Design of a Planar UWB Dipole Antenna With an Integrated Balun Using Surrogate-Based Optimization

Abstract

A design of an ultrawideband (UWB) antenna with an integrated balun is presented. A fully planar balun configuration interfacing the microstrip input of the structure to the coplanar stripline (CPS) input of the dipole antenna is introduced. The electromagnetic (EM) model of the structure of interest includes the dipole, the balun, and the microstrip input to account for coupling and radiation effects over the UWB band. The EM model is then adjusted for low reflection over the UWB band by means of fast simulation-driven surrogate-based optimization. This approach allows us to obtain the final design at low computational costs and at a high-fidelity level of structure description. Measurements of the manufactured optimal design validate the use of the balun as well as the design approach.

Full text

366 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
Design of a Planar UWB Dipole Antenna With
an Integrated Balun Using Surrogate-Based
Optimization
Slawomir Koziel, Senior Member, IEEE, Stanislav Ogurtsov, W. Zieniutycz, and A. Bekasiewicz
Abstract—A design of an ultrawideband (UWB) antenna with an
integrated balun is presented. A fully planar balun configuration
interfacing the microstrip input of the structure to the coplanar
stripline (CPS) input of the dipole antenna is introduced. The elec-
tromagnetic (EM) model of the structure of interest includes the
dipole, the balun, and the microstrip input to account for coupling
and radiation effects over the UWB band. The EM model is then
adjusted for low reflection over the UWB band by means of fast
simulation-driven surrogate-based optimization. This approach al-
lows us to obtain the final design at low computational costs and at
a high-fidelity level of structure description. Measurements of the
manufactured optimal design validate the use of the balun as well
as the design approach.
Index Terms—Microstrip-to-coplanar-stripline (CPS) transi-
tion, numerical optimization, radial line stub, simulation-based
model, surrogate-based optimization, ultrawideband (UWB)
antenna, UWB balun, UWB dipole.
I. INTRODUCTION
PROPER yet simple interfacing of balanced and unbal-
anced transmission lines is critical for ultrawideband
(UWB) circuits, in particular for UWB antennas [1]. A typical
unbalanced line is a microstrip, and a typical balanced line is a
coplanar stripline (CPS), which is commonly used as an input to
the dipole antenna (balanced element). A balun element inter-
faces such lines to each other, e.g., as shown in Fig. 1. Different
balun geometries for UWB antennas have been introduced so
far with various levels of complexity [2]–[6]. Among these,
balun structures delivering acceptable performance, structural
simplicity, as well as compact footprint, all at the same time,
are preferred for UWB antenna applications, e.g., [7].
The UWB band of interest dictates to use full-wave analysis
not only for validation of the final design, but also through the
design adjustment process to account for EM interactions within
the antenna structure. It is also desirable for the electromag-
netic (EM) model of the structure to include the antenna ele-
Manuscript received August 18, 2014; accepted October 14, 2014. Date of
publication October 17, 2014; date of current version February 04, 2015. This
work was supported in part by the Icelandic Centre for Research (RANNIS)
under Grant 141272051.
S. Koziel, W. Zieniutycz, and A. Bekasiewicz are with the Faculty of
Electronics, Telecommunications and Informatics, Gdansk University of Tech-
nology, Gdansk 80-233, Poland (e-mail: koziel@ru.is; wlz@eti.pg.gda.pl).
S. Ogurtsov is with the Engineering Optimization and Modeling Center,
Reykjavik University, Reykjavik 101, Iceland (e-mail: stanislav@ru.is).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LAWP.2014.2363932
Fig. 1. UWB dipole antenna with a balun: layout.
ment, balun, and microstrip input to reliably account for cou-
pling and radiation effects over the UWB band. Manual adjust-
ment of such EM models via simulation sweeps with one pa-
rameter active at a time is tedious, time-consuming, and hardly
feasible even with few design variables. Therefore, we adopted
automated numerical optimization to conduct the design. In par-
ticular, we utilize surrogate-based optimization (SBO) [10] to
conduct design at a high-fidelity level of description yet at low
computational costs.
This letter is organized as follows. The antenna geometry
including the proposed balun, EM models utilized in the de-
sign process, and the design task are described in Section II.
Section III outlines the SBO design process. Section IV presents
