Commercial plaster material (polyacrylate) is used
as an antenna substrate. Two 2.45 GHz patch antennas are
introduced, both designed to be attached directly to the skin.
Measured efficiencies are 70 % in free space and 60 % on-body.
Measured on-body gains of each antenna are 6.2 and 1.4 dBi.
Simulated 1 g specific absorption rates (SAR) of the two antennas
are 2.3 W/kg and 1.6 W/kg using 1 W input power. 10 g SAR
values are 0.6 W/kg and 1.2 W/kg. Antenna feeding using snap-on
buttons is investigated and has been found useful

Jouko Heikkinen (7209941)

Lauri Kettunen (7209944)

Tiiti Kellomaki (7209938)

W.G. Whittow (1223946)

English

Loughborough University Institutional Repository

   This item was submitted to Loughborough’s Institutional Repository (https://dspace.lboro.ac.uk/) by the author and is made available under the following Creative Commons Licence conditions.      For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/  2.4 GHz Plaster Antennas for Health MonitoringTiiti Kelloma¨ki #1, William G. Whittow ∗2, Jouko Heikkinen #3, Lauri Kettunen #4#Department of Electronics, Tampere University of TechnologyP.O.Box 692, FI-33101 Tampere, FINLAND1tiiti.kellomaki@tut.fi, 3jouko.heikkinen@tut.fi, 4lauri.kettunen@tut.fi∗Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leics, UK, LE11 3TU2w.g.whittow@lboro.ac.ukAbstract— Commercial plaster material (polyacrylate) is usedas an antenna substrate. Two 2.45 GHz patch antennas areintroduced, both designed to be attached directly to the skin.Measured efficiencies are 70 % in free space and 60 % on-body.Measured on-body gains of each antenna are 6.2 and 1.4 dBi.Simulated 1 g specific absorption rates (SAR) of the two antennasare 2.3 W/kg and 1.6 W/kg using 1 W input power. 10 g SARvalues are 0.6 W/kg and 1.2 W/kg. Antenna feeding using snap-onbuttons is investigated and has been found useful.Index Terms— Antennas, medical services, microstrip anten-nas.I. INTRODUCTIONMany medical applications require gathering data of thepatient’s condition. Examples include heart rate, breath rate,and blood oxygen level. Usually these data are transmitted viaa cable, which limits the patient’s movements.Freedom of movement is especially important in homenursing and in monitoring athletes’ recovery after training. Asan alternative to cables, the patients could carry a belt-worndevice which records data and transmits the results wirelesslyto the system whenever possible. Plaster-based sensors havebeen developed in the recent years [1]. As printed electronicsevolves, the whole system including the measurement elec-tronics, data gathering, radio transceiver, and antenna couldbe integrated on a single, disposable plaster.In this paper, we examine the feasibility of plaster materialas an antenna substrate, and present two antenna structures.The antennas are to be attached directly to the skin, like regularwound-care plasters. To minimise the effect of the body on theantennas, a ground plane is used. Slits are cut in the antennasto increase flexibility and breathability.The antenna structures as well as the electrical propertiesof the plaster are described in Section II. Section III containsmeasured results for the regular antenna parameters. SimulatedSAR values are presented in Section IV. We describe analternative feeding method using snap-on buttons and examinethe effect of the user sweating on antenna parameters inSection V. Section VI concludes the work.II. ANTENNA STRUCTURESThe goal of the design process was to design antennas ona plaster substrate that cover the frequency band from 2.4to 2.5 GHz. Linear polarisation was desired, because circularpolarisation characteristics would depend too much on antennaTABLE IPATCH ANTENNA DIMENSIONS (IN MILLIMETRES)ground and patch and slits andsubstrate feed stripswg 81 wp 47 wc 3lg 80 lp 48 wb 4ws 57 wf 4.5 wa 1ls 58 lin 15 p 1h 1.36 g 1 l1 10la 5bending, as shown by our ongoing research. The antennas weredesigned to be flexible and breathable.Commercially available Mo¨lnlycke Mefix [2] plaster mate-rial (self-adhesive