thermograph, antenna with no excitation, was subtracted from

each of the 11 remaining thermographs to obtain the increase in

temperature caused by Joule heating for each frequency of the

RF generator.

3. RESULTS

The heating of a planar dipole antenna due to ohmic losses was

visualized through thermographs for different frequencies of the

supplied voltage. Results showed a well-defined increment of

the antenna temperature for each voltage frequency, and allowed

to find the maximum change induced on its surface by visual

comparison.

The more representative thermographs of the heating process

are shown in Figures 2(a)–2(d). Figure 2(a) shows the antenna

at room-temperature without any RF excitation (�25�C) so no

heating was expected. The biggest temperature increase on the

dipole surface was reached by tuning the voltage to 833 MHz

corresponding to Figure 2(c), which agrees with the theoretical

resonance value, at this value a higher current density is

expected on the antenna. Finally, Figures 2(b) and 2(d) were

taken at frequencies below and above the antennas’ resonance

where the transfer of energy between the antennas and source is

not optimum.

The mean temperature increase for each frequency is shown

in Figure 3(a). The results follow the frequency behavior of the

half-wave dipole showing that the induced current increases sig-

nificantly when resonance is achieved around 833 MHz. It is

worth noting that also a thermal drift of the signal is observed

(red line). This is caused by the heating of the substrate which

can be seen in Figure 2.

To remove the substrate heating effect, it is assumed in a

first approximation that the temperature of substrate increases

linearly with the frequency of excitation, so a straight line is fit-

ted to the first four points of the curve and the room-

temperature thermograph with no heating. Figure 3(b) shows the

pure contribution of the antenna where the heating of the sub-

strate was subtracted.

4. CONCLUSION

In this work, a novel method of testing based on infrared ther-

mography was used to find the resonant frequency of a planar

half-wave dipole designed to work at the UHF band. By using

an infrared camera, it is possible to visualize the thermal heating

of an antenna of which the induced currents are responsible.

This makes possible to characterize planar antennas in a nonin-

vasive way, which might be useful for in-situ evaluation and

characterization of antennas where there would be no need to

take the antenna to a special antenna-characterization facility.

REFERENCES

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8. F.J. Zucker, Antenna theory, McGraw-Hill, New York, 1969.

VC 2014 Wiley Periodicals, Inc.

VERY COMPACT PALMATE LEAF-SHAPED CPW-FED MONOPOLEANTENNA FOR UWB APPLICATIONS

Mohammad M. Fakharian and Pejman RezaeiDepartment of Electrical and Computer Engineering, SemnanUniversity, Semnan, Iran; Corresponding author:m_fakharian@sun.semnan.ac.ir

Received 5 November 2013

ABSTRACT: This article presents a very compact coplanar waveguide-

fed planar monopole antenna for ultrawideband (UWB) applications.The antenna consists of a palmate leaf-shaped radiator with a modifiedshaped ground plane on the same side of the substrate. The measured

impedance bandwidth of the proposed antenna is from 3.08 to over 14GHz with a ratio of about 4.6:1 for VSWR� 2. Experimental results

show that the proposed antenna has stably omnidirectional H-planeradiation patterns with low cross-polarization level and average peakgain of 3 dBi across the UWB. The antenna dimensions are restricted to

13.5 3 14.8 3 0.8 mm3. VC 2014 Wiley Periodicals, Inc. Microwave Opt

Technol Lett 56:1612–1616, 2014; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.28395

Key words: ultrawideband; coplanar waveguide; monopole antenna;compact size

1. INTRODUCTION

Recently, ultrawideband (UWB) wireless systems have drawn a

wide range of applications including ground penetrating radars,

high-resolution microwave imaging, communication systems for

military, body area networks, and UWB short pulse radars for

automotive and robotics applications [1–3]. UWB systems are

characterized by low complexity, low operating power level,

high precision ranging, high data rates, very low interferences,

and great capacity. One of the most fundamental parts of the

UWB systems is antennas. They are required to operate in the

ultrawide bandwidth of 3.1–10.6 GHz since the Federal Com-

munications Commission (FCC) released its report in 2002 [4],

be omnidirectional radiation patterns, be simple and compact

with small dimensions and light weight.

The monopole UWB antennas with various shaped planar ele-

ments such as square, elliptical, triangle, circular, annual ring,

pentagonal, and crescent geometries have been studied in previ-

ous literature [5–9]. Most of the UWB antennas mainly focus on

two types of feed structure, that is, microstrip [10, 11] and copla-

nar waveguide (CPW) line. The CPW feed line is preferred due

to its small size, low profile, low radiation loss, and easy integra-

tion with microwave monolithic integrated circuit (MMIC).

Hence, many planar CPW-fed antenna configurations have been

designed and developed [12–14].

