Designs and Measurements of Electrically Small Antennas for Hearing-Aid Applications
Hearing-aids today constitute devices with an advanced technology, and wireless communication integrated into hearing-aids will introduce a range of completely new functionalities. The antenna is an important component in any wireless system, and the demand for compact wireless systems with stringent specifications makes the antenna size reduction a significant challenge. Antenna miniaturization is thus one of the key technologies in designing a successful wireless unit for the hearing-aid application. This dissertation is focused on three areas that are related to the integration of a wire- less communication system into the heading-aids, and these are the antenna miniaturization, the measurement techniques for electrically small antennas and the influence of complex environments on the characteristics of electrically small antennas, respectively. Antenna MiniaturizationIn this dissertation, we present several novel designs of electrically small loop antennas for the hearing-aid application. First antenna design is a two-dimensional (2-D) planar differential-fed electrically small loop. The working mechanism of this antenna is based on the capacitive loading and the induc- tive coupling between two small loops. An analytical model, simulations, fabrications and measurements are presented for this antenna. Second antenna design is a planar two-turn electrically small loop antenna. The work- ing mechanism of this antenna is based on the capacitive loading, and both the capacitive and inductive coupling between two small loops that are of a comparable size are taken into account. An analytical model is provided to give a guidance in the impedance tuning. Third, several three-dimensional (3-D) folded electrically small loop antennas are proposed, the properties of which are significantly improved compared to the 2-D planar electrically small loop antennas. Measurement Techniques for ESAs In this dissertation we proposed two novel measurement techniques for electrically small antennas. A modified Wheeler cap method for the radiation efficiency measurement of balanced electrically small antennas is proposed. This method provides the following advantages. First, no balun is required during the measurement and thus the problems of narrow impedance bandwidth and extra scattering effect caused by the balun are avoided. As a result, the proposed method is valid in a broad frequency band. Second, the application of proposed method and the proper use of the circle fitting for the measured scattering parameters ensure that the cavity resonances do not have any significant effect on the measurement results. By using the Wheeler cap method in the proposed way, most of its limitations and disadvantages are avoided. The method is, therefore, suitable for input impedance and radia- tion efficiency measurement for most types of antennas in a broad frequency band. The antennas under test are not limited to be electrically small and these can be balanced or unbalanced, symmetric or asymmetric type. Moreover, the modified Wheeler cap method for measurements of small antennas in complex environments is further developed. A cable-free impedance and gain measurement technique for electrically small antennas is also proposed. The electromagnetic model of this technique is derived by using the spherical wave expansion, and it is valid for arbitrary electrically small AUT at arbitrary distances between the probe and AUT. The whole measurement setup is modeled by the cascade of three coupled multipleort networks. The electromagnetic model, the simulation results, and the obtained measurement results are presented. Influence of the Complex Environments on ESA Characteristics The influence of complex environments on the characteristics of electrically small antennas is also investigated such as the human head phantom and the hearing-aids. First, the sensitivity analysis of the head phantom parameters on the antenna characteristics is presented, including the influence of the head permittivity and conductivity. Second, the sensitivity analysis of the positions of electrically small antennas is presented, including the orientations and locations of antennas and the distance between the small antenna and head. Third, the influence of the hearing-aid shell material on the antenna characteristics is investigated.
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Breinbjerg, Olav, Sigmund, Ole, Mortensen, N. Asger