polyacrylate) was chosen as the substrate.Its dielectric properties were measured at 2.45 GHz. The di-electric constant is about 1.38 and loss tangent 0.02. Comparedto a PCB material the dielectric constant is very low but lossesare about the same as of a poor PCB.The conducting material of the antennas was copper tape.In the future, the antennas could e. g. be printed on the plaster.Research is going on to find a suitable conducting material forwearable antennas.We must use a ground plane, because the antennas operateless than one millimetre from the body. The reactive near-fieldis then trapped between the radiating element and the groundplane. This reduces the effect of the user on input impedance(resonance frequency) and radiation, and lowers SAR.Two antennas were designed, one a half-wave patch and theother a quarter-wave antenna short-circuited to the ground. Thesubstrate for both is an 8-layer plaster, which is 1.36 mm thick.The dimensions of the antennas are given in Figs. 1, 2, 3, and4, and in Tables I and II.To increase flexibility and breathability, slits were cut bothin the antenna elements and ground planes. The x-directionalslits do not affect the current flow of the radiating mode,however extra horizontal strips had to be added at the feedpoint level to allow for y-directional current at the feed. Theantenna dimensions are a trade-off between antenna efficiencyand breathability.III. MEASURED RESULTSRegular antenna parameters, the input impedance and radi-ation pattern, were measured both in free space and on-body.211Authorized licensed use limited to: LOUGHBOROUGH UNIVERSITY. Downloaded on September 24, 2009 at 07:28 from IEEE Xplore.  Restrictions apply. Fig. 1. Dimensions of the patch antenna–the part above substrate. Thesubstrate is shown dashed.Fig. 2. Dimensions of the patch antenna ground plane. A substrate (showndashed) of width ws, length ls, and height h is placed at the centre of theground plane. Note that the substrate does not cover all of the ground plane.The whole structure lies on two layers of plaster (thickness about 0.3 mm).The slits in the ground plane and the patch coincide.TABLE IIQUARTER-WAVE ANTENNA DIMENSIONS (IN MILLIMETRES)ground and patch and slits andsubstrate feed stripswg 54 wp 36 wa 4.5lg 59 lp 25.5 wb 3.5l1 14 wf 5 wc 1.5ls 28.5 lin 18 wd 14ws 42 lsh 2 lc 17.5h 1.36 g 1 p 1A. Input MatchingThe 2.45 GHz ISM band (Industrial–Scientific–Medical)extends from 2.4 to 2.5 GHz, a bandwidth of 4.1 %. ThisFig. 3. Dimensions of the quarter-wave antenna–the part above substrate. Thelight gray part (length lsh) is bent from above the substrate to the groundplane level, so that the uppermost part (length p) is actually a part of theground plane. The patch is thus short-circuited to the ground. The substrateis shown dashed.Fig. 4. Dimensions of the quarter-wave antenna ground plane. The parts ofwidth p here (upper one) and in Fig. 3 coincide. A substrate (shown dashed)of width ws, length ls, and height h is placed on the ground plane centred inthe y direction. The whole structure lies on two layers of plaster (thicknessabout 0.3 mm). The slits in the ground plane and the patch coincide.can be covered by the 6-dB band of an antenna with a Qvalue of less than 28.We measured the antenna input impedances using a vectornetwork analyser. For on-body measurements the antennaswere attached to the abdomen in order to avoid bending whichwould possibly interfere with the measurement.The human body lowers the resonance frequencies of bothantennas. At the same time the input resistance at resonancefrequency is lowered. The Q value decreases when the antennais placed on the body, indicating losses in the body. Figures 5and 6 illustrate the change. The values in free space and on-body are listed in Table III.212Authorized licensed use limited to: LOUGHBOROUGH UNIVERSITY. Downloaded on September 24, 2009 at 07:28 from IEEE Xplore.  Restrictions apply. TABLE IIIINPUT IMPEDANCE MEASUREMENT RESULTSpatch quarter-waveinput resistance, free space 54 Ω 58 Ωinput resistance, on-body 42 Ω 51 ΩQ, free space 27 23Q, on-body 23 19resonance frequencychange between < 20 MHz < 5 MHzfree space and on-bodyFig. 