Compact Wearable Tunable Printed Antennas for Medical Applications

Transcription

1 Research in Health and Nutrition (RHN) Volume 3, 215 Compact Wearable Tunable Printed Antennas for Medical Applications Albert Sabban *1, IEEE Senior Member Electrical Engineering Ort Braude College, Karmiel, Israel *1 Abstract Biomedical industry is in continuous growth in the last decade. Low profile compact tunable antennas are crucial in the development of wearable human biomedical systems. The antenna resonant frequency may be tuned by using a varactor to compensate variations in antenna resonant frequency at different locations on the human body. Design considerations, computed and measured results of wideband printed antennas with high efficiency on the human body at UHF frequencies are presented in this paper. The proposed antenna may be used in Medicare systems. Keywords Antenna; Tunable Antenna; Medicare Systems Introduction Microstrip antennas are widely employed in communication system and seekers. Microstrip antennas posse's attractive features such as low profile, flexible, light weight, small volume and low production cost. In addition, the voltage controlled varactor may be part of the antenna feed network. Microstrip antennas are widely presented in books and papers in the last decade as referred in J.R. James et al, A. Sabban 1981, A. Sabban 1983 and A. Sabban 211. However, the vicinity of the antenna to human body alters the electrical performance of antenna. RF transmission properties of human tissues have been investigated in several papers such as Lawrence C. Chirwa et al 23 and D.Werber et al 26. wearable antennas was presented by Thalmann T. et al 29 and Gupta B. et al 21. A new class of wideband tunable wearable microstrip antennas for medical applications is presented in this paper. The antennas VSWR is better than 2:1at 434MHz+5%. The antenna beam width is around 1º. The antennas gain is around to 2dBi. A voltage controlled varactor is used to control the antenna resonant frequency at different locations on the human body. Dual Polarized Tunable Printed Antenna A compact microstrip loaded dipole antenna has been designed to provide horizontal polarization. The antenna consists of two layers. The first layer consists of RO335.8mm dielectric substrate. The second layer consists of RT-Duroid 588.8mm dielectric substrate. The substrate thickness affects the antenna band width. The printed slot antenna provides a vertical polarization. The printed dipole and the slot antenna provide dual orthogonal polarizations. The dimensions of the dual polarized antenna are 26cm by 6cm by.16cm. Also tunable compact folded dual polarized antennas have been designed. The dimensions of the compact antennas are 5cm by 5cm by.5cm.varactors are connected to the antenna feed lines as shown in Figure 1. The voltage controlled varactors are used to control the antenna resonant frequency. The antenna may be used as a wearable antenna on a human body. The antenna may be attached to the patient s shirt in the patient stomach or back zone. The antenna has been analyzed by using Agilent ADS software. Dipole Feed Dipole Varactor Coupling stubs Slot Slot feed FIG. 1: DUAL POLARIZED TUNABLE ANTENNA, 26CM LONG. There is a good agreement between measured and computed results. The antenna bandwidth is around 1% for VSWR better than 2:1. The antenna beam width is around 1º. The antenna gain is around 2dBi. Fig. 2 presents the antenna measured S11 parameters without a varactor. Fig. 3 presents the antenna S11 parameters as function of different varactor capacitances. Fig. 4 presents the tunable antenna resonant frequency as function of the varactor capacitance. The antenna resonant frequency varies around 5% for capacitances up to 2.5pF. The antenna 14