optimization and measurement results. Section V concludes the
work.
II. ANTENNA GEOMETRY,DESIGN TASK,AND EM MODELS
The UWB antenna, shown in Fig. 1, should be matched
within the UWB band of 3.1–10.6 GHz. The antenna comprises
a planar dipole with a CPS input of length , a balun of length
,and50 microstrip input. The balun is a microstrip-to-CPS
transition with a ground edge of a linear profile and an open
radial microstrip stub, both as shown in Fig. 1. The radial
stub element was adopted because it allows, in general, more
broadband operation than the microstrip stub; additionally,
it has two degrees of freedom. The radiating element is an
elliptical dipole. The substrate is a 0.76-mm-thick Taconic
RF-35 layer [8]. Metallization is with 70 m copper.
The vector of design variables contains dimensional pa-
rameters of the dipole, CPS section, and the balun, namely
. Other parameters shown
in Fig. 1 are fixed as follows: ,
and , all in millimeters.
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KOZIEL et al.: PLANAR UWB DIPOLE ANTENNA WITH INTEGRATED BALUN USING SURROGATE-BASED OPTIMIZATION 367
Fig. 2. UWB dipole antenna with a balun at a certain design: typical differ-
ences between the low-fidelity EM model and the high-fidelity one
(---). It can be observed that the major types of discrepancies are the fre-
quency shifts and the vertical misalignment. Therefore, frequency scaling and
additive correction are utilized as surrogate modeling techniques.
Fig. 3. Simulated reflection response: the initial design (---)and the finaldesign
without connector ( ) and with connector (—)asinFig.4.
The structure is modeled in the CST MWS environment [9]
and simulated with the MWS transient solver. Two discrete EM
models are defined: the high-fidelity model and the low-fi-
delity model .Models and are utilized in the automated
design process as described in Section III. The mesh density (the
number of meshes per characteristic wavelength) of the high-fi-
delity model is set with preliminary numerical experiments
ensuring that no substantial changes in the reflection response
occurs with further increase of the mesh density. At the initial
design, the model comprises 11 180 680 hexahedral cells and
simulated in 53 min, while the model is with 574 175 cells
and simulated in 1 min 40 s. The coarse model is much faster
than the high-fidelity model . However, it is less accurate, so
to be reliably used in the optimization process, should be
corrected relatively ,e.g.,asdescribedinSectionIII.
It was found with preliminary numerical experiments exe-
cuted in the vicinity of the initial design that lateral extensions
of the finite substrate have no noticeable influence on both re-
flection and radiation responses of the discrete EM models if
dielectric layer extends more than 10 mm beyond metallization
of the dipole. In addition, if the finite dielectric layer of a par-
ticular design extends more than 10 mm beyond metallization
of the dipole, then its reflection response is essentially the same
as that of the same design defined at the substrate modeled with
infinite lateral extends, and the maximal difference of the radi-
ation responses of such models stays within 0.5 dB. Therefore,
dielectric substrate of the EM models is modeled as finite and
extending 15 mm beyond metallization of the dipole.
It was assumed that, in actual applications, the UWB an-
tenna under design should be excited through a microstrip line
Fig. 4. UWB-dipole antenna with balun: manufactured final design.
Fig. 5. Reflection response of the final design: simulated with connector (—)
andmeasured(---)showninFig.4.
of the printed board, i.e., there should be no connector in the
close proximity of the antenna. Consequently, the discrete EM
models to be used in the optimization process were defined with
the waveguide port defined at the microstrip input. At the same
time, an SMA connector interfaces the manufactured antenna
with a vector network analyzer (VNA) in measurements. There-
fore, a high-fidelity model with an SMA connector had been also
defined for verification of the final design and its comparison to
measurement data.
III. DESIGN OPTIMIZATION PROCEDURE
The design task can be formulated as a nonlinear minimiza-
tion problem of the form
(1)
where is an objective function encoding the design specifica-
tions, here minimizing in the UWB frequency range.
Perhaps the most serious bottleneck in solving (1) is the high
computational cost. In order to make the design optimization
process feasible in terms of the CPU time, most operations are
performed on a corrected low-fidelity model, a so-called surro-
gate model The optimization algorithm is an iterative procedure
that yields a sequence of approximations ,of
the optimum design [10]
(2)

