The conformal finite-difference time-domain analysis of the antipodal

Vivaldi antenna for UWB applications

Er-Ping LI, Hong-San and James Mao CAI
Electronic Systems, Institute of High Performance Computing, Singapore,
National University of Singapore, 117528
Email: epleegihpc.a-star.edu.sg

Abstract: The conformal finite-difference

time-domain (CFDTD) method is applied to 2. Antenna prototype
characterize the dual elliptically tapered antipodal The Vivaldi antenna offers great promise for UWB
slot antenna (DETASA) for UWB applications. applications [5]. This planar design is suitable both for
DETASA antenna consists of the metallization on two radar-like and communications applications. The
sides of the substrate and presented by two polygons. conventional Vivaldi antenna typically has at least an
The CFDTD method with uniaxial perfectly match octave bandwidth making it suitable for a wide variety of
layer is suitable to model the patch with curved applications, but can be limited by its radiating slot
surface and edges of these polygons. This paper transition. The antipodal Vivaldi [6-7], in which a
demonstrates the capability of this simulation method microstrip line and its ground plane both gradually flare
for UWB antenna design. This method is examined out, removes the bandwidth limitations of the transition.
with experimental approach. However this kind of antenna is related to the
omnidirectional antennas with relative low gain or their
bandwidth can not cover UWB requirement.
1. Introduction
In the past few years, ultrawide-band (UWB)
technology has gained considerable attention in the
international wireless communication community.
Consequently, the development of ultra wideband
systems e. g. for the wireless multimedia data
communication between different home entertainment
systems (DVD player, flat screen, Internet PC ... )
becomes very appealing since the FCC opened the
spectrum from 3.1 GHz to 10.6 GHz for the unlicensed
low-power use.
Antennas play a critical role in UWB communication
systems since they influence the complexity of the
receiver and transmitter designs. The demand for the
antenna of such a system is not only limited to large
bandwidth but also to the compact size, light weight as
well as other quality criteria like low ringing and gain
stability over the frequency range must be considered.
The study of this kind of antenna could be performed
well by using the time-domain numerical method such as
FDTD and TLM or FVTD.
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In this paper, the characteristics study for a dual 111 7,
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elliptically tapered antipodal slot antenna (DETASA) [1], Sg .Le W 011r(I hOui#r
a-, _ __Q __

using the conformal finite difference time domain taN

l~ FtW-1 .E
(FDTD) techniques[2] with the uniaxial perfectly
-i

matched layers (UPML) absorbing boundaries [3-4] are b ()

presented. The return loss, input impedance and the Figure 1: Configuration of DETASA, (a) main geometry,
far-field radiation pattern are calculated and validated (b) tapered radiating slot, (c) feed line transition
using the measurement results.
In this paper, the dual elliptically tapered antipodal slot subroutine of FDTD code is the UPML updating
antenna (DETASA) is investigated. This DETASA is a equations. These equations include the update equations
modified form of antipodal Vivaldi antenna and built on for ABCs and the update equations for the entire grid
a Duroid substrate with the relative dielectric constant of space.
3.38, loss tangent of 0.002 and the thickness of 0.813
mm. the configuration of antenna is shown in Fig. 1. It In the antenna modeling, the conventional approach to
consists of two parts: tapered radiating slot and feeding extract output parameters such as the input impedance,
transition. The tapered radiating slot shown in Fig. l(b) return loss or VSWR, consists of running the simulation
is configured by two conducting arms which are on twice (or concurrently on two circuits): one run is for
opposite sides of a substrate. The feeding transition is analyzed circuit and the other for a nonreflecting
shown in Fig. l(c), 50Q microstrip line (width Ws) is structure with the identical input. The second simulation
tapered to a parallel offset strip line (width Wp, offset S) provides a pure incident wave. It is evident that this
for feeding tapered slot radiator. The strip line is linearly implementation has a drawback that is the necessity of
tapered while the ground plate is elliptically tapered. All double simulation, which significantly increases the
dimensions of DETASA, in mm, investigated in this employed computer resources and complicates the use of
study, have the following parameters: a1=50, b1=25, the program. An alternative method, implemented to
a2=20, b2=24, Wg 5 1, Lf31, Wp=l, S=0.5, afl=26, overcome this disadvantage as well as to reduce the
af=24, bf-25, W,= 1.86 and R= 15 mm. wasted simulation time, is based on the method of Fang
et al. [9].
3. Methodology
The FDTD method has become the most useful In order to calculate the far-field patterns of antenna, one
numerical method applied in several different areas such post processing subroutine performs a near-to-far-field
as electromagnetic, photonic, biomedical modeling, etc. transformation using frequency domain equivalent
The advantage of FDTD method in comparing to other electric and magnetic currents on the closed surface
numerical method is that it can easily model dielectric surrounding the antenna, which is defined by few cells
and magnetic materials of finite extent. FDTD is a inside of the absorbing boundary [4]. The equivalent
time-domain method thus it possesses the added electric and magnetic currents in the spectral domain are
advantage of being able to provide the frequency calculated by using Fourier Transform of the time
response of the microwave devices in the wide-band domain equivalent currents.
with only one simulation after taking the discrete Fourier
transform of the time domain output signal. This feature 4. Results and Discussions
is even more useful when utilizing to synthesis the UWB The above CFDTD code is used to analysis the
antenna where the extreme wide frequency band needed. DETASA antenna as mentioned in the section 2. Three
One limitation of the FDTD method is its representation cells are used to discretize the thickness of substrate and
of curved media interfaces as a stair-stepped boundary. the offset strip line. The uniform rectangular FDTD
When modeling the complex structure by using the fine space lattice is employed with Ax=0.333mm,
mesh, the error is possibly acceptable but the simulation Ay=0.38mm, Az=0.271mm, and At=0.99 x the Courant
time is too long and the required memory is too large. limit. The space lattice is terminated with 10-cells PML
Otherwise, for the coarse mesh we can save the on all sides. Each PML region is placed only 4 cells from
simulation and the memory but the error becomes the outer dimensions of the antenna geometry.
considerably due to inaccurate approximation of The measured results of return loss are presented in
geometry. In order to overcome this difficulty, we Figure 2, together with the calculated results by using
propose to use the conformal FDTD [2] for accurate our CFDTD programs. Although a bit discrepancy
modeling the perfect electric conductor (PEC) objects between the measurement and the simulation in
with curved surfaces and edges is computationally ultra-wide band, but the return loss of antenna, in the
efficient and numerically stable. This feature is useful in band from 3.1 GHz to 10.6 GHz, is fairly characterized.
modeling the microstrip planar Obviously, it is always difficult to match the return loss
of UWB antenna in the extreme large band of frequency.
The FDTD code was developed for this purpose which
includes the uniaxial PML (UPML) boundary conditions
to model free space environment. The UPML boundary
conditions were first introduced for FDTD method by
Gedney [3]. The UPML is more attractive than split-field
PML because it is based on Maxwellian formulation
rather than a mathematical model [4]. The primary
 90
0-
-10 -

