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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/LAWP.2014.2363852, IEEE Antennas and Wireless Propagation Letters

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. XX, NO. X, DECEMBER 2014 1

Paired Snap-On Buttons Connections forBalanced Antennas in Wearable Systems

Shengjian Jammy Chen,Student Member, IEEE, Christophe Fumeaux,Senior Member, IEEE,Damith Chinthana Ranasinghe,Member, IEEE, and Thomas Kaufmann,Member, IEEE

Abstract—A pair of commercial snap-on buttons is demon-strated as a detachable radio-frequency (RF) balanced connectionbetween a garment-integrated textile dipole antenna and a passivesensor-enabled radio-frequency identification (RFID) tag in awearable wireless system. This arrangement offers reliable, low-cost and easily detachable RF coupling and feeding connectionsfor balanced antennas, as conceptualized in simulations andvalidated through measurements. In addition, a back-to-backbalanced transmission line structure has been designed andmeasured to characterize the RF performance of the proposedsnap-on button connection. The resulting S-parameters indicatethe good performance of the snap-on buttons as RF connectorsfor balanced antennas/transmission lines at least up to 5 GHzwith insertion loss better than 0.8 dB.

Index Terms—Antenna feeding, textile antennas, balancedtransmission line, RFID, wearable electronic system

I. I NTRODUCTION

T HE recent emergence of wearable electronic systemsfor mobile communications, wireless healthcare monitor-

ing/diagnosing and military applications has lead to increasingdemands in body-centric wireless communications compo-nents [1]. Healthcare is one of the main fields of applicationwhere wearable electronic systems are in demand. Sincewearable antennas and sensors are essential to implementwearable applications for healthcare, a significant growthinthe relevant research areas has been observed in the last fewyears [2], [3]. Considering the wearing comfort and ergonomicrequirements for these devices, wearable antennas are re-quired to be flexible, low-cost, lightweight and even washable.To fulfil these characteristics, metallic foils [4], conductiveinks [5], conductive polymers [6], conductive threads [7] andconductive fabrics [8], [9] are the most promising choices asflexible conductor materials. Generally antennas made fromthe latter two materials are garment-integratable and washablethus they offer convenience and re-usability for wearableapplications. The performance of washable antennas such asgain, reflection coefficient and radiation efficiency have beeninvestigated in [10], [11], where reliable and acceptable an-tenna performances have been achieved after several washingcycles. However, the connected electronics are not washablein most scenarios, and thus low-cost and easy-to-use solutions

Manuscript received June xx, 20xx; revised September xx, 20xx.S.J. Chen, C. Fumeaux and T. Kaufmann are with the School of Electrical

and Electronic Engineering, The University of Adelaide, Adelaide 5005, SouthAustralia, Australia

D.C. Ranasinghe is with The Auto-ID Lab, The School of Computer Sci-ence, The University of Adelaide, Adelaide 5005, South Australia, Australia

The authors acknowledge the support of the Australian Research Council(ARC) under Discovery Projects DP120100661 and DP130104614.

Detachable

Tag RFID

reader

Back-end

systems

Tag antenna Reader antenna

Reader interrogating signal

Tag reply (ID & sensor data)

Read rangeSnap-on

buttons

Fig. 1. RFID system configuration with battery-less tag.

of detachable connections for garment-integrated antennas area significant requirement.

A possible solution is utilizing snap-on buttons as detach-able RF connectors. Some dedicated snap connectors designedfor wearable RF applications have been reported in [12].In contrast to specialized solutions, a cheaper and moreconvenient RF connector solution for a 2.4 GHz probe-fedtextile patch antenna has been proposed in [13], by takingadvantage of commercial snap-on buttons commonly used inclothing. Despite affecting the antenna resonance frequencyand return loss, these buttons still exhibit good performance asRF connectors. Further studies investigating the characteristicsof these buttons used as RF connectors in coaxial-to-microstriptransitions [14], [15] and for several plaster microstrip anten-nas [16] have been also reported. The results have demon-strated that the snap-on button is an appropriate connectorfor applications up to 3 GHz. Additionally, a successfulsnap-on-button-based transition between a 2.4 GHz planarinverted-F antenna and an electronic circuit board has alsobeen demonstrated [17]. However, all these research workswere performed in conjunction with unbalanced antennas andtransmission lines.