In this article, a novel leaf-shaped CPW-fed planar monopole

antenna is presented for UWB operation with a compact size

only 13.5 3 14.8 mm2, significantly less than those antennas

1612 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 7, July 2014 DOI 10.1002/mop

reported in Refs. [6–8, 13]218. In Table 1, the proposed

monopole antenna is compared with some recently published

UWB antennas. The proposed antenna consists of a modified

palmate leaf-shaped radiating element and is fed by a modified

CPW feed line.

The palmate leaf resembles a hand with the fingers spread.

This kind of the leaf has been compared with some others of

the leaves such as: pedate, hastate, digitate and so forth. (Fig.

1). This novel configuration could considerably improve its radi-

ation performance, wider bandwidth, and decrease the antenna

size. The Measured results show that the proposed antenna can

achieve a bandwidth of more than 10 GHz, from 3.08 to over

14GHz for VSWR� 2, a relatively stable omnidirectional radia-

tion pattern across the UWB and a good average peak gain. To

find out how the antenna physical dimensions influence its

impedance bandwidth, the need for a parametric study is neces-

sary. All analytical studies presented here are carried out using

Ansoft HFSS [20] that is based on the finite element method in

frequency domain.

2. ANTENNA CONFIGURATION AND DESIGN

The geometrical configuration of the proposed antenna with its

dimensional parameters is depicted in Figure 1. Antenna is

designed on a low-cost FR4 substrate with dielectric constant

TABLE 1 Comparison between Proposed and Recently Reported UWB Antennas

UWB Antennas [(Refs.]) Dimensions (mm3) Compression (%) Band (GHz)

UWB square monopole antenna [6] 18 3 12 31.6 116.22 2.97–12.83

A compact circular-ring antenna [7] 28 3 26 3 1.6 628.73 3.7–18

A compact pentagonal monopole antenna [8] 30 3 11 3 1.6 230.33 2.8–12

Very compact ultrawideband antenna [13] 18 3 10 3 1.6 80.18 3.34–16.3

A CPW-fed inverted L-strip antenna [14] 25 3 25 3 1.6 525.63 2.6–13.04

CPW-fed hexagonal-shape UWB antenna [15] 25 3 23 3 1.6 475.57 2.71–12.61

Printed omnidirectional UWB antenna [16] 30 3 8 3 0.8 20.12 2.75–16.2

Compact-size linearly tapered slot antenna [17] 35 3 36 3 0.8 530.63 3.1–10.6

A very compact CPW-fed circular antenna [18] 12 3 22 3 0.8 32.13 2.9–11.6

Proposed structure 13.5 3 14.8 3 0.8 0 3.08–14.38

Figure 1 Morphology characteristics of some leaves [19]. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]

Figure 2 (a) Geometry of the proposed monopole antenna and (b) idea for the proposed structure: a natural palmate leaf. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 7, July 2014 1613

er 5 4.6 and dielectric loss tangent tan d 5 0.02, and thickness

h 5 0.8 mm. A 50-XCPW feeding technique is adopted to excite

the leaf-shaped monopole antenna. The CPW feed line is modi-

fied and an SMA connector is connected to the port of it.

By examining the current distribution of planar monopole

antenna with regular shapes, that is, square, circular and so

forth, the proposed cuttings in the palmate leaf-shaped monop-

ole antenna are used to enlarge the antenna perimeter which

affects the lower resonant frequency and then increasing the

maximum achieved impedance bandwidth [21].

It is well known that the current distribution is mainly con-

centrated in the edges rather than in the center of the printed

monopole antenna. Thus, by increasing the antenna perimeter p,

the surface current will take longer path and this will be equiva-

lent to a longer length monopole and in turn will decrease the

lowest resonance frequency fL [22]:

eeff �ðer11Þ

2(1)

fLðGHz Þ5300=pffiffiffiffiffiffiffieeff

p(2)

The ground plane of the proposed antenna is designed to

have nonrectangular shape by cutting the upper left and right

corners of the rectangular shape. The truncated ground plane

effectively enhances the antenna impedance bandwidth [12].

On FR4, with improved parameters, size of the antenna is

restricted to 13.5 3 14.8 mm2. Parameters for the proposed

monopole antenna are as follows: W 5 13.5 mm, L 5 14.8 mm,

G1 5 3.35 mm, G2 5 2.4 mm, Wf 5 1.5 m, gf 5 0.15 mm,

W1 5 4 mm, W2 5 6.7 mm, W3 5 5.15 mm, W4 5 2.45 mm,

W5 5 2.1 mm, W6 5 0.8 mm, R1 5 5.6 mm, R2 5 1.8 mm,

R3 5 3.9 mm, R4 5 2.4 mm, R5 5 3.85 mm, R6 5 2.65 mm,

R7 5 3.5 mm, d1 5 0.7 mm, d2 5 1 mm, d3 5 1.4 mm, d4 5 2

mm, d5 5 1.45 mm, d6 5 1.1 mm, and d7 5 0.75 mm.

