1
Ultra-Wideband Antenna for Wireless Body
Area Networks
P.Balachander, S.Vempati
Abstract—Wireless body area networks (WBAN) are an emerging
technology in the healthcare domain, finding applications in
cancer detection, endoscopy and real time imaging. Ultra
wideband (UWB) radio signals have inherent characteristics such
as penetration through obstacles, high precision ranging suitable
for localization and detection purposes. This paper proposes the
design of a pin-fed patch antenna, made on a FR-4 printed circuit
board (PCB) substrate having a resonant frequency of 5GHz in
FEKO. The primary application of this antenna is to monitor the
vital signs in a human body. A section of human chest has been
considered to analyze these effects. It has been modeled as a layer
of dry skin (ε=36) with dimensions 90mm*90mm*1mm.
Index Terms— Patch Antenna, UWB, 5GHz, FR-4, Body Area
Network, Chest Model, FEKO
I. INTRODUCTION
The wireless body area network is of special interest in new
sensing and monitoring devices for healthcare. This paper aims
to design an ultra-wideband microstrip patch antenna as a
transceiver for monitoring vital signs in a human body. This
information is sent to a short-range receiver for further
processing.
Microstrip patch antennas radiate strongly away from the
surface, suitable for free space links [1]. Although, dipole
antennas have a good radiation pattern but they are not feasible
to fabricate due to size constraints. Patch antennas, on the other
hand, are lighter in weight, smaller in dimension and have a
lower cost.
UWB signals have noise like characteristics due to their low
effective isotropically radiated power (EIRP) spectral density
of -41.3 dBm/MHz [2]. This makes them robust to jamming
and interference from neighboring systems. Additionally, these
signals have a greater penetration ability, high data rates
without posing a threat to the patient`s safety.
For the choice of a substrate, FR-4 (ε=4.4) is selected. It is a
popular and versatile choice, having a good strength to weight
ratio. FR-4 substrate does not elicit any adverse response from
the body, proving its biocompatibility.
A coaxial pin feed has been used to feed the microstrip patch
antenna due to the ease in fabrication and low spurious
radiation [3]. An added advantage of using this feed is the
flexibility in placing them at a desired location to match it with
the input impedance.
The organization of the paper is as follows:
In Section II, theoretical geometric parameters of the patch
antenna are calculated using basic design equations [4]. Section
III discusses the simulated values of the antenna parameters
obtained from FEKO [5]. Section IV describes the practical
set-up of the antenna environment. Further improvements in the
antenna geometry and parameters are briefly discussed.
II. ANTENNA DESIGN
The patch antenna is designed to operate at a center frequency
of 5GHz. The relationship between the operation frequency (fc)
and length (L) of the patch is given by:
Substituting the values of εr =4.4; L = 14.31mm is obtained.
The width of the patch is calculated to be W = 19.1mm using
the following equation:
Typically, height of the substrate should be greater than 0.05
times the wavelength. In this design, H = 2.87mm. Since some
of the waves travel in the substrate and some in air, an effective
dielectric constant εeff is introduced to account for fringing and
wave propagation in the line. For W/H>1,
This formula results in εeff = 3.71. Due to fringing fields, the
effective length of the patch is modified as
Where ΔL is given by
From the above two formulae, Leff = 11.75mm is obtained. An
initial estimation to determine the feed coordinates of the
coaxial probe is calculated using the below equations.
Initial values of the x and y coordinates of the pin feed are
4.09mm and 9.55 mm respectively. Table 1 below lists the
antenna parameters obtained from the above equations.
Parameter
Value
fc
5GHz
Leff
11.75 mm
W
19.10 mm
H
2.87 mm
feedx
4.09 mm
feedy
9.55 mm
Table 1: Summary of theoretical antenna parameter values
With these values as reference, a CADFEKO model of the
microstrip patch antenna is constructed, as shown in Fig.1.
Fig.1: Patch Antenna CADFEKO Model
2
As mentioned earlier, the antenna substrate is a FR-4 layer with
dielectric constant 4.4, having dimensions of 90mm * 90 mm *
2.87 mm. The substrate rests on a ground plane of dimensions
50mm*80mm. The patch antenna and the ground plane are
made out of copper, having a high conductivity σ = 5.7*107 S/m
[6].
III. ANTENNA PERFORMANCE ANALYSIS
A. POSTFEKO Simulation Results
For the simulation of antenna on body, a single layer
dry-skin (ε= 36) model [7] was constructed with dimensions
90mm*90mm *0.5mm. These dimensions are considered to
resemble a section of the human chest.
The proposed antenna model shown in Fig.1 is simulated with
modified dimensions as shown in Table 2.
Parameter
Value
Leff
16.87 mm
W
18.56 mm

Impedance Z: 56.12 +j14.2 Ω at 5GHz
H
1.87 mm
feedx
3.8 mm
feedy
9.55 mm
Table.2: Modified dimensions of model
The simulated results obtained are as follows:
 VSWR 2:1 Bandwidth = 643.9 MHz
Fig.3: Plot of Real Impedance
Fig.4: Plot of Imaginary Impedance
Fig.2: Plot of 2:1 VSWR Bandwidth
From Fig.2, VSWR at 5GHz is measured as 1.61. Under FCC
regulations, any spectrum having a bandwidth greater than 500
MHz is classified as ultra-wide band [8]. Based on this
definition, the patch antenna, with a bandwidth >500 MHz can
be categorized as a UWB antenna.
 Reflection Coefficient: -16.84 dB
In order to achieve resonance at 5GHz, an impedance matching
network can be used to match to a 50Ω transmission line system
of the coaxial probe feed line.