In this letter, the use of a pair of commercial snap-onbuttons as detachable RF connectors between a balancedwideband textile dipole antenna and a passive sensor-enabledRFID tag in a wearable elderly activity monitoring systemis presented. Antenna measurements and investigations of thesystem indicate excellent performance of the RF connectorsfor balanced antennas with operating range from 780 MHzto 1030 MHz. The proposed snap-on button pair connectionoffers not only a solid mechanical connection and detacha-bility but also shows a minimal impact on RF performance.Moreover, an investigation of the characteristics of the snap-on button coupling arrangement for balanced transmission linetransitions demonstrates a stable applicability at least up to5 GHz.

II. T HE RFID SYSTEM

The considered application is a wearable RFID systemwith an operating frequency of 923 MHz, aiming for elderly




Fig. 2. A tag with an original rigid dipole antenna.

Fig. 3. Layout of the textile antenna. The design dimensionsare a = 70mm, b = 40 mm andg = 3.5 mm.

monitoring [18], [19]. It consists of at least one battery-free tag with an antenna, a reader with several antennas andback-end systems. A configuration of the system is shownin Fig. 1. The RFID tag is a passive accelerometer-enableddevice with an input impedance of 78 + 16j Ω, containing amicro-controller unit where a unique identification numberisstored. It communicates with a reader through an antenna,where the signal received from the reader is also used asan activating power source. The back-end computer systemscollect, analyse and store the data received from the readerandact as a human interface as well. The proposed paired snap-onbuttons connection provides a solution to detach the tag fromits antenna which can remain integrated with a garment duringwashing.

III. T HE TAG AND ANTENNA

The tag originally comes with a dipole antenna printed on aFR4 substrate [19], as shown in Fig. 2. It is noticeable that theoriginal antenna is not appropriate for wearable applicationsas it is narrowband and not flexible. Thus an alternativewearable wideband antenna has been designed to improvethe wearability and robustness to changing electromagneticenvironments. The new antenna is a flexible wideband textiledipole antenna with an impedance bandwidth for matching to78Ω (|S11| < −10 dB) ranging from 780 MHz to 1030 MHz.To make the antenna flexible and lightweight, a silver-coatednylon RIPSTOP fabric with a sheet resistance of 0.01Ω/ anda thickness of 100µm has been selected as the conductor.

As shown in Fig. 3, the antenna is a planar elliptical dipolewhich has a major axisa = 75 mm and a minor axisb= 40 mm. The elliptical antenna elements offer widebandperformances [20] and the ratio of the major and minor axesa/b determines the taper rate of the slots originating from thefeed (between the ellipses) and consequently the impedancebandwidth. The critical parameter to control the antenna inputimpedance is the feed gap sizeg between the two ellipticalelements. An optimal gap ofg = 3.5 mm is chosen to set theantenna input impedance to be 78Ω to closely match the tag,as obtained through iterative optimizations in CST MicrowaveStudio 2014 (CST). In order to have easy detachability withoutsacrificing the RF performance, commercial snap-on buttons

Fig. 4. Dimensions of the used snap-on button in (mm) and its model insimulation.

are employed as the RF connector for this replacement flexibleantenna.

IV. PAIRED SNAP-ON BUTTONS BALANCED CONNECTION

The snap-on buttons chosen in the present case are of sewn-on type (Fig. 4). As RF connectors, the selected buttons offerexcellent mechanical and electrical performance as verified inthe following through simulations and experiments.