368 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 14, 2015
Fig. 6. Normalized power pattern of the final design: simulated with connector
(—) and measured (---) at selected frequencies: (a) 3, (b) 4, and (c) 5 GHz.
90 on the left corresponds to the normal direction above the antenna.
180 corresponds to “to-the-connector.”
where is the surrogate model at iteration . The surrogate
model is constructed by suitable correction of the low-fidelity
model . Here, we use the following two types of techniques:
1) frequency scaling, and 2) additive response correction. The
reason for this choice is the fact (cf. Fig. 2) that the major types
of misalignment between the responses of and are fre-
quency shifts and vertical discrepancy. Let denote the
explicit dependency of the low-fidelity model on the frequency
(is the set of frequencies of interest at which the model is
evaluated). The surrogate model is defined as
(3)
with
(4)
and
(5)
Fig. 7. Normalized power pattern of the final design: simulated with connector
(—) and measured (---) at selected frequencies: (a) 6 GHz, (b) 7 GHz, (c) 9 GHz,
and (d) 10 GHz. 90 degree on the left corresponds to the normal direction above
the antenna. 180 degree corresponds to ‘to-the-connector’.
being the affine frequency scaling (shift and scaling) [11]. The
frequency scaling parameters are obtained as
(6)
i.e., to minimize the misalignment between the high- and the
scaled low-fidelity model response at . Although the models
are evaluated at a discrete set of frequencies, the information
at other frequencies can be obtained through interpolation. The
misalignment is further reduced by the additive response correc-
tion term (output SM) (4) that ensures zero-order consistency

KOZIEL et al.: PLANAR UWB DIPOLE ANTENNA WITH INTEGRATED BALUN USING SURROGATE-BASED OPTIMIZATION 369
(i.e., ) between the surrogate and [12].
The algorithm (2) working with the surrogate model (3)–(6) typ-
ically requires only a few iterations to yield an optimized design.
IV. NUMERICAL RESULTS AND MEASUREMENTS
The initial design is
,where is in degrees and
other variables are in millimeters. The optimum,
,has
been found in four iterations of the optimization procedure
that was described in Section III. Each iteration required about
120 low-fidelity model evaluations and one evaluation. The
reflection response of the final design is shown in Fig. 3. The
total numerical cost of obtaining this design corresponds to
about 19 simulations of the UWB antenna high-fidelity model.
A photograph of the manufactured design is shown in
Fig. 4. The UWB antenna under test was excited through an
edge-mount SMA connector [13]. Radiation and reflection re-
sponses of the fabricated designs have been have been measured
at the anechoic chamber of Gdansk University of Technology,
Gdansk, Poland, using a setup with a dual-polarized horn
antenna [14] and E5071C ENA Network Analyzer [15]. Sim-
ulated and measured reflection responses are shown in Fig. 5.
The discrepancies between both responses are the result of
manufacturing inaccuracies. Simulated and measured radiation
responses in the plane perpendicular to the antenna substrate
at selected frequencies are shown in Figs. 6 and 7, from where
one sees that the final design stays of omnidirectional radiation
in this plane up to 7 GHz.
V. C ONCLUSION
A fully planar balun configuration interfacing the microstrip
input of the structure to the CPS input of the dipole antenna was
introduced. Simulation-driven design of a UWB antenna with
the balun had been performed using surrogate-based optimiza-
tion. This approach allowed us to obtain the final design at low
computational costs at a high-fidelity level of structure descrip-
tion. Measurements of the reflection and radiation responses of
the manufactured optimal design validate the use of the balun
for UWB-dipole antenna well as the design approach.
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