Conformable patch antenna design for remote health monitoring

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Conformable Patch Antenna Design for Remote
Health Monitoring
Akshat C. Patel, Miral P. Vaghela, Hassan Bajwa, Hassan Seddik
Department of Electrical Engineering
University of Bridgeport, 221 University Ave, Bridgeport, CT, USA
[email protected]
Abstract— In this paper we present a design of a nanostructured
conformable patch antenna array and full system level radio
design that could be integrated with the conformable antenna.
The proposed system designed specifically for biomedical
applications can be used to provide real time remote health
monitoring thus improving patient quality of life. CNT-polymer
nanocomposite antenna is designed on cotton dielectric that
resonate at 2.45 GHz on a 1.57 mm thick substrate with εr=1.6.
To demonstrate antenna performance in dynamic environment
the antenna array was bent outward on a 17mm radius of
curvature. All antenna parameters such as VSWR, return loss,
gain and radiation pattern meet the design criteria. Our
simulated antenna shows a return loss less than -10 dB and
VSWR less than 2 at 2.06 GHz, 2.38 GHz and 2.49 GHz.
Keywords- Conformable Antenna; patch antenna; remote health
monitoring; RF transmitter
I.
INTRODUCTION
Remote health monitoring is emerging as a promising tool
to improve quality of life of cardiac patients by allowing them
to leave hospital and still be monitored by health professionals
[1]. Acquisition of remote physiological signals is becoming
possible with the advancement in communications systems and
compact low power devices. A low power transmitter and an
antenna designed specifically for biomedical application is
required to realize such system.
Recently, the explosive growth of the Carbon Nanotube
(CNT) and energy harvesting technologies have accentuated
the need for miniaturized, high-efficiency conformal antennas
that can operate over a wide range of frequencies, while they
can be integrated in wearable and lightweight configurations
[2, 3]. Extensive research has been carried out in recent years
to fabricate antenna with CNT and conductive polymer inks
instead of metallic conductors. Polymer based antennas using
different fabric as a substrate like cotton, fleece, nylon etc have
been successfully fabricated [4].
Principal requirements for a conformable antenna are low
profile and small size, ability to minimize the radiation
absorption and Reasonable gain. Using carbon nanotube
instead of tradition metal, we can actually create conformal
patch array that provide much more flexibility and can easily
be integrated with convention ECG patch electrode.
Transmitting at 2.45 GHZ frequency also allows antenna to be
very small due to the short wavelength. To give flexibility on
the integration of wearable antennas into ECG electrode, cotton
fabric is selected to serve as the base for conducting patch. It
978-1-4244-5550-8/10/$26.00 ©2010 IEEE
Figure 1. Patch antenna array geometry
has low dielectric constant (εr ≈ 1.1-1.7), which improves
impedance bandwidth of antenna and reduces spurious
radiation loss in substrate.
II.
ANTENNA DESIGN
In this paper we present a design of conformable four patch
array on a curved surface using CNT as a highly conducting
material. Antenna design has been carried out in the 2-3 GHz
ISM band for space, defense and biomedical application. As
shown in Fig. 1 the antenna is made up from textile materials,
where blue portion is highly conductive CNT and rest is cotton
dielectric which has dielectric constant of 2.2 same as that of
RT duroid, a material that has excellent characteristics for
optimal antenna efficiency.
The thickness of the substrate was taken as 1.57mm to
maximize the bandwidth and efficiency. The conducting
ground plane and the antenna consist of a CNT-silver
composite, which is 0.1mm a thin and flexible. The multi-layer
substrate was modeled in order to ensure a well-defined
substrate thickness and to keep the antenna conformal when it
is bent. Microstrip antenna dimensions were calculated using
transmission line model [5]. The patch antenna model has been
created using an Electromagnetic simulator FEKO based on
method of moment.
III.
MATERIAL AND METHODS
The objective of our research is to develop model and
prototype of CNT based conformal patch antenna array. The
approach is to use carbon nanotubes that can be ink-jet printed
on the surface of fabric to build patch antenna array [6].
Appropriate nanotube based conducting ink formulations can
be developed for fabrication. Printing several patterns on the
fabrics and plastic films will be carried out. Useful electrical
measurements such as impedance matching will be conducted
to establish the antenna principles in the materials. To establish
a reasonable interface between cotton and carbon nanotube in
the simulated antenna we assumed cellulose functionalized
carbon nanotube. Alternatively conductive ink of polymer
functionalized CNT also serves as an appropriate formulation
was considered because the polymer functionalized CNT is
impregnated to the cotton reasonably well to have critical
amount of tubes present on the cotton substrate.