A. Mechanical Performance

The button dimensions and its 3D model built in CST areshown in Fig. 4. The base diameters of 5.5 mm (male) and6.2 mm (female) give enough real estate for soldering andsewing while the engaged button height of less than 2.7 mmresults in a low-profile connection with negligible obstructionto movement. There is a spring embedded in the femalebutton to clip and hold the male button when engaged. Thisspring mechanism forms solid and reproducible mechanicaland electrical connections, as presented as a pair of metallicthin rods in the CST model. A mechanical test involving aspring scale and a fixture to experimentally test the requiredforce to pull a pair of engaged buttons apart indicates that aforce of approximately 3 N is needed. This confirms that themechanical connection is very solid.

To attach to fabrics and textiles, there are four holes on thebase of both male and female buttons for sewing propose. Asshown in Fig. 5a) and b), two female buttons are sewed onthe textile dipole using conductive threads, while two malebuttons are soldered on the tag printed circuit board, onemale button being cut to fit the electronics. Consequently theantenna can be easily detached from and reattached to thetag. The mechanical compatibility of these buttons with textilematerials and most electronics is clear since they can be sewedor soldered on them respectively.

B. Electrical Performance

The proposed snap-on buttons must have satisfactory elec-trical performance to be good RF connectors. In order toinvestigate this aspect, two identical textile ellipticalantennashave been fabricated and experimentally characterized withdifferent connection methods. The first one employs the bal-anced paired snap-on buttons feeding arrangement as shownin Fig. 5 b) while the second one adopts a permanent directcontact method realized with conductive epoxy. For dedicatedantenna measurements (i.e. without the tag), a wideband(0.7 GHz - 6 GHz) balun has been adapted from [21], [22]




Fig. 5. The snap-on button feeding arrangement. a) A tag withsoldered malesnap-on buttons, b) Proposed antenna with sewed female snap-on buttons, c)A 50 Ω to 78Ω impedance balun with soldered male snap-on buttons, d) Atag with proposed antenna worn on human body.

Fig. 6. Simulated and measured reflection coefficient|S11| of the proposedantenna. The simulated|S11| parameters for two cases are included: i) theantenna fed with the 50 to 78Ω balun (with balun); and ii) directly with a78 Ω port (w/o balun).

to provide a transition from a 50Ω microstrip line to 78Ωbalanced coplanar strips. As shown in Fig. 5 c), the snap-onbuttons have been soldered on the impedance transition to feedthe antenna.

1) S-parameters: The simulated and measured reflectioncoefficient of the two antennas with different realizationsofthe connection are illustrated in Fig. 6. Simulations with andwithout the 50 to 78Ω balun are both shown. The slightlylower reflection coefficient observed in the operation bandwhen using the balun is a consequence of small reflectionsand dielectric losses introduced by the balun. The resultsfrom simulation indicate that|S11| is only slightly affectedby the connection with the snap-on button connectors com-pared to the results with direct connection. In particular,forthe simulations without the balun, the resonance frequencyincreases from 876 MHz to 878 MHz while the impedancebandwidth shifts from 782 - 1038 MHz to 783 - 1042 MHz.The experimental results obtained with the balun are in good

|S11| (

dB

)

Frequency (GHz)

-17

-15

-13

-11

-9

0.75 0.8 0.85 0.9 0.95 1 1.05 1.1

Gap - 150 μmGap - 350 μm

Gap - 10 μm

Gap - 550 μm

Gap

Fig. 7. Simulated reflection coefficient|S11| of the proposed antenna withdifferent gaps between female and male snap-on buttons.

agreement and confirm the simulated findings: the measuredresonance frequency only has a 9 MHz increment from 867MHz and the impedance bandwidth moves slightly towardsthe higher frequency end, namely from 773 - 1007 MHz to805 - 1012 MHz. Hence, with snap-on buttons, the antennastill maintains its bandwidth and matching to the system.