5. The S11 of the patch antenna. User affects the resonance frequencybut the change Q value (bandwidth) is small. Some of the observed frequencychange may be due to antenna bending on-body.Although the antennas are designed to be flexible especiallyregarding bending about the x axis, they are both sensitiveto strong bending. Wrapping the antennas about a styrofoamcylinder of diameter 94 mm caused the resonance frequencyto drop by 40 MHz. The prototypes described here may beused only on flat parts of the body, such as on the chest orback. If the antennas are placed on arms or other such placesto imply antenna curvature, it is necessary to redesign theelement length to attain the desired resonance frequency.B. On-Body Radiation PatternsThe on-body radiation patterns were measured in an ane-choic chamber. The antennas were attached to the abdomen.We only measured the radiation patterns in the yz plane.The inaccuracy associated with the on-body results is about±2 dB. The antennas were also measured in free space, using aSatimo StarLab system [3]. From the free-space measurementswe get the radiation efficiency, and from the change in Qvalue between free-space and on-body we can estimate thebodyworn efficiency.Table IV summarises the measured free-space and on-bodyresults. Example radiation patterns are presented in Figs. 7and 8, which show the horizontal plane when the antenna feedcable points downwards.Measurements show that both antennas perform well on-body. The radiation efficiency in free space (about 70 %)is quite large considering that the antennas were made on aplaster substrate. Even a non-uniform ground plane preventsFig. 6. The S11 of the quarter-wave antenna. The resonance frequency doesnot change significantly but the bandwidth grows.Fig. 7. Patch antenna radiation patterns at 2.5 GHz, in the yz plane, x-directionally polarised. Radial scale is in dBi.TABLE IVRADIATION PATTERN MEASUREMENT RESULTS.patch quarter-waveradiation efficiency in free space 70 % 72 %maximum gain in free space 6.7 dBi 4.2 dBimaximum gain when worn 6.2 dBi 1.4 dBiradiation efficiency on-body (estimate) 60 % 60 %bodyworn eff. (estimated from Q) 85 % 83 %the radiation efficiency from dropping too much on the body.The bodyworn efficiency can be calculated frombodyworn efficiency = ηon-bodyrad /ηfree spacerad = Qfree space/Qon-bodywhere the ηrad is the radiation efficiency and Q is the Qvalue [4]. The right side of the equation assumes that onlyone mode is present both in free space and on-body and thatonly the loss Q changes.IV. SIMULATED SPECIFIC ABSORPTION RATEThis section considers the specific absorption rate (SAR) at2.4 GHz from the two plaster antennas. SAR is the standardcriteria for measuring the amount of electromagnetic energy213Authorized licensed use limited to: LOUGHBOROUGH UNIVERSITY. Downloaded on September 24, 2009 at 07:28 from IEEE Xplore.  Restrictions apply. Fig. 8. Quarter-wave antenna radiation patterns at 2.5 GHz, in the yz plane,x-directionally polarised. Radial scale is in dBi.absorbed in the body and is calculated asSAR = σ|E|2ρwhere |E| is the rms magnitude of the electric field strengthvector, ρ is the mass density of the material in kg/m3 and σis the electrical conductivity in S/m. [5]Note that the maximum 1 g SAR, defined by internationalANSI/IEEE standards (used by the FCC in the USA) is1.6 W/kg [6]. The 10 g SAR is comparable to the EuropeanICNIRP safety standards of 2.0 W/kg [7].The SAR values in this paper were simulated in CST [8] byplacing the antenna next to a digital human body model. Themodel is from The Visible Human Project [9]. To reduce thememory constraints and improve the computation speed, thebody was truncated to leave a section of the torso and also byusing one homogenous tissue, see Fig. 9. Homogeneous phan-toms are often used for SAR studies as they are well knownto give conservative SAR values [5], [10]. SAR measurementsand simulations generally agree to within 10 % [5].The electrical