07-05-2014, 03:54 PM
Flexible Bow-tie Antennas
Introduction
The Flexible Display Center (FDC) at Arizona State University (ASU) was founded
in 2004 as a partnership between academia, industry, and government to collaborate
on the development of a new generation of innovative displays and electronic circuits
that are flexible, lightweight, low power, and rugged [1]. Due to the increasing need
for flexible and lightweight electronic systems, FDC aims to develop materials and
structural platforms that allow flexible backplane electronics to be integrated with
display components that are economical for mass-production [2]. Currently, FDC
is focusing on the incorporation of antenna structures, which can function coopera-
tively with the other flexible circuit elements. Design of flexible antennas, as a part
of flexible electronic circuits, may have a very wide spectrum of applications in mil-
itary and civilian wireless communication, which can allow people to wear antenna
structures instead of carry them. Hence, flexible and fluidic antennas have a great
potential [3]. In this paper, the design, fabrication, simulation and measurements of
a bow-tie antenna with a flexible substrate is discussed. The antenna is modeled and
simulated with Ansoft HFSS, and the simulations are compared with measurements
performed in the Electromagnetic Anechoic Chamber (EMAC) at ASU.
Antenna Design
Antenna design using flexible substrates is a new research topic for the ASU FDC.
Hence a broadband element was chosen to initiate this research. A bow-tie antenna
[4] was selected as a first design because of its simple geometry and broadband
characteristics when compared to a cylindrical dipole. Furthermore, bow-tie anten-
nas are expected to be more directive than dipole antennas because of the larger
radiating area [5]. The fabricated antenna and its design geometry are illustrated
respectively, in Figures 1 and 2 where all of the dimensions are in millimeters.
In order to properly feed the bow-tie antenna, a microstrip-to-coplanar feed network
(CPFN) transition is necessary. For this purpose a microstrip-to-CPFN balun was
designed which provides an odd mode in the coupled microstrip line while suppress-
ing the even modes [6, 7]. This balun introduces a 180◦ phase difference between
the coupled microstrip lines near the center frequency.
Simulation and Measurement Results
The designed antenna was modeled and simulated using the commercial software
HFSS [9]. Furthermore, after the fabrication of the antenna at the FDC facilities,
the radiation patterns, the gain and the return loss of the antenna were measured
in the EMAC. In order to check the validity of the design, the simulation results
were compared with measurements.
The comparison of the return loss is shown in Figure 4. It can be observed that
the agreement between the simulations and measurements is very good. The center
frequency is 7.66 GHz where the return loss is maximum.
In addition to return loss, 2D amplitude radiation patterns in the principal E-
and H-planes are also compared at the measured center frequency (7.66 GHz). The
normalized radiation patterns are illustrated in Figure 5, together with the principal
plane definitions. It can be seen that the measured radiation patterns are in excellent
agreement with the simulated ones in all of the three principal planes. The absolute
gains obtained by simulations and measurements are 2.33 and 2.5 dBi, respectively.
Conclusions
A bow-tie antenna was designed and fabricated using the unique flexible material of
the ASU FDC. The HFSS simulations were compared with measurements performed
in the EMAC. The radiation patterns, return loss and absolute gain comparisons
showed that the measurements and the HFSS simulations are in very good agree-
ment. These affirmative indications of the initial research on flexible FDC substrates
will lead to different antenna designs with the flexible FDC material that might find
a wide range of applications in wireless communication.