A directly modulating antenna incorporating a reconfigurable Frequency Selective Surface (FSS) for arbitrary phase modulation is designed and simulated. A 4-layer FSS is used to minimise constellation distortion, with only 1.5dB variation in transmitted magnitude for 360° phase change. Low loss substrates and Barium Strontium Titante (BST) variable capacitors are simulated to reduce the antenna loss to 1.3dB in the filter pass band and achieve an average total efficiency of 65% when producing an 8-PSK constellation

Ford, K.L.

Henthorn, S.D.

O'Farrell, T.

S. Henthorn

K.L. Ford

T. O'Farrell

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A low-loss reconfigurable Frequency Selective Surface based antenna for Direct Antenna Modulation

White Rose Research Online

This is a repository copy of A low-loss reconfigurable frequency selective surface based antenna for direct antenna modulation.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/131947/Version: Accepted VersionProceedings Paper:Henthorn, S.D., Ford, K.L. orcid.org/0000-0002-1080-6193 and O'Farrell, T. (2018) A low-loss reconfigurable frequency selective surface based antenna for direct antenna modulation. In: Antennas & Propagation Conference (LAPC 2017), Loughborough. Antennas & Propagation Conference (LAPC 2017), Loughborough, 13-14 Nov 2017, Loughborough, UK. IET . ISBN 978-1-78561-700-3 https://doi.org/10.1049/cp.2017.0233eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. A low-loss reconfigurable Frequency Selective Surface basedantenna for Direct Antenna ModulationStephen Henthorn, Kenneth Lee Ford, Timothy O’FarrellDept. of Electronic and Electrical Engineering, University of Sheffield, Sheffield, United Kingdom, sdhenthorn1@sheffield.ac.ukKeywords: Frequency selective surfaces, Direct antennamodulation, Phase shift keying, reconfigurable antennasAbstractA directly modulating antenna incorporating a reconfigurableFrequency Selective Surface (FSS) for arbitrary phase mod-ulation is designed and simulated. A 4-layer FSS is used tominimise constellation distortion, with only 1.5dB variation intransmitted magnitude for 360o phase change. Low loss sub-strates and Barium Strontium Titante (BST) variable capacitorsare simulated to reduce the antenna loss to 1.3dB in the filterpass band and achieve an average total efficiency of 65% whenproducing an 8-PSK constellation.1 IntroductionModern communications systems make use of complex digitalmodulation schemes, producing waveforms which are broad-band and often have significant peak to average power ratios(PAPRs). In conventional transmitters, this places a require-ment on the power amplifier (PA) to be linear across the inputpower and frequency ranges in order to avoid distorting the sig-nal [1]. This often results in the PA being operated at back-offfrom its saturation point, reducing the power efficiency signifi-cantly. Recently, considerable effort has been made to improveefficiency and reduce distortion, including PA designs such asthe Doherty amplifier [2], and predistortion of the signal [3],both of which add complexity to the transmitter.A recent approach to the problem is Direct Antenna Modula-tion (DAM), where an amplified carrier wave is modulated atthe antenna. This means only the carrier wave is amplified,rather than the broadband modulated waveform, allowing thePA to operate in its efficient non-linear region without addingdistortion. Several approaches have been taken, including inte-grating active components into the antenna to produce simplemodulation schemes [4], or surrounding the antenna by passiveswitched reflectors to produce directional complex modulation[5]. We recently, [6], proposed an antenna loaded with a three-layer reconfigurable Frequency Selective Surface (FSS), usingvaractor diodes to vary the FSS centre frequency, and hence thetransmitted phase from the antenna (Fig. 1). Its performanceas a modulator is evaluated in an end-to-end Quadrature PhaseShift Keying (QPSK) communications system in [7].Fig. 1: Concept of a DAM modulator using an FSSFig. 