I_Probe
I_IF_In
The CNT particles are evenly distributed in the patch
region.
•
Conductivity of CNT and cotton are taken as 109 S/m
and 10-3 S/m respectively. The theoretical
conductivity of the CNT can rise as high as 106 – 109
[7-9] for pure metallic tube because of the structural
quantization. For our simulation to create an upper
bound we used ballistic conductivity value of CNT.
•
V_Mix1_Out
vm_mc_ZLW-1WSH_19930601
MIX1
F_LO=LO1_Freq
TYPE=UPPER
V_Mix 2_In
V_Filt1_Out
I_Probe
I_Mix 2_In
I_Probe
I_Filt1_Out
vf_mc_SBLP-467_19930426
va_mc _AMP-75_19930601 S2
RF_AMP1
V_Mix 2_Out
v f_mc_SHP-500_19930426
S1
V_Filt2_Out
I_Probe
I_Mix2_Out
DA_SBFilter1_tRANSMITTER_W_pRACTICAL_ELEMNTS2
DA_SBFilter1
Subst="MSub1"
Fs1=1.9 GHz
Fp1=2 GHz
Fp2=3 GHz
Fs2=3.5 GHz
Ap=0.1 dB
As=20 dB
N=0
Res pons eType=Chebyshev
StubConfig=Two Parallel Stubs
StubType=Short Circ uit Quarter Wave
Zo=50 Ohm
D=1
Finf=1.0 GHz
Delta=0 mil
V_RF_Out
I_Probe
I_Filt2_Out
va_hp_AMT4071_19930601
RF_AMP2
I_Probe
I_Mix1_Out
Term
I_Probe
Term3
I_RF_OutTerm2
Num=3
Num=2
Z=50 Ohm
Figure 2. RF transmitter system schematic design
•
2nd IF Stage:
Mixer Mini-Circuit ZLW-1WSH_19930601
High-Pass Filter Mini-Circuit SHP-19930426
Low-pass filter Mini-Circuit SBLP-467_19930426
RF-AMP Mini-Circuit AMP-75_19930601
•
3rd RF Stage:
Mixer Mini-Circuit ZEM-4300MH_19930601
RF-AMP HP-AMT4071_19930601
Microstrip line bandpass filter at the last stage of the
transmitter.
RF TRANSMITTER DESIGN
In this section we present RF transmitter system schematic
and simulation using Agilent Advanced Design System (ADS).
For RF transmitter design suitable chipset are used from ADS.
The component models are selected from RF/Analog library. A
50 Ohm terminal in harmonic balance library is used to
terminate the transmitter output. To match required
performance we designed a micro-strip band-pass filter in RF
stage. For simulation purpose we used a single tone power
source as input signal with frequency 200Hz, which is in the
range of our ECG output signal frequencies. The magnitude of
received signal from ECG montor is taken as 0.5-2mV, and
frequency range of ECG signal is taken as 0.1 - 250 Hz [10].
The RF transmitter consists of two IF and RF stage. The
components of each stage of transmitter are as follows:
•
V_Mix1_In
v a_mc_ZFL-1HAD_19930601
IF_AMP
vm_mc_ZEM-4300MH_19930601
MIX2
F_LO=LO2_Freq
TYPE=UPPER
Thickness of CNT is taken as 0.1mm.
IV.
V_IF_In
P_1Tone
PORT1
Num=1
Z=50 Ohm
P=polar(dbmtow(Power_IF),0)
Freq=IF_Freq
Due to the complex structure of antenna, following
assumptions have been made here.
•
I_Probe
I_Mix1_In
A. Filter Design
For the RF stage of the transmitter we designed microstrip
line band pass filter to match design requirements. The goal of
the band pass filter is just to pass the carrier frequency 2.4GHz
and our modulating signal which has a very small bandwidth.
For the RF stage of the transmitter we designed microstrip line
band pass filter to match design requirements.
1st IF Stage:
IF Amplifier Mini-Circuit ZFL-1HAD_19930601
.