properties of the body simulating tissue wereσ = 1.88 S/m, r = 37.97, and ρ = 1000 kg/m3. The VisibleHuman model was imaged with the subject lying on a flatmetallic bench and consequently the phantom has a flat back.The antenna designs in this paper are planar and therefore thecentre of the lower back was a convenient place to locate theantenna. In future work, we will bend the antenna so that itcan be added to the arm or leg. All results in this section havebeen calculated with 1 W input power.To make the SAR simulation easier, the simulated patchantenna model was slightly different from the measured one.The centre slit of the simulated model is 7 mm longer andthe metal strips adjacent to it are 1 mm narrower than inthe measured one. The effect of this difference on SAR isnegligible. The measured and simulated quarter-wave antennasare identical.The 1 g SAR of the patch antenna was 2.27 W/kg and the10 g SAR was 0.59 W/kg. Therefore, with a 1 W input powerthe antenna would be below the limits in Europe but not inthe USA. If this antenna were to be used in the USA, theinput power would have to be limited to 0.7 W. Fig. 10 showsthat the largest 1 g SAR volume is located near the feed ofthe antenna and this is much larger than other 1 g volumes.Fig. 11 shows that the 10 g SAR distribution is more uniform.The quarter-wave antenna has a smaller 1 g SAR of1.58 W/kg, see Fig. 12. This means that this antenna would notbreach the IEEE safety standards. However the 10 g SAR of1.21 W/kg, see Fig. 13, was twice as large as with the patchantenna. The 1 g and 10 g SAR plots of the quarter-waveantenna have a similar pattern to each other.Fig. 9. Patch antenna on the back of the torso model. Feed line pointsupwards.Fig. 10. The 1 g SAR of the patch antenna. Feed line points upwards.Fig. 11. The 10 g SAR of the patch antenna. Feed line points upwards.Fig. 12. The 1 g SAR of the quarter-wave antenna. Feed line pointsdownwards.214Authorized licensed use limited to: LOUGHBOROUGH UNIVERSITY. Downloaded on September 24, 2009 at 07:28 from IEEE Xplore.  Restrictions apply. Fig. 13. The 10 g SAR of the quarter-wave antenna. Feed line pointsdownwards.V. ANTENNAS IN USEFinally, the antennas were tested in real operating condi-tions. To make the RF connection cheaper, we investigated theusability of snap-on buttons. The antennas were also subjectedto a harsh environment, namely a sweating user.A. Snap-On Buttons in FeedRF connectors are expensive and thus not suitable for dis-posable single-use antennas. Instead, we experimented usingsnap-on buttons, a technique proposed in [11]. In [11], theantennas were fed using a pin through the ground plane. Theexact feeding position was found difficult to control.The antennas proposed in this paper employ an inset mi-crostrip feed line. The snap-on buttons are connected to themicrostrip instead of the patch. This gives us more control ofthe feed impedance, and additionally it is easier to connect thecoaxial cable to the edge of the antenna rather than throughit. Problems arising from our approach include a very longtransition from the coaxial cable to the microstrip, which addsinductance in the feed, and the fact that the coaxial cable maymove in use.We measured the time domain response of a button-fed 50-ohm microstrip line. The reflection from the button was betterthan –14 dB up to 3 GHz. This indicates that snap-on buttonsare suitable for use in cheap commercial devices.B. Sweating EffectsThe product sheet of the plaster material describes theplaster as nonabsorbent, but states that air and water vapourwould pass through [2]. Thus it was expected that sweatingwould not alter the antenna parameters significantly.We attached the antennas to the back of a male who thenjogged for 45 minutes, sweating heavily. No cables wereattached to the antennas during the exercise. The radiationpatterns and input impedances were measured afterwards. Theantennas were not wet to touch after they had been shaken dry.The glue had however partially failed. The user’s shirt was dryat the places where the antennas had