 Gain: 0.17 dBi
Fig.5: Plot of Gain
3
VSWR vs Frequency
3
Since the base station receiver is placed at close proximity (10
to 15m) from the patient monitoring system, a low gain will not
critically affect the performance of the communication system.
 Specific Absorption Rate(SAR): 385 mW/kg
2.8
2.6
2.4
VSWR
2.2
2
1.8
1.6
1.4
1.2
1
4.85
4.9
4.95
5
5.05
5.1
F (GHz)
Fig.8: Plot of VSWR Bandwidth
 vs Frequency
Fig.6: Plot of SAR
-10
-15
 (dB)
Specific Absorption Rate (SAR) is critical to antenna design for
biomedical applications. It is a measure of the absorbed
transmitted energy by the human tissue. According to FCC
regulations, the critical SAR limit in USA is 1.6 W/kg and 2
W/kg in Europe [9]. In this design, the obtained SAR values are
measured in mW/kg, which is much below the critical limit.
-5
-20
-25
-30
B. Validation
To verify the above simulation results, a reference model of a
patch antenna with the dimensions from Table.2 is constructed
in SONNET software [10]. The construction of the antenna
model is shown in Fig.7.
X: 5
Y: -36.13
-35
3.5
4
4.5
5
F (GHz)
5.5
6
6.5
Fig.9: Plot of Reflection Coefficient (s11)
Re(Zin) vs Frequency
80
70
in
Re(Z ) ( )
60
X: 5
Y: 49.6
50
40
30
20
4.85
Fig.7: SONNET EM Model of Patch Antenna
Simulated plots of VSWR bandwidth, reflection coefficient
(s11) and real part of input impedance are shown in Fig.8 to 10
respectively.
4.9
4.95
5
5.05
5.1
F (GHz)
Fig.10: Plot of Real Impedance
A comparison between the simulated results of the VSWR
Bandwidth, Reflection Coefficient and real part of input
impedance of SONNET model in Fig 8 to 10 and FEKO model
has been tabulated in Table. 3.
Parameter
VSWR (5 GHz)
SONNET Model
(Reference)
1.02
FEKO Model
1.61
4
Bandwidth
185MHz
643.9MHz
Real Impedance
Ω
56.12Ω
Reflection
-36.13dB
-16.84dB
Coefficient
Table 3: Comparison of Antenna Parameters
the course ECEN 5134: “Principles of Electromagnetic
Radiation & Antennas”.
REFERENCES
[1]
IV. PRACTICAL REALISATION OF THE PIN FED
MICROSTRIP ANTENNA
A. Monitoring Vital Signs of a Patient
The designed pin fed microstrip patch antenna will be used as
an important component in the overall healthcare monitoring
system as described in the block diagram in Fig.11.
Fig.11: Block diagram showing coaxial pin fed microstrip
patch antenna used in healthcare monitoring system
The block diagram in Fig.11 consists of a monitoring device for
monitoring the vital signs of a human body such as body
temperature, pulse rate, blood pressure, respiratory rate, level
of consciousness, urine output [11].
The analog (biomedical signal) information consisting of the
vital signs of the human body is converted to digital
information using the appropriate ADC (Analog to Digital
Converter). This digital information is converted to its
corresponding analog form using the DAC (Digital to Analog
Converter), which is then transmitted by the patch antenna to
the receiver located at a short range (10 to 15 m) from the
transmitter. The analog signal received by the antenna is then
sent to the processing device which converts it to a biomedical
form suitable for display.
CONCLUSION
A coaxially fed ultra wide band microstrip patch antenna,
resonant at 5GHz has been designed in FEKO to monitor the
vital signs of a human body. This design has been subsequently
validated in SONNET, verifying the reliability of the FEKO
simulation results. VSWR 2:1 bandwidth = 643.9 MHz and an
input impedance of 56.12+j14.2 Ω was calculated.
ACKNOWLEDGMENT
This paper was part of the Final Antenna Project Assignment of
Samal, S., Dwari, S., Dutta, A., & Reddy, S. P. (2012, December). A
Microstrip Patch antenna for biomedical applications at 2.45 GHz.
In Computers and Devices for Communication (CODEC), 2012 5th
International Conference on (pp. 1-4). IEEE.
[2] M.R Kamarudin, Y.I.Nechayev, P.S.Hall on “ Performance of Antennas
in the On-Body Environment,” Antennas and Propagation Society
International Symposium, 2005 IEEE 475-478 vol.3A, 2005
[3] Rajeshkumar, V., et al. "Design and Comparative Study of Pin feed and
Line feed Microstrip Patch Antenna for X-band Applications 1." (2012).
[4] Balanis, Constantine A. Antenna theory: analysis and design. John Wiley
& Sons, 2012.
[5] FEKO Suite 6.0 (www.feko.info), EM Software & Systems - S.A. (Pty)
Ltd, PO Box 1354, Stellenbosch, 7599, South Africa.
[6] Lu, Lei, et al. "Ultrahigh strength and high electrical conductivity in
copper."Science 304.5669 (2004): 422-426.
[7] Shrestha, Sudhir, et al. "Flexible microstrip antenna for skin contact
application." International Journal of Antennas and Propagation 2012
(2012).
[8] ECE, IIIII. "Ultra-wideband technology for short-or medium-range
wireless communications." (2001).
[9] Evaluating compliance with FCC guidelines for human exposure to
radiofrequency electromagnetic fields, 1997 :OET Bull. 65
[10] SONNET, Version 10.53, 2005 by Sonnet Software Inc.
[11] Elliott, Malcolm, and Alysia Coventry. "Critical care: the eight vital signs
of patient monitoring." Br J Nurs 21.10 (2012): 621-625.