Impedance matching and tuning of non-resonant mobile terminal antennas

Risto Valkonen

    Research output: ThesisDoctoral ThesisCollection of Articles


    The volume reserved for antennas in a mobile handset is very limited. Furthermore, the laws of physics state that the efficiency and/or the bandwidth of a small antenna are also limited. Despite this fact, the antennas of the future mobile terminals should operate efficiently at increasingly many frequency bands. This doctoral thesis contributes to solving some of the main challenges arising from the theoretical and practical limitations of handset antennas. Non-resonant capacitive coupling element (CCE) based antennas are an attractive option for future mobile handsets, having many useful characteristics such as compact size and broad frequency tunability. A circuit model improving the physical understanding of the operation of a capacitive coupling element antenna is developed in the thesis work. This work also advances the usability of CCE antennas by developing impedance matching and tuning solutions as enablers of future mobile communication systems, including software defined radio. The properties of non-self-resonant antennas are utilized in developing novel multi-band antennas for mobile terminals. A method for implementing simultaneous multi-band operation of single-element CCE antennas is developed, based on a novel diplexing impedance matching circuit and a distributed CCE feeding structure. The realized prototypes operate at 4th generation mobile communication system (LTE-A) frequency bands 698—960 MHz and 1710—2690 MHz, reaching state-of-the-art performance. This versatile CCE structure is shown to operate equally well with single- or dual-feed RF front-end interfaces. Broadband tunable antenna prototypes are realized as a proof of concept regarding CCE tunability. Also design rules for avoiding excess losses and nonlinearity in tuning circuits are given, based on numerical and experimental analyses. An antenna in a mobile terminal is also subject to electromagnetic interaction with the user of the handset. This interaction deteriorates the operation of the antenna through absorption of power and detuning of the antenna impedance. In this work, methods for avoiding the impedance detuning are developed. CCE geometry design and antenna selection diversity areshown to decrease the impedance mismatch and improve radiation efficiency by up to 4 dB in total in the case of some typical hand grips. Also matching circuits that result in improved robustness against the mistuning are studied and developed.
    Translated title of the contributionMobiilipäätelaitteiden resonoimattomien antennien impedanssisovitus ja –säätö
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • Vainikainen, Pertti, Supervising Professor
    • Räisänen, Antti, Supervising Professor
    • Icheln, Clemens, Thesis Advisor
    Print ISBNs978-952-60-5054-6
    Electronic ISBNs978-952-60-5053-9
    Publication statusPublished - 2013
    MoE publication typeG5 Doctoral dissertation (article)


    • mobile antennas
    • capacitive coupling element
    • impedance matching
    • frequency tuning
    • equivalent circuits
    • electromagnetic user interaction


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