2 Research in Health and Nutrition (RHN) Volume 3, beam width is 1º. The antenna cross polarized field strength may be adjusted by varying the slot feed location. The computed radiation pattern is shown in Fig. 4. The antennas was analyzed by using 3D fullwave Momentom software. 9mm. Small tuning bars are located along the feed line to tune the antenna to the desired resonant frequency. Fig. 7 presents the antenna computed S11 and S22 parameters. The computed radiation pattern of the folded dipole is shown in Fig. 8. FIG. 2: MEASURED S11 ON HUMAN BODY m5 f req= 418.MHz f req= 423.MHz db(dpant_1h2pf _shirt..s(1,1))=-4.4 db(dpant_1h1pf 5_shirt..S(1,1))= E Mag. [db] E_co -1-2 E_cross -3 Linear Polarization THETA FIG. 5: ANTENNA RADIATION PATTERN 5.5cm ( p ( )) db(s(1,1)) pF Capacitor 2.5pF Capacitor -4 1pF Capacitor m5 f req= 413.MHz 2pF Capacitor db(dpant_1h2pf 5_shirt..S(1,1))= freq, MHz f req= 432.MHz db(dpant_1h_shirt..s(1,1))= freq= 427.MHz db(s(1,1))= FIG. 3: S11 PARAMETER AS FUNCTION OF VARACTOR CAPACITANCE Capacitor (pf) Resonant Frequency (MHz) FIG. 4: RESONANT FREQUENCY AS FUNCTION OF VARACTOR CAPACITANCE Folded Dual Polarized Tunable Antenna The dimensions of the folded dual polarized antenna presented in Fig. 6 are 7x5x.16cm.. The length and width of the coupling stubs in Fig. 5 are 12mm by Mag. [db] Dipole Feed FIG. 6: TUNABLE FOLDED DUAL POLARIZED ANTENNA freq=438.mhz S11 S22 Frequency FIG. 7: FOLDED ANTENNA COMPUTED S11AND S22 RESULTS Mag. [db] Tuning bars E_co Varactor Slot feed E_cross Linear Polarization 4cm Coupling stubs -5 E THETA FIG. 8: FOLDED ANTENNA RADIATION PATTERN 15

3 Research in Health and Nutrition (RHN) Volume 3, 215 Antenna S11 Variation as Function of Dinstace From Body The Antennas input impedance variation as function of distance from the body had been computed by employing ADS software. The analyzed structure is presented in Fig. 9. Properties of human body tissues are listed in Table 1 see Lawrence C. Chirwa et al 23 and D.Werber et al 26. Fig. 1 presents S11 results for different belt and shirt thickness, and air spacing between the antennas and human body. If the air spacing between the sensors and the human body is increased from mm to 5mm the antenna resonant frequency is shifted by 5%. There is good agreement between measured and calculated results. The voltage controlled varactor may be used to tune the antenna resonant frequency due to different antenna locations on a human body. Ground Belt Senso =2-4 Air Shirt =2-4 FIG. 9: ANALYZED STRUCTURE FOR IMPEDANCE CALCULATIONS dbs(1,1) air 8mm Belt 4mm -25 air 8mm Belt 3mm -3 air 4mm Belt 3mm -35 air mm Belt 4mm air 5mm Belt 4mm S11 freq, MHz FIG. 1: S11 RESULTS FOR DIFFERENT ANTENNA POSITIONS TABLE 1 PROPERTIES OF HUMAN BODY TISSUES Tissue Parameter 434 MHz 6 MHz Stomach Colon, Muscle Lung Skin Air Body =4-5 o Free Space 15-3mm mm.-5mm.5-.8mm.-2mm S11 3 FIG. 11: FOLDED ANTENNA S11 RESULTS FOR DIFFERENT ANTENNA POSITION RELATIVE TO THE HUMAN BODY TABLE 2 EXPLANATION OF FIG. 11 Picture # Line type Sensor position 1 Dot Shirt thickness.5mm 2 Line Shirt thickness 1mm 3 Dash dot Air spacing 2mm 4 Dash Air spacing 4mm 5 Long dash Air spacing 1mm 6 Big dots Air spacing 5mm Fig. 11 presents S11 results for different position relative to the human body of the folded antenna shown in Fig. 6. Explanation of Fig. 16 is given in Table 2. If the air spacing between the sensors and the human body is increased from mm to 5mm the antenna resonant frequency is shifted by 5%. Varactors Tuning varactors are voltage variable capacitors designed to provide electronic tuning of microwave components. Varactors are manufactured on silicon and gallium arsenide substrates. Gallium arsenide varactors offers higher Q and may be used at higher frequencies than silicon varactors. Hyperabrupt varactors provide nearly linear variation of frequency with applied control voltage. However abrupt varactors provide inverse fourth root Frequency dependence. MACOM offers several gallium arsenide hyperabrupt varactors such as MA46 series. Fig. 12 presents the C-V curves of varactors MA4654 to MA4656. Fig. 13 presents the C-V curves of varactors MA46H7 to MA46H76. Figure 14 presents a compact tunable antenna with a varactor. Figure 15 presents measured S11 as function of varactor bias voltage. We may conclude that 6 16