C -20 V
--30-
0
~40 -SmAlo
Measuremien

0 2 4 6
8 10 12 14 16 18 20
Frequency (GHz)
Figure 2: Comparison of simulated and measured 270
1 1-parameter (a)
The input impedance of antenna is also calculated and 90
plotted in Fig. 3. It is found that the impedance, in the
band of 3. I GHz-10.6GHz, varies from 40 6to 87 2 for
the real part and from -40 to u12 e for the imaginary
part.
150
IA
0
w 5
B
E
lC n
w^ A - %aA -

: -60 1 /
ivYv
I--Real_Zin
270

(b)
-100---1-X
1mg ZIn I
Figure 4: Simulated (- Co-pol, --- X-pol) and
0 2 4 & 8 10 12 14 16 18 20 measured (xxx Co-pol, ooo X-pol) radiation pattern for
Frequency (GHz) proposed DETASA at 7 GHz, (a) XY-plane (E-plane), (b)
Figure 3: Real part and imaginary part of input YZ-plane (H-plane).
impedance simulated by CFDTD
As shown in Fig. 4, the typical radiation patterns at 7 Acknowledgments
GHz of antenna are calculated by CFDTD and compared The authors would like to thank Dr. Xianming Qing,
to the measurement. It is seen that, the CFDTD and Dr. Zhi Ning Chen and Dr. Michael Yan Wha Chia,
measured results for the prototype are in close agreement. from the Institute for Infocomm Research, for their help
The slight difference could be due to the calculation of in providing the prototype and the measurement result of
radiation pattern by CFDTD does not perform in the this antenna.
resonant frequency as well as the infinite substrate is
considered for simulation. References

5. Conclusion [1] X. Qing, Z. N. Chen and M. Y. W. Chia, Dual

Based on the conformal FDTD method, characteristics elliptically tapered antipodal slot antenna (DETASA)
of a dual elliptically tapered antipodal slot antenna are loaded by curved terminations for UWB applications,
studied in this paper. The CFDTD code, including accepted to publish in Radio Science.
UPML, is developed and has been verified by [2] W. Yu and R. Mittra, Accurate modeling of planar
measurement results. The results have shown that the microwave circuit using conformal FDTD algorithm,
proposed antenna features good impedance and radiation Electronics Letters, 4 (2000).
performance and has the potential in UWB applications. [3] S. D. Gedney, An anisotropic PML absorbing media
Future studies will be focused on the optimization of the for the FDTD simulation of fields in lossy and
radiation pattern parameters for array of DETASA for dispersive media, Electromagnetics, 16, p399 (1996).
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Electrodynamics: the finite-difference time-domain
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[6] J.D.S. Langley, P.S. Hall and P. Newham, Balanced
Antipodal Vivaldi Antenna for Wide Bandwidth
Phased Arrays, IEE Proc. Antennas Propag., 2, p97
(1996).
[7] Hung Loui, J. P. Weem and Z. Popovic, A Dual-Band
Dual3-Polarized Nested Vivaldi Slot Array With
Multilevel Ground Plane, IEEE Trans. Antennas and
Propagation, 9, p2168 (2003).
[8] J. Fang and D. Xeu, Numerical errors in the
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