2: Polar plot of transmitted signal of LC bandpass filtersof different orders with changing capacitanceThis paper further develops the work of [7], presenting anDAM for QPSK, continuous phase and polyphase signals [8].The design uses low-loss materials and devices to improve theantenna efficiency, and a 4-layer FSS for an improved modula-tion performance. The antenna design and its performance asan antenna and a modulator are discussed.2 Frequency Selective Surface designThe reconfigurable FSS for a PSK modulating antenna shouldbe broadband, such that there is little change in transmittedmagnitude across 360◦ of change in transmitted phase. Toachieve this, multiple layers of FSS are required. This isdemonstrated in Fig. 2, where the magnitude and phase trans-mitted through different numbers of layers of LC bandpassFSS are shown with changing capacitance at a fixed frequency.1Fig. 3: FSS unit cell design1 1.5 2 2.5Frequency (GHz)-40-35-30-25-20-15-10-50Magnitude (dB)S11, 0.6pFS11, 1pFS11, 1.4pFS21, 0.6pFS21, 1pFS21,1.4pFFig. 4: Reflection and transmission coefficients of FSS designin free spaceThe 4 layer design was selected as it provides a maximum ofless than 1dB variation around the whole constellation, and ina real system there is a trade-off with loss increasing linearlywith the number of layers.A square-loop aperture design for the FSS was chosen for itsstability with angle of incidence, its relatively broadband fre-quency response and its ease of implementation. The unit cellgeometry was designed using the square-loop design equationsin [9] and Babinet’s principle. In order for the FSS to resonateat 1.8GHz when variable capacitors are integrated, the geome-try was designed for 3.4GHz. The design in Fig. 3, producesan equivalent inductance and capacitance of 24pH and 93pF,respectively.The design was validated in CST Microwave Studio, usingFloquet boundaries in free space to simulate four infiniteFSS spaced by λ0/4. The unit cell geometry was simulatedas PEC, and each layer had a 1.6mm TLY5 fibreglass sub-strate, chosen for its low loss tangent at microwave frequen-cies (ǫr2.2, tanδ = 0.0009). Variable capacitors were inte-grated across the gaps in the unit cell to allow tuning of the(a)(b) (c)Fig. 5: (a) Side-on view of final antenna design, (b) Front-onview of antenna design, (c) Reverse of each FSS layerFSS to produce modulation. STPTIC-27G2 variable capaci-tors, which are based on a Barium Strontium Titanate mate-rial, were chosen for their high Q-factor at the frequency ofinterest and good linearity [10]. The capacitors were simu-lated as lumped elements with a tuning range of 0.6 to 3pFand an Equivalent Series Resistance (ESR) of 0.6Ω. However,only the lower half of the capacitance range is explored, asincreasing C increases the current through the variable capac-itor, leading to higher resistive losses in the component. Thisis marginally offset by a lower ESR for a similar Q-factor atlarger capacitances, but losses through the surfaces were foundto still increase. The simulation shows between 1.1dB and0.3dB minimum transmission loss at various capacitances (Fig.4).3 DAM Antenna designTo ensure all fields pass through all layers of the FSS and tolimit the antenna form factor, the FSS was integrated into acavity-backed antenna with a monopole feed (Fig. 5a). Thisincreases the centre frequency of the FSS by 350MHz com-pared with the FSS in free space. Each layer consists of a5 × 5 grid of unit cells (Fig. 5b), allowing the cavity to belarge enough to propagate fields at 1.8GHz while restrictinghigher order modes. As the FSS is no longer in free space,a layer spacing of λ0/4 = 41.7mm will not produce an opti-mum filter response at the operating frequency. However, asthe cavity is not an infinite length waveguide, the theoretically20.9 1 1.1 1.2 1.3 1.4 1.5Capacitance (pF)-20-15-10-50Normalised farfield at boresight (dB)(a)0.9 1 1.1 1.2 1.3 1.4 1.5Capacitance (pF)050100150200250300350Farfield phase at boresight (Degrees)(b)Fig. 6: (a) Magnitude and (b) phase variation of E-field atboresight in the farfield with varying capacitancederived guide quarter-wavelength, λg/4 = 76mm, also doesnot produce an optimal filter. Simulation in CST found thata spacing of λs/4 = 57mm gives an optimally flat filter re-sponse at 1.8GHz. The capacitors are modelled as before, andare placed in alignment with the antenna E-field to minimisethe number of components required. 