We demonstrate that diode-switching can be used to introduce frequency agility into antennas produced by thermal transfer printing. Our particular example is a triangular Sierpinski fractal pattern with two PIN diodes to switch between operation optimised for the 800 MHz UHF band (diodes on) and the 2400 MHz ISM band (diodes off). Our measured results show an improvement in S11 in the UHF band from -2 dB to -28 dB, and from -7 dB to -30 dB at 2400 MHz, when switching the diodes appropriately. The measured bandwidth is 200 (1000) MHz, and the measured directivity is 3.1dB (5.2dB) while the measured gain is -5.2dB (6.7dB) for the diodes on(off)

Christodoulou C.

Kirsch N.

M. Kgwadi

Mantysalo M.

T.D. Drysdale

Tan L.

Whyte G.

English

Crossref

Electronics Letters

Diode‐switched thermal‐transfer printed antenna on flexible substrate

Kgwadi, M.

Drysdale, T. D.

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDiode-switched thermal-transfer printed antenna onflexible substrateJournal ItemHow to cite:Kgwadi, M. and Drysdale, T. D. (2016). Diode-switched thermal-transfer printed antenna on flexible substrate.Electronics Letters, 52(4) pp. 258–260.For guidance on citations see FAQs.c© 2016 Institution of Engineering and TechnologyVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://dx.doi.org/doi:10.1049/el.2015.3060http://dx.doi.org/10.1049/el.2015.3060Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDiode-switched thermal-transfer printedantenna on flexible substrateM. Kgwadi and T. D. DrysdaleWe demonstrate that diode-switching can be used to introduce frequencyagility into antennas produced by thermal transfer printing. Our particularexample is a triangular Sierpinski fractal pattern with two PIN diodes toswitch between operation optimised for the 800 MHz UHF band (diodeson) and the 2400 MHz ISM band (diodes off). Our measured results showan improvement in S11 in the UHF band from -2 dB to -28 dB, and from-7 dB to -30 dB at 2400 MHz, when switching the diodes appropriately.The measured bandwidth is 200 (1000) MHz, and the measured directivityis 3.1 dB (5.2 dB) while the measured gain is -5.2 dB (6.7 dB) for thediodes on(off).Introduction: Rapid and inexpensive antenna manufacturing is requiredto support the burgeoning interest in the Internet of Things (IoT). IoTis heavily reliant upon wireless communications between, ultimately,trillions of devices [1]. For any IoT antenna, it is important to be ableto handle a dynamic frequency allocation, at least in part determined byregulatory authorities according to location [2] but also influenced bythe local propagation environment favouring some channels over othersdue to fading mechanisms associated with building geometry e.g. [3] andother radio noise sources. Hence, reconfigurable antennas are attractiveboth for prototyping IoT communications networks in new locations,but also during deployment for responding to changes in the optimalchannel choice. Reconfigurable antennas often require less non-returnableengineering effort compared to designing multi-band antennas, and cango further than frequency agility to include polarisation diversity [4] orbeamforming capability [5]. Switching is known to introduce frequency[6] and polarisation [7] agility into planar antennas produced usingconventional printed circuit board techniques on a variety of dielectricsubstrates. However, these manufacturing techniques are relatively slowcompared to thermal transfer printing of metallic films, which prints ata rate of approximately 5cm/s and requires no curing to reach minimumsheet resistance [8]. Flexible substrates give IoT system designers moreoptions for integrating nodes unobtrusively into built environments [9].Antenna design and implementation: An IoT antenna needs to be ableto switch between two or more different bands depending on spectrumallocation from regulatory authorities, spectrum availability due to otherusers, or deleterious effects in the local propagation environment. Asequentially reconfigurable frequency agile Sierpinski antenna using RFmicroelectro-mechanical systems (RF MEMS) switches was achieved in[10]. The RF MEMS in [10] require DC bias voltage of up to 17 Vto achieve switching, which may not be cost effective in low powersensor nodes. Therefore PIN diodes are an attractive alternative forour approach because they can be switched with voltages lower than1 V DC (nominal value 0.95 V) [11]. We propose a Sierpinski gasket-based tunable antenna using NXP semiconductor’s general purpose PINdiodes ( BAP50-03) as switches, so as to allow a sensor node to switchbetween ISM band (2400 MHz) communications and the television whitespace (TVWS) bands centered at (800 MHz) by using a flexible, low-costfrequency agile antenna. Thus an associated sensor node could switchbetween the two frequency bands depending on availability and/or networktraffic conditions. Figure 1(a) shows a schematic of our design, with aphotograph of the final printed version shown in Figure 1(b). The antennawas printed using thermal-transfer printing by a Zebra S4 300 dpi, ona Flexcon THERMfilm R© SELECT 21944E which is a polyester withrelative dielectric constant (εr) of 3.2 [8]. The metalization was achievedby IIMAK’s MetallographTM conductive thermal transfer ribbon whichyields a 260 nm aluminium layer. The antenna is switched between the‘ON’ and ‘OFF’ state by switch S, via a voltage supply fed through thevector network analyser DC-bias port. On a microcontroller-based sensornode, the same can be achieved by programming one of the output pins.For a 3.3 V output pin voltage, a 47 Ω resistor in series with the PIN diodewill be required obtain the required forward bias current of 50 mA. In the‘ON’ state the diodes are forward biased and the whole of the antennais in operation, resonating at 800 MHz. In the ‘OFF’ state the diodes areat zero bias and the antenna elements beyond the cathodes of the diodesare effectively disconnected, yielding a shorter antenna that resonates at(a) (b)Fig. 1 Schematic (a) of the reconfigurable Seirpenski antenna (dimensions:CPW center strip 3.2 mm, CPW gap 0.35 mm, CPW ground width 18 mm )and picture (b) of the fabricated antenna.(a)(b)Fig. 2 Plots of S11 parameter for the (a) simulated and (b) measured antennas,for the diodes ON and OFF. With the diodes OFF, operation is at 2.4 GHz andON at 800 MHz (larger effective structure size).2400 MHz. In practice, some residual loading remains because the diode isnot a perfect open circuit. Two surface-mount inductors are used to isolatethe RF signal from the ground.Simulation and measurements: The antenna was simulated inopenEMS which is an open source, finite-difference time-domainbased electromagnetic field solver [12]. The metallisation was modelledas a perfect electrical conductor of infinitesimal thickness to givereasonable simulation times. This does not affect the radiation patternor operating frequency but prevents us from predicting the efficiency.Lumped components were used to model the PIN diodes and inductors inthe simulations. The reverse bias capacitance of the diodes was modelledas a 0.4 pF lumped capacitor in the ‘OFF’ state, while the forward biasresistance was modelled as a 3 Ω lumped resistor in the ‘ON’ state. Theinductor was modelled as lumped 4.7µH inductance in series with a 3 Ωresistance. Figure 2(a) shows the simulated reflection coefficient in thetwo cases when the diodes are ‘ON’ and ‘OFF’. Figure 2(b) shows themeasured S11, with the desired resonance in the