4 Research in Health and Nutrition (RHN) Volume 3, varactors may be used to compensate variations in the antenna resonant frequency at different locations on the human body. We may conclude that varactors may be used to compensate variations in the antenna resonant frequency at different locations on the human body. Capacitance (Pf) MA4655 MA4656 Voltage (v) Fig. 16. Three to four tunable folded dipole antennas may be assembled in a belt and attached to the patient stomach. The cable from each antenna is connected to a recorder. The received signal is routed to a switching matrix. The signal with the highest level is selected during the medical test. The antennas receive a signal that is transmitted from various positions in the human body. Folded tunable antenna may be also attached on the patient back in order to improve the level of the received signal from different locations in the human body. In several applications the distance separating the transmitting and receiving antennas is less than 2D²/λ, where D is the largest dimension of the source of the radiation. λ is the wavelength. freq= 375.MHz db(folded_novarc..s(1,1))= freq= 386.MHz db(folded_varc_xv..s(1,1))= FIG. 12: VARACTOR CAPACITANCE AS FUNCTION OF BIAS VOLTAGE Capacitance (Pf) MA46H74 Voltage (v) db(folded_varc_xv..s(1,1)) db(folded_novarc..s(1,1)) db(s(1,1)) NO VARACTOR 9V 8V freq, MHz 7V freq= 396.MHz db(folded_varc_7v..s(1,1))= freq= 384.MHz db(s(1,1))= MA46H7 FIG. 13: VARACTOR CAPACITANCE (PF) AS FUNCTION OF BIAS VOLTAGE FIGURE 15. MEASURED S11 AS FUNCTION OF VARACTOR BIAS VOLTAGE In these applications the amplitude of the electromagnetic field close to the antenna may be quite powerful, but because of rapid fall-off with distance, they do not radiate energy to infinite distances, but instead their energies remain trapped in the region near the antenna. Thus, the near-fields only transfer energy to close distances from the receivers. The receiving and transmitting antennas are magnetically coupled. Change in current flow through one wire induces a voltage across the ends of the other wire through electromagnetic induction. The amount of inductive coupling between two conductors is measured by their mutual inductance. In these applications we have to refer to the near field and not to the far field radiation. FIGURE 14. TUNABLE ANTENNA WITH A VARACTOR Medical Applications for Tunable Antennas An application of the proposed antenna is shown in The proposed tunable wearable antennas may be placed on the patient body as shown in Fig. 17a. The patient in Fig. 17b is wearing a wearable antenna. The antennas belt is attached to the patient front or back body. 17