M LSC
TL4
Subs t="M Sub1"
W=104.835 m i l
L=409.819 m il
M CROS
Cros 1
Subs t="M Sub1"
W1=15.95 m i l
W2=104.835 m i l
W3=17.601 m i l
W4=104.835 m i l
M TEE
Tee1
Subs t="M Sub1"
W1=9.914 m i l
W2=15.95 m i l
W3=16.141 m i l
Port
P1
Num =1
M LIN
TL2
Subs t="M Sub1"
W=15.95 m il
L=454.417 m i l
M LSC
TL1
Subs t="M Sub1"
W=16.141 m il
L=454.142 m i l
M LSC
TL7
Subs t="M Sub1"
W=104.776 m i l
L=409.831 m il
M CROS
Cros 2
Subs t="M Sub1"
W1=17.601 m i l
W2=104.776 m i l
W3=16.029 m i l
W4=104.776 m i l
M LIN
TL5
Subs t="M Sub1"
W=17.601 m il
L=452.115 m i l
M LSC
TL3
Subs t="M Sub1"
W=104.835 m i l
L=409.819 m il
M LSC
TL10
Subs t="M Sub1"
W=106.155 m i l
L=409.575 m il
M CROS
Cros 3
Subs t="M Sub1"
W1=16.029 m i l
W2=106.155 m i l
W3=15.708 m i l
W4=106.155 m i l
M LIN
TL8
Subs t="M Sub1"
W=16.029 m i l
L=454.304 m il
M LSC
TL6
Subs t="M Sub1"
W=104.776 m i l
L=409.831 m il
M LSC
TL13
Subs t="M Sub1"
W=106.155 m i l
L=409.575 m il
M CROS
Cros 4
Subs t="M Sub1"
W1=15.708 m i l
W2=106.155 m i l
W3=16.029 m i l
W4=106.155 m i l
M LIN
TL11
Subs t="M Sub1"
W=15.708 m i l
L=454.771 m il
M LSC
TL9
Subs t="M Sub1"
W=106.155 m i l
L=409.575 m il
M LSC
TL16
Subs t="M Sub1"
W=104.776 m i l
L=409.831 m il
M CROS
Cros 5
Subs t="M Sub1"
W1=16.029 m il
W2=104.776 m il
W3=17.601 m il
W4=104.776 m il
M LIN
TL14
Subs t="M Sub1"
W=16.029 m i l
L=454.304 m il
M LSC
TL12
Subs t="M Sub1"
W=106.155 m i l
L=409.575 m il
Figure 3. Microstrip bandpass filter
M LSC
TL19
Subs t="M Sub1"
W=104.835 m i l
L=409.819 m il
M CROS
Cros 6
Subs t="M Sub1"
W1=17.601 m il
W2=104.835 m il
W3=15.95 m il
W4=104.835 m il
M LIN
TL17
Subs t="M Sub1"
W=17.601 m i l
L=452.115 m il
M LSC
TL15
Subs t="M Sub1"
W=104.776 m i l
L=409.831 m il
M TEE
Tee2
Subs t="M Sub1"
W1=15.95 m il
W2=9.914 m il
W3=16.141 m il
M LIN
TL20
Subs t="M Sub1"
W=15.95 m i l
L=454.417 m il
M LSC
TL18
Subs t="M Sub1"
W=104.835 m i l
L=409.819 m il
Port
P2
Num =2
M LSC
TL21
Subs t="M Sub1"
W=16.141 m il
L=454.142 m i l
Fs1
Input Param eters
Fp1
1.900
2.000
CF-Des
Perform ance
CF-Actual
2.500
Marker M2
Fs2
3.000
3.500
Dev-PB
2.500
F
Marker M1
Fp2
58.049
S11 (dB)
As
Ap
20.000
MA-LSB
-68.331
|S11|^2
0.100
MA-USB
-64.972
S21 (dB)
2.00
0.00
1.00
-58.05
2.00
0.00
1.00
-58.05
Fs1: Lower Stop-Band Edge
Fp1: Lower Pass -Band Edge
Fp2: Upper Pass -Band Edge
Fs2: Upper Stop-Band Edge
As: Atten at Stop-Band Edge
Ap: Atten PB Edge or Ripple
CF: Center Frequency (Desired or Actual)
Dev-PB: Deviation in Pass-Band
MA: Minim um Atten. Lower/Upper Stop-Band
F: Frequency
1/2: Input/Output Ports
Spec: Frequency Specification
Figure 4. Microstrip bandpass filter characteristics
Fig. 3 shows the center frequency of the filter and the
attenuation magnitude at different part of the filter spectrum,
like Low/Upper stop band and the attenuation in the pass
band. Table 1 also shows markers M1and M2 from the
input/output characteristics diagrams. Customized RF
microstrip line filter helps regulate the characteristic of the
patch antenna [12].