been.The radiation patterns were measured right after sweating,and the input impedances about one hour after it.Sweating reduced the antenna efficiencies by between 2 and8 %. The radiation patterns were unaffected. The measuredresonance frequencies were not changed by sweating. Theinput impedance decreased by about 8 ohms (patch) or 6 ohms(quarter-wave). The Q value of the quarter-wave antennareduced from 28 to 24, whereas for the patch antenna thechange was smaller than measurement uncertainty.VI. CONCLUSIONAntennas made on a plaster substrate have been described.The antennas employ a ground plane between the radiatingelement and the user to reduce losses. Slits cut in both the el-ements and the ground planes add flexibility and breathability.The bandwidth of the plaster antennas covers the 2.4 GHzISM band. The input impedance is not significantly affectedby the body presence. The gain of the half-wave patch antennais 6.2 dBi and the quarter-wave patch 1.4 dBi, both measuredwith the antenna attached to the abdomen. Estimated body-worn efficiencies (radiation efficiency on-body compared tothe free-space value) of the antennas are 85 %.The SAR values of the antennas were quite high. Particu-larly the patch antenna would break the U.S. SAR limits ifmore than 0.7 W is fed into the antenna. The SAR could bereduced by adding more metal to the antennas, especially inthe areas of high current density near the feed point.Of the two antennas described here, the larger one (patch)was seen to be better, especially in terms of the on-body gain.However, if smaller antennas are desired, the quarter-waveantenna has proven useful as well.ACKNOWLEDGMENTTiiti Kelloma¨ki wishes to thank Timo Kelloma¨ki for excel-lent performance as a sweating anthropomorphic mannequin.Tiiti Kelloma¨ki’s research was funded by The Graduate Schoolin Electronics, Telecommunications and Automation (GETA).William Whittow thanks Shahid Bashir for his CST expertise.REFERENCES[1] T. Vuorela, J. Ha¨nnika¨inen, and J. Vanhala, ”Plaster like PhysiologicalSignal Recorder – Design Process, Lessons Learned,” in Proceedingsof the Ambience 08 Smart Textiles – Technology and Design, p. 89–96.Bora˚s, Sweden, June 2–3, 2008.[2] Mefix. [Online]. Available: http://www.molnlycke.com/item.asp?id=924[3] SATIMO [Online]. Available: http://www.satimo.com/[4] W. G. Scanlon and N. E. Evans, ”Numerical analysis of bodyworn UHFantenna systems,” Electronics & Communication Engineering Journal,vol. 13, no. 2, p. 53–64, Apr 2001.[5] W. G. Whittow, C. J. Panagamuwa, R. M. Edwards, and J. C. Var-daxoglou, ”On the effects of straight metallic jewellery on the specificabsorption rates resulting from face illuminating radio communicationsdevices at popular cellular frequencies,” Physics in Medicine and Biol-ogy, vol. 53, p. 1167–1182, 2008.[6] ANSI/IEEE 1992 IEEE standard for safety levels with respect tohuman exposure to radio frequency fields 3 kHz to 300 GHz. StandardC95.11992.[7] ICNIRP 1998 Guidelines for limiting exposure to time-varying electric,magnetic and electromagnetic fields (up to 300 GHz), Health Phys., 74,494–522.[8] CST [Online.] Available: http://www.cst.com/[9] The National Library of Medicine’s Visible Human Project [Online].Available: http://www.nlm.nih.gov/research/visible/visible human.html[10] O. P. Gandhi and Jin-Yuan Chen, ”Electromagnetic absorption in thehuman head from experimental 6-GHz handheld transceivers,” IEEETrans. Electromagn. Compat., vol. 37, no. 4, p. 547–558, Nov 1995.[11] I. Belov, M. Chedid, and P. Leisner, ”Investigation of snap-on feedingarrangements for a wearable UHF textile patch antenna,” in Proceedingsof the Ambience 08 Smart Textiles – Technology and Design. Bora˚s,Sweden, June 2–3, 2008.215Authorized licensed use limited to: LOUGHBOROUGH UNIVERSITY. Downloaded on September 24, 2009 at 07:28 from IEEE Xplore.  Restrictions apply. 

2.4 GHz plaster antennas for health monitoring

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2.4 GHz plaster antennas for health monitoring

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