coplanar strip fed uwb antenna with a step cut

Coplanar Strip Fed UWB Antenna with a Step Cut

Ayman S. Al-zayed, V. A. Shameena

Department of Electrical Engineering, Kuwait University, Kuwait

Received 16 December 2013; accepted 28 April 2014

ABSTRACT: A novel compact coplanar strip fed dipole antenna with a step cut suitable for

ultrawideband (UWB) application is developed. The antenna is evolved from an open ended

slot line by symmetrically etching out two rectangular metallic parts from its upper inner

corners. The antenna has 210 dB reflection coefficient from 3.1 to more than 12 GHz that

covers the Federal Communication Commission (FCC) specified UWB frequency range.

From the simulation and experimental studies, it is found that the proposed antenna deliv-

ers moderate gain and stable radiation patterns over the operating band. Time domain

analysis on the proposed antenna has been conducted and was found that the antenna can

be used for UWB applications. The proposed antenna occupies a compact size of 28.5 3 10

3 1.6 mm3. VC 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:665–672, 2014.

Keywords: coplanar strip fed; uniplanar dipole antenna; ultrawideband; transient analysis

I. INTRODUCTION

Ultrawideband (UWB) radio technology has been attract-

ing the attention of many researchers due to its advantages

such as high data rate and low probability of detection for

military communications. Applications of such bandwidth

ranges from imaging systems, communications and mea-

surement systems, and vehicular radar systems [1].

With the fast development of this technology, there are

many articles discussing the high performance of UWB

antennas. Among the UWB antennas reported in literature,

printed dipole antennas are considered to be highly attrac-

tive for their merits such as compact size, low fabrication

cost, and suitability for integration with feed network [2].

Different UWB planar dipole designs have been reported

in the literature [2–8]. However, most of the designs

aforementioned have complicated structures. Different

microstrip UWB antennas are also reported [9, 10]. These

designs have large and complex geometries and poor radi-

ation performance.

The coplanar strip fed (CPS) also called slot line fed

may be considered as a complementary of the coplanar

wave guide. The main advantage of this uniplanar trans-

mission line is the ease of mounting active and passive

components to it. Slot line fed antennas have been investi-

gated in [11, 12]; both of these designs have smaller

bandwidth than that required for UWB performance.

Many consumer electronic devices require compact

antennas. However, as the size reduces, performance of

the antenna degrades, and therefore, designing compact

antennas with good performance continue to be a chal-

lenging and interesting task.

Since UWB technology is based on pulse transmission,

time domain analysis of the UWB antenna is equally

important as the frequency domain analysis. Therefore,

time domain parameters such as group delay and fidelity

must be considered and analyzed to assure good time

domain performance for the UWB antenna.

In this article, we present a step cut slot line fed UWB

dipole antenna. The antenna is evolved from an open

ended slot line by symmetrically removing two rectangu-

lar metallic parts from its upper inner corners. The

antenna has moderate gain and stable radiation patterns.

To analyze whether any mismatch occurs when the

antenna is connected to an unbalanced coaxial SMA con-

nector, performance of the antenna is investigated with

and without a balun feed. It was found that antenna gives

similar performances in both cases. Simple design equa-

tions are developed so that antenna can be redesigned for

desired substrates and operating frequencies. Excellent

time domain response is obtained for the antenna which

confirms its suitability for pulsed UWB applications.

Compared to antennas aforementioned the proposed

design has simple geometry, small area, improved radia-

tion patterns, higher radiation efficiency, and flat group

Correspondence to: A. S. Al-zayed; e-mail: ayman.alzayed@

ku.edu.kw.

DOI: 10.1002/mmce.20810

Published online 17 May 2014 in Wiley Online Library

(wileyonlinelibrary.com).

VC 2014 Wiley Periodicals, Inc.

665

delay. The antenna has a compact overall dimension of

28.5 3 10 3 1.6 mm3.

II. ANTENNA GEOMETRY

The geometry of the step cut UWB antenna is shown in

Figure 1. This novel antenna consists of a slot line with

two symmetrical rectangular metal pieces of dimensions L3 W separated by a gap (G). Two symmetrical rectangles

of dimension L1 3 W1 are removed from the upper part.