V.
SIMULATION RESULTS
Here we present simulation results, node names and
calculation at each node. Measurements of Noise Figure (NF),
power and power gain for each node is conducted. As shown
below parameters for the simulations are set as variables in
ADS.
Var
Eqn
VAR
VAR1
LO1_Freq =450 MHz
LO2_Freq =2000 MHz
Power_IF = -50 _dBm
IF_Freq =200 Hz
FirstIF =LO1_Freq+IF_Freq
RF_Out=FirstIF+LO2_Freq
Noise_BW=1 Hz
NoiseTemp=16.85 _C
HB NOISE CONTROLLER
HARMONIC BALANCE
HarmonicBalance
HB1
MaxOrder=5
Freq[1]=LO1_Freq
Freq[2]=LO2_Freq
Freq[3]=IF_Freq
Order[1]=3
Order[2]=3
Order[3]=1
Noisecon[1]="IF_In"
Noisecon[2]="FirstIF"
Noisecon[3]="RF_Out"
NoiseConMode=yes
NoiseCon
RF_Out
FreqForNoise=RF_Out
NoiseNode[1]=V_Filt2_Out
NoiseNode[2]=V_Mix2_Out
NoiseNode[3]=V_RF_Out
BandwidthForNoise=Noise_BW
HB NOISE CONTROLLER
HB NOISE CONTROLLER
NoiseCon
FirstIF
FreqForNoise=FirstIF
NoiseNode[1]=V_Mix1_Out
NoiseNode[2]=V_Filt1_Out
NoiseNode[3]=V_Mix2_In
BandwidthForNoise=Noise_BW
NoiseCon
IF_In
FreqForNoise=IF_Freq
NoiseNode[1]=V_Mix1_In
BandwidthForNoise=Noise_BW
OPTIONS
Options
Options2
Temp=NoiseTemp
Tnom=25
Figure 5. ADS Simulation tools
20.0
m4
freq=2.450GHz
dBm(V_RF_Out)=12.260
LO1_Freq and LO2_Freq are the oscillators’ frequencies for
the mixers. Harmonic Balance simulator HB1 is used to
investigate the performance of the system output spectrum. For
harmonic balance simulation, the base band frequency is taken
as 200 Hz. The frequencies of the two local oscillators are
450MHz and 2000 MHz.
ADS simulation tools and simulation results are shown in Fig.4
and Fig.5. The most dominant spectrum component for the
output RF is 2.45GHz which is our transmitted carrier signal.
Power, gain and the noise figure values of each node and for
every component are calculated and shown below in Tables 2.
dBm(V_RF_Out)
m4
4.3
m5
freq=2.450GHz
dBm(V_RF_Out)=-13.180
m5
-15.7
0
1
2
3
4
freq, GHz
Figure 6. Output port spectrums
5
6
7
TABLE II. POWER AND CASCADE POWER GAIN AT EACH NODE
TABLE IV. CASCADE NF AT EVERY STAGE
Cascaded Noise Figure at Nodes (dB)
Power at Nodes (dBm)
Power_IF
-50
Mix1_In_dBm
-39.285
Mix1_Out_dBm
-53.073
Filt1_Out_dBm
-56.889
Mix2_In_dBm
-35.564
Mix2_Out_dBm
-44.232
Filt2_Out_dBm
-44.232
RF_Out_dBm
-13.180
Cascaded Power Gain at Nodes with Respect to the Input
Signal Power(dB)
Gain_Mix1_In
10.715
Gain_Mix1_Out
-3.073
Gain_Filt1_Out
-6.889
Gain_Mix2_In
14.436
Gain_Mix2_Out
5.768
Gain_Filt2_Out
5.768
Gain_Sys
36.052
NF_Mix1_In
-0.266
NF_Mix1_Out
10.474
NF_Filt1_Out
10.675
NF_Mix2_In
7.596
NF_Mix2_Out
11.636
NF_Filt2_Out
20.356
NF_Sys
1.788
A. IMD and IP3 Simulation
To calculate Inter modulation distortions (IMD) and the 3rd
order intercept points (IP3), Harmonic Balance (HB)
simulation was performed in Agilent ADS.