5 Research in Health and Nutrition (RHN) Volume 3, 215 Fig. 18 presents several compact tunable Antennas for medical Applications. A voltage controlled varactor may be used also to tune the loop antenna resonant frequency at different antenna locations on the body. Medical System Data recorder Wearable Tunable antennas Belt FIG. 16. TUNABLE WEARABLE ANTENNA Antennas Cables Medical System Belt Antennas FIG. 17: A. MEDICAL SYSTEM WITH PRINTED WEARABLE ANTENNAS. B. PATIENT WITH PRINTED WEARABLE ANTENNA Varactors Loop Antenna With GND Patient Shirt Cables FIG. 18. TUNABLE ANTENNAS FOR MEDICAL APPLICATIONS Conclusions This paper presents wideband tunable microstrip antennas with high efficiency for medical applications. The antenna dimensions may vary from 26cm by 6cm by.16cm to 5cm by 5cm by.5cm according to the medical system specification. The antennas bandwidth is around 1% for VSWR better than 2:1. The antenna beam width is around1º. The antennas gain varies from to 2dBi. If the air spacing between the dual polarized antenna and the human body is increased from mm to 5mm the antenna resonant frequency is shifted by 5%. A varactor is employed to compensate variations in the antenna resonant frequency at different locations on the human body. REFERENCES J.R. James, P.S Hall and C. Wood, Microstrip Antenna Theory and Design,1981. A. Sabban and K.C. Gupta, Characterization of Radiation Loss from Microstrip Discontinuities Using a Multiport Network Modeling Approach, I.E.E.E Trans. on M.T.T, Vol. 39,No. 4,April 1991, pp A. Sabban, Wideband Microstrip Antenna Arrays, I.E.E.E Antenna and Propagation Symposium MELCOM, Tel- Aviv,1981. A. Sabban, A New Wideband Stacked Microstrip Antenna, I.E.E.E Antenna and Propagation Symp., Houston, Texas, U.S.A, June A. Sabban, E. Navon A MM-Waves Microstrip Antenna Array, I.E.E.E Symposium, Tel-Aviv, March R. Kastner, E. Heyman, A. Sabban, Spectral Domain Iterative Analysis of Single and Double-Layered Microstrip Antennas Using the Conjugate Gradient Algorithm, I.E.E.E Trans. on Antennas and Propagation, Vol. 36, No. 9, Sept. 1988, pp A. Sabban, "Microstrip Antenna Arrays", Microstrip Antennas, Nasimuddin Nasimuddin (Ed.), ISBN: , InTech, p-antenna-arrays, pp , 211. Lawrence C. Chirwa*, Paul A. Hammond, Scott Roy, and David R. S. Cumming, "Electromagnetic Radiation from Ingested Sources in the Human Intestine between 15 MHz and 1.2 GHz", IEEE Transaction on Biomedical eng., VOL. 5, NO. 4, April 23, pp

6 Research in Health and Nutrition (RHN) Volume 3, D.Werber, A. Schwentner, E. M. Biebl, "Investigation of RF transmission properties of human tissues", Adv. Radio Sci., 4, pp , 26. Gupta, B., Sankaralingam S., Dhar, S.,"Development of wearable and implantable antennas in the last decade", Microwave Symposium (MMS), 21 Mediterranean 21, pp Thalmann T., Popovic Z., Notaros B.M, Mosig, J.R.,"Investigation and design of a multi-band wearable antenna", 3rd European Conference on Antennas and Propagation, EuCAP 29. pp Dr. A. Sabban (M'87-SM'94) received the B.Sc degree and M.Sc degree Magna Cum Laude in electrical engineering from Tel Aviv University, Israel in 1976and 1986 respectively. He received the Ph.D. degree in electrical engineering from University at Boulder, USA, in Dr. A. Sabban reasearch interests are microwave, antenna engineering, electromagnetic theory and system engineering. In 1976 he joined the armament development authority RAFAEL in Israel. In RAFAEL he worked as a senior researcher, group leader and project leader in the electromagnetic department till 27. He leaded several research programs. He developed wideband mm-wave antennas, wearable antennas, MIC and MMIC modules, MEMS detection array, and microwave components. He developed an iterative spectral domain analysis of single and double-layered microstrip antennas using the conjugate gradient Algorithm. From 1987 to 1991 he joined the University of Colorado at Boulder where he was a research assistant and studied toward his Ph.D. degree. His research topic was "Multiport Network Modeling for Evaluating Radiation Loss and Spurious Coupling among Microstrip Discontinuities in Microstrip Circuits". He developed a planar lumped model for evaluating spurious coupling and radiation loss among coupled microstrip discontinuities, a spectral domain algorithm to analyze microstrip lines and coupled microstrip lines. In 27 he retired from RAFAEL. From 28 to 21 he worked as an RF Specialist and project leader in Hitech companies. From 21 to date Dr. A. Sabban is a senior lecturer and researcher in Ort Braude College in Israel in the electrical engineering department. Dr. A. Sabban developed a model to analyze antennas on human body. He Invented several new compact wearable printed Antennas for Medical Applications. He published books, chapters in books and over 6 research papers and hold a patent in the antenna area. 19