The signal is fed by a coaxial cable connected to F1 and

the ground is connected to F2. The antenna is made of

commercially available FR4 epoxy substrate having

dielectric constant (er) of 4.4, and thickness (h) of

1.6 mm. The overall dimension of the antenna is 28.5 3

10 3 1.6 mm3 which is very compact. Dimensions of the

antenna are given by L 5 10 mm, W 5 14 mm,

L1 5 2 mm, W1 5 4.5 mm, G 5 0.5 mm, and h 5 1.6 mm.

III. PARAMETRIC ANALYSIS AND DESIGN

To analyze how the reflection coefficient is varied with

various dimensional parameters of the antenna, a detailed

parametric analysis on the antenna is carried out. All the

simulations are performed using full-wave electromagnetic

simulation software Agilent ADS Momentum.

A. Effect of CPS length L on Reflection CoefficientThe effect of variation of reflection coefficients with the

length of the CPS L is shown in Figure 2. From the figure

it shows that there are three resonances produced by the

antenna and all the three resonances are affected by L.

The second resonance shows a down shift in frequency

with increase in L, whereas the first resonance shows a

very little higher shift in frequency with increase in L.

This shifting of frequency to higher values is due to the

concentration of fringing field near the step cut. It is to be

mentioned that for the first two values of L, only the first

two resonances are obtained and third resonance appears

when L is larger than 10 mm. Thus, it is found that opti-

mum UWB performance is obtained for a value of

L 5 10 mm. The other parameters of the antenna are fixed

as W 5 14 mm, L1 5 2 mm, W1 5 4.5 mm, G 5 0.5 mm,

and h 5 1.6 mm.

B. Effect of CPS width W on Reflection CoefficientThe width of the CPS W is varied and the corresponding

reflection coefficients are plotted in Figure 3. All of the

three resonances are strongly affected as W is varied. It

can be seen from the figure that the resonances decrease

in frequency with increase in W. This is due to the

increase in the length of surface current path as the

parameter W increases. For optimum matching and

bandwidth, optimal value of W was found to be 14 mm.

Other parameters of the antenna are given by

L 5 10 mm, L1 5 2 mm, W1 5 4.5 mm, G 5 0.5 mm, and

h 5 1.6 mm.

C. Effect of Step Cut Length L1 on Reflection CoefficientFigure 4 shows the effect of varying the length L1 of the

step cut on the reflection coefficient. This parameter

seems to have strong influence on the bandwidth. All the

resonances are strongly affected by this parameter with

Figure 1 Antenna geometry. (a) Top view. and (b) Side view.

Figure 2 Reflection coefficients for different L values.

Figure 3 Reflection coefficients for different W values.

666 Al-zayed and Shameena

International Journal of RF and Microwave Computer-Aided Engineering/Vol. 24, No. 6, November 2014

the first and third resonances show a lower shift in fre-

quency, whereas the second resonance shows a higher

shift in resonant frequency with increase in L1. For opti-

mum performance L1 is optimized to be 2 mm. Other

parameters of the antenna are fixed as L 5 10 mm,

W 5 14 mm, W1 5 4.5 mm, G 5 0.5 mm, and h 5 1.6 mm.

D. Effect of Step Cut Width W1 on Reflection CoefficientTo complete this parametric study, variation of reflection

coefficients of the antenna with step cut width W1 is con-

ducted and is given in Figure 5. The first and third

resonances show a lower shift with increase in W1,

whereas the second resonance was found to increase to

higher frequency with increase in W1. The variation in

resonant frequencies is found to be similar to that in the

case of L1. Optimum matching and bandwidth is obtained

when the value of W1 is equal to 4.5 mm. The other

parameters of the antenna are fixed as L 5 10 mm,

W 5 14 mm, L1 5 2 mm, G 5 0.5 mm, and h 5 1.6 mm.

From the parametric analysis performed, simple design

equations for the proposed antenna are developed as

follows

L50:38k0

�eeff

; (1)

W50:52k0ffiffiffiffiffiffiffi

eeffp ; (2)

L150:07k0ffiffiffiffiffiffiffi

eeffp ; (3)

W150:17k0

�eeff

: (4)

where eeff is the effective dielectric constant, and k0 is the

free space wavelength at the center frequency, where the

center frequency is set to be in the middle of 3.1–

10.6 GHz band. It is to be pointed out that these equations

can be used to provide initial values of the step cut

antenna parameters to be designed to satisfy UWB fre-

quency response using desired substrates. The developed

design equations are validated by generating four different

antenna designs having different substrate parameters such

as dielectric constant and dielectric thickness. The sub-

strate parameters and computed dimensional parameters

are given in Table I.

