a fractional bandwidth of 188%
for the simulation result, and 183% for the measured result. The reection coecients and the
measured VSWR of the proposed miniaturized antenna are shown in Figure 4. The simulated
and measured group delay response depicted in Figure 5 shows the group delay variation is
under 0.25 ns for a large frequency range up to 16 GHz.
Results of the parametric study are shown in Figure 6. It is evident from these results that
the antenna’s gain and radiation eciency can be improved by increasing the number of
CRLH unit cells. The peak gain and radiation eciency are obtained at around 8 GHz. Gain
of greater than 4 dBi from 2.9–12.6 GHz is achieved for a unit cell of four. Over this frequency
range, the radiation eciency exceeds 50%. As the proposed antenna had to t within an area
of 23 × 6 mm2, the number of unit cells selected in the design was therefore four.
The simulated and measured gain and radiation eciency of the antenna at various frequen‐
cies are given in Tables 1 and 2. The results in these tables show a beer performance obtained
from the antenna over higher frequencies than at the lower frequencies. The simulated three‐
dimensional (3‐D) and measured two‐dimensional (2‐D) radiation paerns of the compact
CRLH antenna at various frequencies are shown in Figures 7 and 8, respectively. The results
show the cross‐polarization is comparable or higher than copolarization at certain frequen‐
cies, however, over a small angular range in both the E‐ and H‐planes. This is observed in
L
R
/2 R
R
/2
2G
L
2C
L
G
R
C
R
R
L
L
L
L
R
/2R
R
/2
2G
L
2C
L
Figure 3. Equivalent circuit model of the CRLH unit cell employed in the proposed antenna.
Trends in Research on Microstrip Antennas44

other CRLH antennas too [25–31] which present scope for improvement of such antennas.
Cross‐polarization can be reduced by loading the shorting pins, which has been demonstrated
for patch antennas [32–35]. The shorting pins are located in the centerline of a square patch to
strengthen the surface current density near the feeding point at edge. Because of symmetric
arrangement of these two shorting pins, surface current density on the patch is maintained as
the odd‐symmetric property with respect to the H‐plane, thus tremendously degrading the
cross‐polarization level. Such a solution is worthy of investigation in CRLH antennas.
The measured gain and radiation eciency of the CRLH antenna is shown in Figure 9. The
antenna has a peak gain of 6.5 dBi and radiation eciency of 88% at around 8 GHz. The
antenna has a gain that exceeds 4 dBi over 3.5–12.5 GHz, and radiation eciency over this
frequency range is greater than 53%.
(a)
(b)
Figure 4. Reection coecient and measured VSWR response of the CRLH transmission line antenna. (a) Simulated and
measured reection coecient response. (b) Antenna’s measured VSWR response.
Miniature Planar Antenna Design for Ultra-Wideband Systems
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Table 3 shows the proposed antenna’s salient features. The antenna is highly compact and
operates over an ultra‐wideband (UWB). In addition, it can be easily integrated with RF cir‐
cuits making it suitable for UWB wireless communication systems. Characteristics of the
CRLH antenna are compared with other recently reported antennas in Table 4.
Frequency (GHz) 0.5 3 5 8 11.3
Gain (dBi) 1.7 3.8 5.1 7 6.1
Eciency (%) 25 50 62 91 79
Table 1. Simulated gain and radiation eciency.
Figure 5. Group delay response of the CRLH transmission line antenna.
Figure 6. The antenna’s simulated radiation gain and eciency response as a function of number of unit cells.
Trends in Research on Microstrip Antennas46

Frequency (GHz) 0.5 3 5 8 11.3
Gain (dBi) 1.5 3.4 4.8 6.5 5.7
Eciency (%) 20 45 57 88 73
Table 2. Measured gain and radiation eciency.
@500 MHz @3GHz
@5GHz @8GHz
@11.3GHz
Figure 7. Simulated 3‐D radiation paerns of the CRLH antenna.
Miniature Planar Antenna Design for Ultra-Wideband Systems
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47

(a)
(b)
Figure 8. The measured radiation paerns of the CRLH transmission line antenna in the E‐ and H‐planes. (a) Measured
E‐plane co and cross radiation paerns (Co: red line, Cross: blue line). (b) Measured H‐plane co and cross radiation
paerns (Co: red line, Cross: blue line).
Trends in Research on Microstrip Antennas48

Dimensions 22.6 × 5.8 × 0.8 mm3
0.037 λ0 × 0.009 λ0 × 0.001 λ0 at 0.5 GHz
Bandwidth 10.8 GHz (500 MHz–11.3 GHz)
(Fractional bandwidth = 183%)
Gain (dBi) 6.5 (max) at 8 GHz
≥4 from 2.9–12.6 GHz
Eciency (%) 88 (max) at 8 GHz
≥50% from 2.9–12.6 GHz
Table 3. Measured characteristics of the CRLH antenna.
Reference Dimensions Fractional
bandwidth (%)
Peak gain (dBi) Max. eciency (%)
[2] 0.051 λ0 × 0.016 λ0 × 0.002 λ0123.8 2.8 70
[6] with 7 unit cells 0.556 λ0 × 0.179 λ0 × 0.041 λ087.16 3.4 68.1
[6] with 8 unit cells 0.564 λ0 × 0.175 λ0 × 0.02 λ084.23 2.35 48.2
[23] 0.047 λ0 × 0.021λ0 × 0.002 λ0104.76 2.3 62
Proposed antenna 0.037 λ0 × 0.009 λ0 × 0.001 λ0183 6.5 88
Table 4. Comparison of the CRLH antenna with other reported antennas.
Figure 9. Measured gain and radiation eciency response of the CRLH antenna.
Miniature Planar Antenna Design for Ultra-Wideband Systems
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To summarize, this chapter presented the design and measured results of a novel antenna
that is based on CRLH transmission lines. The antenna is highly compact planar structure
with dimensions of 22.6 × 5.8 × 0.8 mm3 and possesses desirable characteristics of ultra‐wide‐
band performance (500 MHz–11.3 GHz) with gain and radiation eciency of 6.5 dBi and
88%, respectively, at 8 GHz. The low cost antenna is simple to design and easy to fabricate
using standard manufacturing techniques. The CRLH transmission line unit cell constituting
the antenna is realized by etching L‐ and T‐shaped dielectric slots inside a rectangular patch,
which is grounded through a high impedance transmission line. By cascading together sev‐
eral unit cells, the desired bandwidth and radiation characteristics can be obtained. In addi‐
tion, the antenna can be easily integrated with RF electronics.
Author details
Mohammad Alibakhshikenari1*, Mohammad Naser‐Moghadasi2, Ramazan Ali Sadeghzadeh3,
Bal Singh Virdee4 and Ernesto Limiti1
*Address all correspondence to: alibakhshikenari@ing.uniroma2.it
1 Electronic Engineering Department, University of Rome “Tor Vergata”, Rome, Italy
2 Faculty of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran
3 Faculty of Electrical Engineering, K. N. Toosi University of Technology, Tehran, Iran
4 London Metropolitan University, Center for Communications Technology, London, UK
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