HARMONIC BALANCE
HarmonicBalance
HB2
MaxOrder=5
Freq[1]=LO1_Freq
Freq[2]=LO2_Freq
Freq[3]=IF_Freq+Freq_Spacing/2
Freq[4]=IF_Freq-Freq_Spacing/2
Order[1]=3
Order[2]=3
Order[3]=3
Order[4]=3
UseKrylov=yes
Gain of each stage is calculated by the following formulas
from ADS [13] and is shown in Table 3 and Table 4.
•
•
•
•
•
•
FirstIF_Gain = Mix1_In_dBm-FirstIF_dBm
Mix1_Gain=Mix1_Out_dBm-Mix1_In_dBm
IF_Stage_Gain=Filt1_Out_dBm-Mix1_Out_dBm
Mix2_Gain=Mix2_Out_dBm-Mix2_In_dBm
Last_Stage_Filter_Gain=Filt2_Out_dBmMix2_Out_dBm
Last_Stage_Amp_Gain1=RF_Out_dBmFilt2_Out_dBm.
TABLE III. POWER GAIN AT EVERY STAGE
Absolute Power Gain at Every Stage
(dB)
Mix1_Gain
-13.788
Mix2_Gain
-8.668
IF_Stage_Gain
-3.816
Last_Stage_Filter_Gain
-2.496E-05
Last_Stage_Amp_Gain1
31.052
Cascaded noise figure values are shown in the Table IV.
To access the performance of the system it is necessary to
calculate carrier-to-intermodulation performance. Carriers to
Intermodulation measurements and Third Order intercept
values from ADS are shown in the table below.
TABLE V. IMD And IP3
Carrier to Intermodulation (dBc)
Mix1_In_CIMD
139.636
Mix1_Out_CIMD
135.784
Filt1_Out_CIMD
130.319
Mix2_In_CIMD
115.619
Mix2_Out_CIMD
115.619
Filt2_Out_CIMD
115.619
RF_Out_CIMD
66.643
Third Order Intercept Point (dBm)
Mix1_In_TOI
30.533
Mix1_Out_TOI
14.819
Filt1_Out_TOI
8.271
Mix2_In_TOI
22.245
Mix2_Out_TOI
13.578
Filt2_Out_TOI
13.578
RF_Out_TOI
20.134
VI.
ANTENNA RESULTS
The antenna is designed to resonant at 2.455 GHz (λ= 122.44
mm) on an h= 1.57 mm thick substrate with εr=2.2. To
demonstrate antenna performance in dynamic environment, the
antenna array was bent outward on a 10mm raddius of curvature.
The resulting return loss and VSWR reesponse for this
configuration is shown in Fig.7 and Fig.8 respeectively.
Figure 9. 3D Far-field radiattion pattern at 2.45 GHz
Figure 7. Plot of Return loss (dB) v/s Freequency
Figure 10. Simulated 2D gaain pattern at 2.45GHz
Figure 8. Plot of VSWR v/s Frequenncy
The antenna operating frequency, bandwidth and efficiency
were obtained from the return loss measurement. The
impedance matching result illustrattes the frequency detuning
which shows the effect of flexiible substrate on antenna
performance. The far field radiation
n pattern and 2D gain of the
proposed antenna array is show
wn in Fig.9 and Fig.10
respectively. Due to high conform
mability and conductivity of
CNT, all antenna parameters like VSWR,
V
return loss, gain and
radiation pattern meet design criteriaa.
TABLE VI. ANTENNA CHA
ARACTERISTICS
Frequency
(GHz)
Impedance Z
(ohm)
Return
S11 (dB)
loss
VSWR
2.06
87
-10.79
1.9
2.38
35
-14.43
1.5
2.49
26
-10.29
1.8
ylindrical surface is found to
The conformal patch array on cy
be resonating at three different frequ
uencies. The results at these
frequencies are summarized in table 6.
VII. CONCLUSION
We report our investigation on the design of a conformable
CNT patch antenna that lead to improved antenna parameters
such as VSWR, return loss, gain and radiation pattern and
efficient energy harvesting. CNT owing to its unique electronic
property is a potential choice for metal less conformable
polymer-CNT nanocomposite antenna. We also investigated
and present design of RF transmitter that can transmit ECG
signal wirelessly and can be integrated with conformable
antenna to provide remote health monitoring of patients.
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