Miniaturisation and Efficiency in Compact and Electrically Small Tuneable UHF Antennas

  • Oliver N James

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Many wireless communications systems require access to multiple frequency bands in compact form factors. The capability of an antenna to achieve high efficiency and wide bandwidth is linked to its physical size. Efficient wide-band antennas can be too large for compact multi-band systems in the low UHF range (300-1000 MHz). Electrically small antennas (ESAs) can work around the spectrum requirements if they are frequency agile, but efficiency degradation associated with the tuning mechanism is often overlooked. There is also no consistent approach to reporting the performance enhancement over an integrated bandwidth from the efficiency perspective, upon which the user experience ultimately depends. Three topical tuning mechanisms have been investigated in the UHF band, each underpinned by identification of candidate technologies from the literature, design of a frequency agile demonstrator antenna, and measurement of the input response, 3D radiation pattern and realised efficiency. First, the ability of an RF-MEMS reconfigurable impedance matching network to preserve system efficiency has been challenged. Improved impedance matching in both free-space tuning and hand effect detuning scenarios was achieved, but the radiation performance of the whole system highlighted that previously undocumented losses given by emerging RF MEMS capacitors should not be overlooked when assessing tuner deployment in communication systems. Second, a Co2Z magneto-dielectric substrate material was found to offer profound miniaturisation (novel material achieved frequency reduction of 6.4× compared to Duroid RT5880) and susceptibility to magnetic biasing (frequency tuning 32% for S11 < -3 dB). The measured efficiency was found to be around 1% for this iteration of the material. Nonetheless, the miniaturisation and tuneability aspects warrant ongoing review for use in applications that are miniaturisation-critical, since the novel material offers significant board area savings as well as radiation pattern stability. Third, an improved novel approach to quantifying ‘tuneability’ in compact narrow band antennas (including electrically small antennas) has been formulated, emphasising the perspective of combined realised efficiency and frequency agility. Favouring efficiency aspects in the new approach increases relevance to power consumption, battery life, data rates and carbon emissions at the network level. Metrics for bandwidth of assured efficiency, average efficiency and integrated efficiency-bandwidth product have been given, reducing the commonplace over-reliance on tuneable S11 in isolation as a predictor of tuning performance. Demonstration of the approach has been given by application to a set of fixed-frequency/varactor-tuneable loop ESAs. The improved approach provides a platform for objective, quantified performance grading of a diverse range of antenna tuning technologies in future work, alleviating the problem of identification and grouping of tuning technologies by application. This in turn could reduce wasted design effort as well as promoting efficiency as a design concern in tuneable antenna solutions.
Date of Award28 Nov 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorGeoff Hilton (Supervisor) & Mark A Beach (Supervisor)

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