Figure 3 The current distributions on the antenna at: (a) 4 GHz, (b) 7 GHz, and (c) 10 GHz. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com]

Figure 4 Simulated VSWR curves for various ground planes. [Color

figure can be viewed in the online issue, which is available at wileyonli-

nelibrary.com]

Figure 5 Photograph of the antenna prototype. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com]

Figure 6 Measured and simulated VSWR of the proposed antenna.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com]

1614 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 7, July 2014 DOI 10.1002/mop

3. RESULTS AND DISCUSSIONS

To study the effects of variations in critical design parameters

on the impedance bandwidth of the presented antenna, the sur-

face current distributions on the antenna are investigated. Fig-

ure 2 shows the simulated current distribution on the antenna

at 4, 7, and 10 GHz. Figure 2(a) shows the surface current dis-

tributions at 4 GHz. As shown in this figure, the electric cur-

rent is intensified around the ground plane and in the lower

part of the leaf-shaped radiating patch. Consequently, the per-

formance of the antenna is dependent on these sections mainly

near the resonance frequency at 4 GHz. It is observed in Fig-

ure 2(b) that the current distributions are more concentrated on

the ground plane than the radiating patch at 7 GHz frequency.

Conversely, the current distribution on the entire ground plane

has a significant effect on the antenna impedance bandwidth.

Figure 2 (c) shows that the current distributions are concen-

trated on the ground plane and on the outer sides of the radiat-

ing patch with the exclusion of the top and bottom parts at 10

GHz. In the following, one of the critical design parameters is

examined on the antenna impedance bandwidth. The effect of

the various truncates in the ground plane on the impedance

bandwidth is shown in Figure 3. As shown in Figure 3, without

truncation in the ground plane (a), the center edge of 10 dB

frequency bandwidth decay and causes the antenna impedance

bandwidth to split in two bands. While for designs (c) and (d)

a wider impedance bandwidth are achieved. So, the truncation

in the ground plane plays an important role in obtaining UWB

behavior.

Figure 7 Measured radiation patterns of the proposed antenna at (a) 4 GHz, (b) 7 GHz, and (c) 10 GHz for H- and E-planes. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 7, July 2014 1615

A prototype of the proposed monopole antenna based on

the optimized parameters is fabricated and tested, as shown

in Figure 4. The simulated and measured VSWR curves of

the proposed antenna are in good agreement as shown in Fig-

ure 5. The little difference between them results may be due

to the soldering effect of the SMA connector and its manu-

facturing tolerance, which have been neglected in our simula-

tions. It is observed from the measured results that the

designed UWB antenna exhibits an impedance bandwidth of

more than 10 GHz starts from 3.08 GHz to over 14 GHz for

VSWR� 2.

The measured radiation patterns of the proposed UWB

antenna at frequencies 4, 7, and 10 GHz in E- and H-planes are

shown in Figure 6. As expected, the radiation patterns of the

proposed antenna in the H-plane are omnidirectional and almost

bidirectional in the E-plane with stable radiation patterns with

frequency.

The measured and simulated peak gain of the proposed

antenna is shown in Figure 7. It can be noticed that the UWB

antenna gain is almost stable over the whole frequency band. It

can also see that its gain is about 2–4 dBi in most of its imped-

ance bandwidth due to its compact size and truncated ground

plane. The gain bandwidth of the proposed antenna is mainly

limited by its low gain below 5 GHz, which may be caused by

its small size.

4. CONCLUSION

In this article, a compact palmate leaf-shaped planar monopole

antenna with a size of 13.5 3 14.8 mm2 has been presented,

which is fed by a tapered CPW feed line. Measurement results

have shown that the proposed antenna can achieve a wide band-

width (VSWR� 2) of more than 128% (from 3.08 GHz to more

than 14 GHz), a stable omnidirectional H-plane radiation pattern

and an average gain of 3 dBi. These achievements make the

proposed antenna an excellent candidate for UWB or other com-

mercial communication systems.

ACKNOWLEDGMENTS

This work has been supported financially by the Office of

Brilliant Talents at the Semnan University. The authors would

like to thank all the members of the Antenna Laboratory at

Iran Telecommunication Research Center, especially Mr.

Solat, Mr. Akhlaghpasandi, and Mr. Mirabdollahi, for their

cooperation.

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VC 2014 Wiley Periodicals, Inc.

Figure 8 Measured and simulated peak gain of the proposed antenna.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com]

1616 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 7, July 2014 DOI 10.1002/mop