2) RFID System Performance: A tag attached to a garment-integrated antenna using snap-on buttons and worn by a humansubject is shown in Fig. 5 d). In this configuration, one of themost important parameters of the RFID system is the readrange, i.e. the maximum distance from the reader antenna atwhich the RFID tag is activated (i.e. successfully powered)and communication is successful. In the present case, as theoperational frequency band of the wearable RFID system isgenerously covered by the antenna impedance bandwidth forboth types of connections (direct and snap-on), similar perfor-mances are expected. This is validated in the experiments witha female human subject, where identical read ranges, namely3.0 m, have been obtained for the tag with direct and snap-onconnections to the antenna. It is noted that this read range canbe further enlarged with appropriate techniques to isolatethedipole radiation from the human body.

3) Effect of the Gap between Engaged Female and MaleButtons: The presented simulation results have been obtainedfor the ideal snap-on button connection configuration, whichconsiders the maximum and flattest contact surface for thecomponents connected. However, the thickness of the mate-rials touching both female and male buttons can influencethe gap and consequently the capacitance between male andfemale connector parts. To investigate how this can affect RFperformance, a simulation-based study has been conducted.Asdemonstrated in Fig. 7, the simulated|S11| is not significantlyaffected when increasing the gap from 10µm to 550 µmwhich is the maximum possible gap obtained via empiricalobservations of the buttons. This confirms the robustness ofthe snap-on button connection in the proposed design.

V. I NSERTIONLOSS

To characterize the RF performance of the paired snap-onbuttons connections in balanced structures up to 5 GHz, apair of connected back-to-back 125Ω coplanar strips wereused for testing. For reference, a through coplanar strip linewith the same length was used. As shown in Fig. 8, boththe test and reference structures consist of two baluns and apair of transmission lines of same length made from silverfabric. The impact on the RF performance from the button




c)

b)

a)

l

d w

Fig. 8. The realized back-to-back balanced transmission line structure. a)Dimensions of the coplanar strips,l = 100 mm,d = 0.55 mm,w = 6 mm,b) Textile through coplanar strips transmission line, c) Transmission line withsnap-on buttons connection.

|S21| (

dB

)

Frequency (GHz)

-3

-2

-1

0

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

-5

0

-4

W/O buttons

With buttons

Fig. 9. Measured transmission coefficient of the reference and test back-to-back balanced coplanar strips structures.

connections can be extracted through S-parameters compari-son, with time gating used to isolate the impact on the snap-onconnections from the parasitic effects of the baluns. Basedonthese measurements, the resulting transmission coefficients areshown in Fig 9. The test structure holds a slightly lower trans-mission coefficient than the reference, which demonstratesagood RF performance of the proposed paired snap-on buttonconnection, with insertion losses from 0.29 dB to 0.76 dB upto 5 GHz.

VI. CONCLUSION

A practical and affordable solution using a pair of commer-cial snap-on buttons as detachable RF connectors has beenpresented for balanced textile structures. This solution hasbeen applied to connect a wideband elliptical dipole antennain a 923 MHz wearable RFID system. Two identical antennashave been fabricated using silver fabric, and experimentallycharacterized to compare direct contact connection to the pro-posed balanced snap-on connection. The simulated and mea-sured reflection coefficients indicate that only small variationsand an insignificant performance degradation are introducedwhen using the paired snap-on buttons as RF connectors. Thisis confirmed by a system test where identical read ranges havebeen obtained when employing these two types of connections.Further results from a simulation-based study indicate that thegap between engaged female and male buttons has negligibleimpact on the RF performance. Moreover, measurements ona back-to-back balanced transmission line structure suggestthat the button connectors are usable for similar applicationsat least up to 5 GHz. The utilization of low-cost commercialsnap-on buttons offers a practical solution for detachableRF

connectors without sacrificing the RF performance in balancedconfigurations.

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