Figure 6 shows the simulated reflection coefficient

responses of the four antennas developed with the

Figure 4 Reflection coefficients for different L1 values.

Figure 5 Reflection coefficients with different W1 values.

TABLE I Antenna Description and ComputedParameters

Antenna A Antenna B Antenna C Antenna D

Laminate Rogers

5880

FR4

Epoxy

Rogers

RO3006

Rogers

6010LM

H (mm) 1.57 1.6 1.28 0.635

er 2.2 4.4 6.15 10.2

G (mm) 0.1 0.5 0.65 0.775

L (mm) 13 10 8.68 6.9

W (mm) 18 14 12 9.6

L1 (mm) 2.57 2 1.71 1.36

W1 (mm) 5.83 4.5 3.89 3.1

Figure 6 Simulated reflection coefficients of the developed

antennas with parameters given in Table.I.

CPS fed UWB Antenna with a Step Cut 667

International Journal of RF and Microwave Computer-Aided Engineering DOI 10.1002/mmce

parameters given in Table I. It can be seen that all the

antennas are operating in UWB region with three

resonances.

Electric field distributions of the antenna at three

resonances are given in Figure 7. Figure 7a shows the

electric field distributions of the antenna at the first reso-

nance. It can be realized from the figure that at first reso-

nance, E field is concentrated on the entire surface of the

antenna and a half wavelength variation is observed

through the entire surface. Electric field distribution at the

second resonance shown in Figure 7b indicates that field

is mainly concentrated on the step cut portions of the

structure. This can be verified from the directive nature of

radiation pattern at the second resonance. Figure 7c shows

the electric field distributions of the antenna at the third

resonance. It can be seen from the figure that field is con-

centrated on the upper edges of the structure and also on

both side of the slot. Similar observation was obtained

from the parametric analysis conducted previously which

illustrated strong impact of all dimensional parameters on

the third resonance.

Since the structure of the antenna is symmetric, it is

necessary to analyze whether any mismatch occurs when

the antenna is connected to an unbalanced coaxial SMA

connector. A well-defined simulation procedure of UWB

balun is specified in [13] and we opted this method to

design the balun. Figure 8 shows the reflection coeffi-

cients of the antenna with and without the balun feed. It

can be seen that both responses demonstrate similar per-

formance indicating the possibility of using the proposed

antenna with and without a balun feed. This feature adds

another advantage to this compact and simple antenna.

IV. EXPERIMENTAL RESULTS

The antenna is made of FR4 epoxy substrate having

dielectric constant (er) of 4.4, and thickness (h) of

1.6 mm. Figure 9 shows a photograph of the antenna. The

reflection coefficient responses of the step cut UWB

antenna are plotted in Figure 10. The 10 dB return loss

bandwidth of the antenna ranges from 3.1 to more than

Figure 7 Simulated electric field distributions of the antenna at

(a) 3.61GHz, (b) 6.52GHz, and (c) 10GHz.

Figure 8 Simulated reflection coefficient of the antenna with

and without balun.

Figure 9 Photograph of the proposed antenna.

Figure 10 Simulated and measured reflection coefficients of

the step cut UWB antenna.



12 GHz with three resonances centered at 3.53, 6.5, and

11 GHz.

The radiation patterns of the antenna in two principle

planes for the three resonant frequencies are measured

and are shown in Figure 11. The crosspolarization for all

of the three resonances are plotted in the same figure. It

can be seen from the plots that good crosspolar isolation

levels are obtained for all radiation patterns and at the

first resonance antenna shows almost omni directional pat-

tern. It can also be noticed that for the second and third

resonances the radiation patterns shows slight directional

behavior.

Figure 11 Measured principle plane radiation patterns of the antenna at (a) 3.53 GHz, (b) 6.5 GHz, and (c) 11GHz.



Figure 12 gives the simulated and measured bore sight

gain of the step cut antenna. The antenna has an average

gain of 4.5 dBi over the operating bandwidth. It can also

be noted that the gain reaches a peak value of 6 dBi at

11GHz.

The radiation efficiency of the antenna is also shown

in Figure 13. A very good efficiency is observed in the

entire operating band. Simulated efficiency is also shown

for comparison.

