ROBUST ELECTRONICSYSTEMS RESEARCH

Live Wire
Researchers designed an unusually compact wearable antenna that covers
the entire instrument, scientific and measurement band.
By Kyeol Kwon, Ph.D. Candidate, and Jaehoon Choi, Professor, Department of Electronics & Computer Engineering,
Hanyang University, Seoul, Korea

A

wireless body area network (WBAN) consists of a network of computing devices worn on the body that
communicate with a server over a wireless network.
WBANs can interact with sensors that monitor the wearer’s medical status and provide an early warning of health problems (andthis is only one application of the technology). Traditionally in
WBAN design, circularly polarized (CP) microstrip patch antennas are used, since they can maintain performance in spite ofmovement by the wearer. However, CP antennas tend to haverelatively narrow bandwidth. It is particularly difficult to obtainbroad bandwidth with a compact CP antenna.

Researchers at Hanyang University set out to design a compact CP antenna with broad-enough bandwidth to cover the entire
instrument, scientific and measurement (ISM) band, which runs
from 2.4 GHz to 2.485 GHz. The antenna was designed as asquare slot etched into the bottom plane of an FR-4 substratewith a pair of Y strips connected to ground with an inverted
L-shaped microstrip feed line. A square slot is etched on thebottom plane of the 36 x 36 x 1.6 mm substrate with a relative
permittivity of 4.4. To reduce the dimensions of the antenna, aportion of the inverted L-shaped feed line is meandered orfolded over itself to provide the required resonant length but

Researchers set out to design a compact CP antenna
with a broad bandwidth.
© 2014 ANSYS, INC.

ANSYS ADVANTAGE Volume VIII | Issue 1 | 2014

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ROBUST ELECTRONICSYSTEMS RESEARCH

within a smaller area. Other key design
parameters include thickness of the feedline and width of each slotted section ofthe feed line. The research team considered adding strips connected to ground tobroaden the 3 dB axial ratio bandwidth.
They also considered I-shaped, T-shaped
and Y-shaped strips.
Researchers faced the challenge ofconfiguring geometric parameters to optimize the antenna to achieve ideal returnloss and axial ratio bandwidths. The
return loss bandwidth consists of the frequency range over which the loss of signal power caused by the reflection at adiscontinuity in the transmission line —such as mismatch with the terminatingload — is below a certain value expressedin decibels. The axial ratio bandwidth isa measure of the quality of the circularpolarization of the antenna. A circularly
polarized field is made up of two orthogonal E-field components of equal amplitude that are 90 degrees out of phase.
The axial ratio is the ratio of these components. The ideal value is 0 dB, and theaxial ratio bandwidth is typically quoted
as the bandwidth over which orthogonalcomponents differ by no more than 3 dB.
Optimizing the design of the antennausing physical measurements is very
time-consuming and expensive due tothe high cost and long lead time involvedin building each prototype iteration. So
Hanyang researchers used ANSYS HFSS
electromagnetic field simulation software to evaluate performance of the
proposed antenna using a wide range of
geometric parameters (listed below) prior
to building hardware. They created basic
parametric geometry by drawing it inHFSS and assigning material properties.
By utilizing the integral automatic adaptive meshing capability of HFSS, theygenerated a mesh conformal to the 3-Dstructure and appropriate for the electromagnetic problem. Researchers assigned
boundary conditions and excitations andthen set up the analysis and frequency
sweep. The final step was to run thesimulation and view the results including
antenna parameters.
The key parameters whose geometries were varied in the simulation were Ith(x-axis dimension of feed line), w2 (thickness of feed line), w3 (width of each slotted section of feed line), and ws (distance
between verticals on Y-strip), as shown inthe diagram. The results for return loss
© 2014 ANSYS, INC.

 Photograph of fabricated antenna including (left) top view and (right) bottom view

 Configuration of proposed antenna including (a) top view, (b) bottom view, and (c) side view.

Ith = x-axis dimension of feed line,  Itv = y-axis dimension of feed line, w2 = thickness of feed line,
w3 = width of each slotted section of feed line, ws = distance between two tops of Y-strips.

10 dB ReturnLoss (GHz)Ith (mm)w2

w318180

3.12 — 3.7

3220.52.62 — 3.12

50

10.5

2.87 — 3.5

66

0.5

0.5

2.13 — 3.12

 Simulation predictions of return loss bandwidth for various lth values

Strip

3 dB ARBWNo Strips

0I-Shaped Strips

2.43 – 2.63

T-Shaped Strips

2.43 – 2.56

Y-Shaped Strips

2.15 – 2.8

 Simulation predictions of axial ratio bandwidth for different shaped stripsWs (mm)3 dB ARBW

32.3 – 2.7

3.5

2.2 – 2.84

2.15 – 2.8

4.5

2.55 – 2.65

 Simulation predictions of axial ratio bandwidth for various Y-strip widths

ANSYS ADVANTAGE Volume VIII | Issue 1 | 201438

They faced the challenge of configuring geometricparameters to optimize the antenna to achieve ideal
return loss and axial ratio bandwidths.

 Simulated and measured axial ratio of proposed antenna

vs. frequency for different values of Ithshow that the resonant frequency of theantenna is reduced as Ith increases. Basedon these results, the proposed antenna
uses Y-shaped strips, as it has the widest axial ratio bandwidth (ARBW): 3 dB.
When ws increases from 3 mm to 4.5mm, the 3 dB bandwidth becomes wider.
However, the enhanced axial ratio performance deteriorates as ws becomes larger
than 4 mm.
The proposed antenna was fabricatedusing the optimized design parameters
determined through simulation: L (internal side length) = 24 mm, G (external sidelength) = 36 mm, ltv = 19 mm, w2 = 0.5mm, w3 = 0.5 mm, and ws = 4 mm. The
measured return loss characteristics ofthe fabricated antenna closely matched
the simulation predictions. The antenna
has a 10 dB return loss bandwidth of
1,120 MHz (from 2,170 MHz to 3,290MHz) and a 3 dB ARBW of about 29 percent with respect to the center frequencyof 2,450 MHz, which is wide enough tocover the full ISM band. The overall sizeof the antenna is small: 36 x 36 x 1.6 mm.
The new antenna is more compact thansimilar antennas presented in the past,
yet the bandwidth is broader. These features make this new antenna a goodcandidate for modern WBAN systems
that require high performance, small
size, low weight and low production costs.
This optimized design may well be thecatalyst for a wide variety of safe, comfortable body-worn devices for future medical and consumer product applications.

ADDITIONAL RESOURCESANTENNA DESIGN AND PLATFORM
INTEGRATION ANALYSIS WITH HFSS

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 Simulated and measured return loss of proposed antenna© 2014 ANSYS, INC.

ANSYS ADVANTAGE Volume VIII | Issue 1 | 201439