ISM band (WLAN,2400-2500 MHz) with an S11 better than -30 dB through the entire bandand no resonance at 800 MHz in the ‘OFF’ state. In the ‘ON’ state, theresonance in UHF band is observed at 730 MHz with an S11 of -28 dBand operational bandwidth of 200 MHz (S11 ≤-10 dB). The 150 MHzshift in the resonance frequency between simulation and measurementsis attributed to discretization of the diagonal lines into a staircaseapproximation and the presence of a 56 µm glassine liner which preservesthe acrylic adhesive and thus makes the structure electrically largerthan simulated (hence the resonances shifts to the lower frequencies).ELECTRONICS LETTERS 24th August 2015 Vol. 00 No. 00(a) (b)(c) (d)Fig. 3 Simulated normalized current densities, (a) at 800 MHz in the ‘OFF’state,(b) 2400 MHz in the ‘OFF’ state,(c) 800 MHz in the ‘ON’ state , and (d)2400 MHz in the ‘ON’ state.The glassine liner, whose dielectric permittivity is unknown to us, wasomitted in the simulations since it is expected to be peeled off duringantenna deployment. Our measured results are sufficient to demonstratethe principle, although we would expect some fine tuning to the designwould be needed to take into account the precise conditions under whichit was manufactured in volume.Figure 3 shows the normalised surface current distribution in both ‘ON’and ‘OFF’ cases at the two design frequencies. In the ‘OFF’ state thecurrent density at 800 MHz is limited to the lower part of the antenna asshown in Figure 3(a). The current distribution at 2400 MHz shown Figure3(b) shows a higher intensity in the active part of the antenna compared to800 MHz in Figure 3(a) signifying resonance. Some currents also appearon the inactive part of the antenna in the ‘OFF’ state, due to fringingcapacitance. In the ‘ON’ state, the 800 MHz current is distributed throughout the whole antenna as shown in Figure 3(c) and shows higher intensitythan in the ‘OFF’ state. This shows the antenna resonating at 800 MHz inthe ‘ON’ state. Figure 3(d) shows the current distribution at 2400 MHz.The radiation pattern was measured using an NSI 2000 nearfield spherical scanner in a 4.5x4.5x4.0 m anaechoic chamber. Thenormalized simulated far-field radiation patterns for 800 MHz (‘ON’) and2400 MHz (‘OFF’) are shown in Figure 4(a) and Figure 4(b) respectively.The corresponding normalized measured far-field radiation patterns at800 MHz (‘ON’) and 2400 MHz (‘OFF’) are shown in Figure 4(c) andFigure 4(d) respectively. The measured radiation patterns in the ‘ON’ stateare omnidirectional with a measured gain of -5.3 dBi and a directivity of3.1 dB at 800 MHz. The low measured gain is attributed to the conductorlosses due to thin metallization (0.07 of the skin depth) typical of printedantennas [13] and losses in the resistances of the forward biased PIN diodesand RF blocking inductors. The radiation pattern is however consistentwith the simulated radiation pattern in Figure 4(a). The measured radiationpattern at 2400 MHz (‘OFF’) shows omnidirectional radiation pattern asshown in Figure 4(d), with a measured gain of 6.7 dB and directivity of5.2 dB. There is an unexpected null in the measured pattern in the E-Planeat 150o which we attribute to the presence of bias line of length 2 cm at2400 MHz (0.16λ) although this is of negligible consequence at 800 MHz(0.05λ).Conclusion: We present a flexible, low-cost, frequency-agile triangularSierpesnki gasket-based antenna fabricated by thermal transfer printing foruse by wireless sensor nodes in IoT applications. The antenna employstwo general purpose PIN diodes, two inductors and a single bias lineto switch between two modes of operation. By switching the diodes onor off, the desired frequency agility between the UHF band (800 MHz)and the WLAN band (2400-2500 MHz) is achieved with bandwidthsof 200 MHz and 1000 MHz respectively. The radiation pattern isomnidirectional with measured directivity of 3.1 dB (800 MHz/‘ON’) and5.2 dB (2400 MHz/‘OFF’) and measured gains of -5.2 dB (800 MHz/‘ON’)and 6.7 dB (2400 MHz/‘OFF’) suitable for use in wireless sensor nodes.We thus show that the benefit of thermal transfer printing can be combinedwith switching elements to make low-cost, flexible, reconfigurableantennas.