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 . 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 . 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 . 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 . 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 . 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 . 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=LO1_Freq Freq=LO2_Freq Freq=IF_Freq Order=3 Order=3 Order=1 Noisecon="IF_In" Noisecon="FirstIF" Noisecon="RF_Out" NoiseConMode=yes NoiseCon RF_Out FreqForNoise=RF_Out NoiseNode=V_Filt2_Out NoiseNode=V_Mix2_Out NoiseNode=V_RF_Out BandwidthForNoise=Noise_BW HB NOISE CONTROLLER HB NOISE CONTROLLER NoiseCon FirstIF FreqForNoise=FirstIF NoiseNode=V_Mix1_Out NoiseNode=V_Filt1_Out NoiseNode=V_Mix2_In BandwidthForNoise=Noise_BW NoiseCon IF_In FreqForNoise=IF_Freq NoiseNode=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=LO1_Freq Freq=LO2_Freq Freq=IF_Freq+Freq_Spacing/2 Freq=IF_Freq-Freq_Spacing/2 Order=3 Order=3 Order=3 Order=3 UseKrylov=yes Gain of each stage is calculated by the following formulas from ADS  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. REFERENCES           Liszka, K.J., et al., Keeping a beat on the heart. Pervasive Computing, IEEE, 2004. 3(4): p. 42-49. A.Patel, M.V., H.Bajwa and P.Patra, . Conformable Patch Antenna Arrays for Energy Harvesting. MRS fall meeting, 2009. Burke, P.J., S. Li, and Z. Yu, Quantitative theory of nanowire and nanotube antenna performance. Nanotechnology, IEEE Transactions on, 2006. 5(4): p. 314-334.    Hertleer, C., H. Rogier, and L. Van Langenhove. 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