It is known that the fundamental radiation mode of a mobile terminal chassis enables wideband coverage below 1 GHz. However, in theory, the bandwidth of this chassis mode extends infinitely beyond 1 GHz. In this paper, we investigate the use of inductive coupling element (ICE) to significantly increase the antenna bandwidth obtainable from the fundamental mode. Using characteristic mode analysis, a dual-ICE configuration is proposed and optimized to excite the fundamental mode, giving a simulated impedance bandwidth of nearly 100%

Aliakbari Abar, Hanieh

Lau, Buon Kiong

Lund University Publications

LUND UNIVERSITYPO Box 117221 00 Lund+46 46-222 00 00Wideband excitation and matching of fundamental chassis mode using inductivecoupling elementAliakbari Abar, Hanieh; Lau, Buon KiongPublished in:2020 IEEE International Symposium on Antennas and Propagation and North American Radio Science Meeting2020Document Version:Peer reviewed version (aka post-print)Link to publicationCitation for published version (APA):Aliakbari Abar, H., & Lau, B. K. (2020). Wideband excitation and matching of fundamental chassis mode usinginductive coupling element. In 2020 IEEE International Symposium on Antennas and Propagation and NorthAmerican Radio Science Meeting IEEE - Institute of Electrical and Electronics Engineers Inc..Total number of authors:2General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portalRead more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.Wideband Excitation and Matching of FundamentalChassis Mode Using Inductive Coupling ElementHanieh Aliakbari, Buon Kiong LauDepartment of Electrical and Information Technology, Lund University, Lund, Swedenhanieh.aliakbari_abar@eit.lth.se, buon_kiong.lau@eit.lth.seAbstract—It is known that the fundamental radiation mode ofa mobile terminal chassis enables wideband coverage below 1GHz. However, in theory, the bandwidth of this chassis modeextends infinitely beyond 1 GHz. In this paper, we investigate theuse of inductive coupling element (ICE) to significantly increasethe antenna bandwidth obtainable from the fundamental mode.Using characteristic mode analysis, a dual-ICE configuration isproposed and optimized to excite the fundamental mode, giving asimulated impedance bandwidth of nearly 100%.Keywords—Terminal antenna; characteristic modes; inductivecoupling element; matching network; feed designI. INTRODUCTIONMobile terminal antennas were first designed for frequencybands below 1 GHz, and only a small bandwidth was required.However, as mobile communications became popular, manymore frequency bands were added to meet growing demand. Inparticular, it is challenging to obtain more bandwidth for thebands below 1 GHz, due to space limitation constraining theantenna elements to be electrically small. Nonetheless, antennadesigners have achieved bandwidths far beyond those expectedof electrically small antennas. This phenomenon was found tobe due to the terminal chassis, which is electrically muchlarger, becoming the main radiator [1]. The antenna elementhas instead taken the role of coupling power into the chassis,effectively exciting the fundamental mode of the chassis [2].Today’s terminal antennas offer multi-band coverage, withthe low-band covering most of the range 0.7–1 GHz and thehigher bands extending from around 1.7 GHz. Although theseantennas meet current requirements, the low-band design doesnot exploit the full potential of the fundamental mode, which intheory has a bandwidth that extends to infinity (see Fig. 1).In this paper, the design of low-band terminal antenna isrevisited from the perspective of characteristic mode analysis(CMA) and impedance matching. It is shown that inductivecoupling elements (ICEs) [3] can be applied to attain a muchlarger bandwidth than 0.7–1 GHz. To this end, key parametersfor the excitation and impedance matching of the fundamentalchassis mode were identified and optimized. Using theoptimized structure and a suitable feeding network, the 6 dBimpedance bandwidth of 98% (0.78-2.3 GHz) is achieved.II. CHASSIS EXCITATION: SIMULATION RESULTSAltair FEKO 2018 was used for CMA and BetaMatchFig. 1. Characteristic eigenvalues of solid chassis (solid lines) and slottedchassis (dashed lines). W = 60, L = 120, a = 1, b = 3 (units: mm). (The feedingstrip at the outer end of the slot is shorted in characteristic mode analysis.)Fig. 2.  (a) Normalized current of the fundamental mode at 0.9 GHz, andtotal currents in the rectangular plate with feeding at (b) one horizontal slot,(c) two horizontal slots, (d) two horizontal /vertical slots.Version 3.7.6 for bandwidth potential [1] estimation. In Fig.  1,the characteristic eigenvalues for an infinitely thin, 120 mm 60 mm perfectly conducting chassis are shown. It can be seenthat eigenvalue of the fundamental dipole mode (mode 1) isclose to zero (resonant) from ~0.9 GHz onwards. Capacitivecoupling element (CCE) can be used to excite this mode, but itwas found in [4] that the use of CCE can severely limit themode’s bandwidth potential. Apart from CCE, ICE can also beused for modal excitation [3], through coupling power into thecharacteristic current. Figure 2(a) shows that the current ofmode 1 is strongest along the two longer sides of the chassis,indicating that ICEs should be placed in these locations. Figure1 shows that a simple ICE (a slot with a feeding strip) does notchange the chassis’ resonant modes in the observed frequencyrange. This means that the fundamental mode retains itsinfinite bandwidth, despite having the ICE.This work was supported by the Swedish Research Council under grantno. 2010-468 and 2018-04717.Fig. 3. (a) Bandwidth potential and (b) modal excitation coefficient of arectangular chassis fed by an ICE with different slot length b.Fig. 4. (a) Modal excitation coefficent, (b) bandwidth potential,  (c) real partof admittance and (d) imaginary part of admittance, for dual-ICE (b = 3 mm).However, due to impedance mismatch, the bandwidthpotential for the ICE is lower than 3% at 0.8 GHz, with 50source impedance (see Fig. 3(a)). To further increase theexcitation of mode 1, the slot length b can be increased (seeFig. 3(b)). This in turn improves the bandwidth potentialbecause the radiation resistance will be increased, especially atthe lower frequencies. Figure 2(b) shows the current on thechassis when the feeding strip of the ICE is excited.However, the single-ICE design is unsuitable for widebandrealization, as it also excites other modes that become resonantnear 2 GHz in a manner that limits its bandwidth potential.Therefore, a dual-ICE design, with another ICE added to theother location of high current and excited in phase, is adopted.Figure 2(c) shows the current distribution when the dual-ICE isexcited. This new design effectively limits the excitation ofhigher order modes and increases the modal excitationcoefficient of mode 1. Despite the better performance of thedual-ICE, the bandwidth potential is still limited to 15% or less(Fig. 3(a)), due to the impedance mismatch at the feeding port.Borrowing the idea of matching stubs, vertical slots of length dcan be added to the ICEs. As can be seen in Fig. 4, the verticalslots primarily improve the impedance matching, retainingalmost the same level of excitation. As shown in Figs. 4(c) and4(d), the vertical slots improve the bandwidth potential mainlyby changing the imaginary part of the admittance, while thereal part remains almost the same. However, the rate ofincrease of the bandwidth potential decreases as d increases(see Fig. 4(b)), as the feed is becoming resonant in thefrequency range of interest. Based on these insights, the keyparameters of b and d were optimized for both excitation andimpedance matching.III. FINAL STRUCTURE      The final structure (with d = 10 mm and b = 17 mm) isshown in Fig. 5(a). A passive matching network (twocapacitors and one inductor) was developed in CST 2018 tomatch the two ICEs to 50Ω over the frequency band. A T-divider is used for combining the two paths. Fig. 5(b) showsthat the operating band is from 0.79 GHz to 2.3 GHz at -6 dBreflection coefficient. Moreover, the radiation pattern is stableand resembles the dipole pattern in the entire operating band.Figure 2(d) shows the current distribution when the final ICEdesign is excited.Fig. 5. (a) Final antenna structure with feed network and (b) reflectioncoefficient S11.IV. CONCLUSIONIn this paper, the systematic design of ICEs for theexcitation of a rectangular terminal chassis is described. TwoICEs consisting of T-shaped slots are used as a probe to purelyexcite the fundamental dipole mode of the chassis. The keydimensions of the ICEs were identified using CMA andbandwidth potential, and then optimized to achieve a largeimpedance bandwidth. More analysis on CMA will beprovided in the full paper.REFERENCES[1] J. Villanen, J. Ollikainen, O. Kivekäs, and P. Vainikainen, “Couplingelement based mobile terminal antenna structures,” IEEE Trans.Antennas Propag., vol. 54, no. 7, pp. 2142-2153, Jul. 2006.[2] H. Li, Y. Tan, B. K. Lau, Z. Ying, and S. He, “Characteristic modebased tradeoff analysis of antenna-chassis interactions for multipleantenna terminals,” IEEE Trans. Antennas Propag., vol. 60, no. 2, pp.490-502, Feb. 2012.[3] R. Martens, and D. Manteuffel, “Systematic design method of a mobilemultiple antenna system using the theory of characteristic modes,” IETMicrow. Antennas Propag., vol. 8, no. 12, pp. 887 – 893, 2014.[4] H. Aliakbari, and B. K. Lau, “On modal excitation using capacitivecoupling elements and matching network,” in Proc. IEEE Int. Symp.Antennas Propag. (APS’2019), Atlanta, GA, Jul. 7-12, 2019.

Wideband excitation and matching of fundamental chassis mode using inductive coupling element

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