V. TIME DOMAIN ANTENNA ANALYSIS

The group delay gives a measure of the average time

delay of the input signal at each frequency; it is also a

measure of the dispersive nature of the device. Low

value of group delay is an indication of good UWB sys-

tem [14]. The group delay is measured for both face to

face and side by side orientations of the antennas.

Obtained values of group delay responses of the antenna

are shown in Figure 14. A variation in group delay is

less than 0.5 nanosecond for both orientations, which

indicates that the antenna offers a good time domain

performance.

The transfer functions of the antennas will be deter-

mined from the measured values of transmission coeffi-

cient S21 in the frequency domain. Using two identical

antennas with receiving antenna oriented along various

angles, the following relation is used to find the transfer

function [15]

H xð Þ5ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2pRcS21 xð ÞejxR=cð Þ=jx

q(5)

where H(x) is the transfer function, the free space veloc-

ity is c, the distance between the two identical antennas is

denoted by R. A fourth-order Rayleigh pulse [14] is used

as the source pulse to excite the antenna. The impulse

response can be calculated from the transfer function by

taking the Inverse Fast Fourier Transform. The received

waveform is obtained by convoluting the transmitted

waveform with the impulse response. The transmitted and

received waveforms in the bore sight direction of the step

cut antenna at free space are shown in Figure 15.

The transmitted and received waveforms maintain sim-

ilar shape which indicates that the antenna does not distort

the information contained in the transmitted signal.

Figure 12 The simulated and measured gain of the step cut

UWB antenna.

Figure 13 Simulated and measured efficiency of the antenna.

Figure 14 Group delay of the antenna.

Figure 15 Transmitted and received pulses of the antenna.



Correlation between the transmitted and received

pulses or fidelity is an important measure of quality of an

UWB antenna system [16]. The fidelity factor F is

defined by

F5max

Ðx tð Þ:

Ðy t2sð ÞdtffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiÐ

jx tð Þ2j:dtÐjy tð Þ2j

qdt

(6)

Fidelity factor will be deduced from the measurements

for different receiving antenna orientations, with y(t) as

the incident and x(t) as the received waveform.

The measured fidelity factor of the antenna in all

azimuth angles is given in Figure 16. The step cut

UWB antenna was found to exhibits good fidelity in the

entire azimuthal orientation with an average value of

93.5%.

VI. CONCLUSIONS

A simple novel compact CPS step cut dipole antenna

capable of UWB performance is designed and tested.

From the simulation and experimental studies, it was con-

clude that the step cut antenna provides moderate gain

and stable radiation patterns. The performance of the

antenna is analyzed with and without a balun feed and it

was found that the antenna gives similar UWB performan-

ces in both cases. Using the design equations antenna can

be easily redesigned for desired substrates and operating

frequencies. Low value of group delay and high value of

fidelity indicates that the proposed antenna imposes negli-

gible effects on the transmitted pulse.

ACKNOWLEDGMENT

The authors would like to thank Kuwait University for

providing financial assistance. This work was supported

by Kuwait University Research Grant no [EE02/12].

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Figure 16 Measured fidelity factor of the antenna.



BIOGRAPHIES

Ayman S. Al-Zayed Received the B.Eng.

(Honours) degree in Communication and

Electronic Engineering from the Univer-

sity of Northumbria at Newcastle in 1995.

In 2000, he obtained the M.S. degree in

Electrical Engineering from the Univer-

sity of Hawaii at Manoa. In 2004, he

earned the Ph.D. degree in Electrical

Engineering from North Carolina State

University. In February, 2004, he joined the Department of Electri-

cal Engineering at Kuwait University as an Assistant Professor,

where he was promoted to Associate Professor in March 2012. His

research interests include microwave and millimeter-wave active

and passive devices, power combining, quasi-optical devices,

antennas, phased arrays, and radars.

V. A. Shameena was born in India. She

received her B.Sc. degree in Physics from

Calicut University, Kerala, India and

M.Sc. and Ph.D. degrees in Electronics

from Cochin University of Science And

Technology (CUSAT), Kerala, India in

2003, 2005, and 2012, respectively. Cur-

rently she is working as a Research Asso-

ciate in Kuwait University. Her research

interest includes ultrawideband antennas and planar antennas.



coplanar strip fed uwb antenna with a step cut

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