(a) (b)(c) (d)Fig. 4 Simulated and measured radiation patterns. (a) simulated radiationpattern at 800 MHz in the ‘ON’ state and (b) simulated radiation patterns at2400 MHz in the ‘OFF’ state; (c) corresponding measured radiation pattern inthe ‘ON’ state at 800 MHz and (d) measured radiation pattern in the ‘OFF’state at 2400 MHz. The arrow marks a null caused by the bias line.Acknowledgement: We would like to thank IIMAK for donating theMetallograph conductive ribbon.M. Kgwadi (Division of Electronics and Nanoscale Engineering,University of Glasgow, Glasgow, G12 8LT,UK. )T. D. Drysdale (Department of Engineering and Innovation, The OpenUniversity, Milton Keynes, MK7 6AA, UK.)E-mail: timothy.d.drysdale@ieee.orgReferences1 Tan, L. and Wang, N.: ‘Future Internet: The Internet of Things’, Adv. Comp.Theory and Eng. (ICACTE), Aug. 2010 3rd Int. Conf., 2010, 5, pp. 476-380.2 Ofcom: ‘TV White Spaces: Approach to coexistance’,http://stakeholders.ofcom.org.uk/binaries/consultations/white-space-coexistence/summary/white-spaces.pdf, accessed 28 May 2014.3 Austin, A., Neve, M., Rowe, G., and Pirkl, R.: ‘Modelling the effects ofnearby buildings on inter-floor radio-wave propagation’, Anten. and Prop.,IEEE Trans., 2009, 57,(7), pp. 2155-2161.4 Yang, X., Shao, B., Yang, F., Elsherbeni, A. and Gong, B.: ‘A PolarizationReconfigurable Patch Antenna With Loop Slots on the Ground Plane’,Anten. and Wireless Prop. Letters, IEEE, 2012, 11, pp. 69-72.5 Kang, W., Lee, S., and Kim, K.: ‘Design of Symmetry Beam PatternReconfigurable Antenna’, Electronic Letters, 2010, 46, pp. 1536-1537.6 Christodoulou, C., Fieldner, L., Zachou, V., and Anagnostou, D.: ‘PlanarReconfigurable Antennas’, Anten. and Prop., EuCAP 2006, First Europ.Conf. on,, Nov. 2006, pp. 1-7.7 Cao, Y., Cheung, S., and Yuk, T.: ‘A Simple Planar PolarizationReconfigurable Monopole Antenna for GNSS/PCS’, Anten. and Prop.,IEEE Trans., 2015,63, (2), pp. 500-507.8 Whyte, G., Harrison, D., Cumming, D., and Drysdale, T.: ‘Direct Printingof Flexible Metallic Millimetre-wave Frequency Selective Surfaces’, Anten.and Prop. Soc. Int. Symp. (APSURSI), July 2010, pp. 1-4.9 Kirsch, N., Vacirca, N., Kurzweg, T., Fontecchio, A., and Dandekar, K.:‘Performance of Transparent Conductive Polymer Antennas in a MIMO ad-hoc Network’, Wireless and Mobile Computing, Networking and Comms.(WiMob), 2010 IEEE 6th Int. Conf.,Oct. 2010, pp. 9-14.10 Kingsley, N., Anagnostou, M., Tentzeris, M., and Papapolymerou,J.: ‘RF MEMS Sequentially Reconfigurable Sierpinski Antenna ona Flexible Organic Substrate With Novel DC-Biasing Technique’,Microelectromechanical Sys., Jrnl of, 2007,16, (5), pp. 1185-1192.11 NXP Semiconductors: ‘ BAP50-03 General Purpose PIN Diode -Product Data Sheet’, http://www.nxp.com/documents/data_sheet/BAP50-03.pdf. accessed July 2015.12 Liebig, N.: ‘OpenEMS - Open Electromagnetic Field Solver’ ,http://openEMS.de, accessed June 2015.13 Mantysalo, M., and Mansikkamaki, P.: ‘An Inkjet-Deposited Antennafor 2.4GHz Applications’, AEU- Int Jrnl of Electronics and Comms.,2009,63,(1), pp. 31-35.2

Diode-switched thermal-transfer printed antenna on flexible substrate

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Diode-switched thermal-transfer printed antenna on flexible substrate

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