1mm wide bias lines aremodelled on the reverse of the FSS substrate, and are mostlyorthogonal to the cavity E-field, with the two vertical lines to-ward the edges of the antenna to minimise their effect on thetransmitted signal, as shown in Fig. 5c.4 Antenna performanceThe full antenna was simulated in CST Microwave Studio.Fig.6 shows the boresight normalised farfield magnitude andphase received at 1.8GHz with changing capacitance. It showsa nearly flat magnitude response between 0.97 and 1.39pF,with a highly linear phase change in this region. Between 0.97and 1.38pF, there is 360◦ of phase change for only 1.5dB vari-ation in magnitude, giving the maximum difference in magni-tude between two arbitrary PSK constellation points.0.9 1 1.1 1.2 1.3 1.4 1.5Capacitance (pF)020406080Total efficiency (%)Fig. 7: Total simulated efficiency of antenna with varying ca-pacitanceThe total efficiency of the antenna is simulated as varying be-tween 54.5% and 73% in the filter pass region, with a meanof 65% (Fig. 7). Losses occur in the substrate and the para-sitic resistance of the variable capacitors. However, at 0.98pF,there is a decrease in antenna efficiency but an increase in themagnitude of the signal received at boresight. This is due toan increase in peak directivity at this point. As such, the con-stellation in directions other than boresight must be consideredwhen selecting constellation patterns, as reflected non-Line-of-Sight (LoS) paths with a distorted constellation may inter-fere in a less determinate way with a boresight constellationat a receiver. Fig. 8 shows an 8-PSK constellation at vari-ous observation angles within the maximum 3dB beamwidth,80◦, in both E- and H-planes, where each complete constella-tion is normalised. The constellation is completely stable inthe H-plane, and the different points have only 14% variationin absolute amplitude. In the E-plane, there is more variation,with up to 26% difference in amplitude and 11◦ in phase at aviewing angle of 40◦. However, this constellation is transmit-ted with 3dB less power than the boresight constellation, soany interference from reflected impaired constellations shouldbe small.5 ConclusionA directly phase modulating antenna incorporating a reconfig-urable FSS has been designed and simulated. The design issimulated as able to produce an 8-PSK constellation with only14% variation in constellation magnitude. The antenna lossis 1.3dB in the filter pass band and achieves an average totalefficiency of 65% across an 8-PSK constellation.AcknowledgementsThis work is funded by an EPSRC iCASE award with indus-trial partners NEC and BT. The data for this work can be ac-cessed at https://figshare.com/s/96c1f68ee99a868ee1b2.3030609012015018021024027030033000.20.40.60.81-40-2002040(a)030609012015018021024027030033000.20.40.60.81-40-2002040(b)Fig. 8: Simulated 8-PSK constellation from antenna at various observation angles in (a) E-plane (b) H-planeReferences[1] Raghavan, A., Srirattana, N., Laskar, J.: “Modeling andDesign Techniques for RF Power Amplifiers”, (Wiley-IEEE Press, 2008)[2] Raghavan, A., Srirattana, N., Laskar, J.: “Efficiency En-hancement of RF Power Amplifiers”, in ‘Modeling andDesign Techniques for RF Power Amplifiers’ (Wiley-IEEE Press, 2008), pp 173–198.[3] Lei, D., Zhou, G. T., Morgan, D. R., Ma Zhengxiang,Kenney, J. S., Jaehyeong, K., Giardina, C. R. “A robustdigital baseband predistorter constructed using memorypolynomials”, IEEE Transactions on Communications,52, pp. 159-165, (2004).[4] Daehee, P., Minhoe, K., Dong-Ho, C. “Novel single-RFMIMO system based on repetitive Pulse Width Modula-tion”, IEEE Communications Letters, 20, pp. 165–168,(2016).[5] Babakhani, A., Rutledge, D. B., Hajimiri, A. “Trans-mitter architectures based on near-field Direct AntennaModulation”, IEEE Journal of Solid-State Circuits, 43,2674–2692, (2008).2017 11th European Conference on Antennas and Prop-agation (EUCAP), Paris, France, 2017, pp. 731-734.[6] Henthorn, S., Ford, K. L., O’Farrell, T. “Frequency Se-lective Surface loaded antenna for Direct Antenna Mod-ulation”, 2017 11th European Conference on Antennasand Propagation (EUCAP), Paris, France, (2017), pp.731-734.[7] Henthorn, S., Ford, K. L., O’Farrell, T. “Bit error rateperformance of quadrature modulation transmission us-ing reconfigurable Frequency Selective Surfaces”, IEEEAntennas and Wireless Propagation Letters, PP(99), pp.1–1, (2017).[8] O’Farrell, T. “New signature code sequence designtechniques for CDMA systems”, Electronics Letters,27, pp. 371–373, (1991).[9] Langley, R. J., Parker, E. A. “Equivalent circuit modelfor arrays of square loops”, Electronics Letters, 18, pp.294–296, (1982).[10] STMicroelectronics. “STPTIC-27G2Parascan tunable integrated capacitor”,http://www.st.com/content/ccc/resource/technical/-document/datasheet/3a/d0/ac/4a/ab/72/46/aa/-DM00217084.pdf/files/DM00217084.pdf/jcr:content/-translations/en.DM00217084.pdf, accessed 23 May2017.4

A low-loss reconfigurable frequency selective surface based antenna for direct antenna modulation

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A low-loss reconfigurable frequency selective surface based antenna for direct antenna modulation

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