Antenna Miniaturization in Complex Environments

Jiaying Zhang
, PhD student
Olav Breinbjerg
, Professor
Ole Sigmund, Professor
Niels Asger Mortensen, Associate professor

1 January, 2008 - 31 December, 2010.

Technical University of Denmark.

Hearing aids already constitute an advance technology and wireless coupling to hearing aids will open for a range of completely new functionalities. The project "Wireless Coupling in Small Autonomous Apparatus" is carried out in collaboration between Teknologisk Institut, Widex, and Technical University of Denmark, and is approved for funding by Højteknologifonden from the year 2007 to 2010. This Ph.D. project concerns the antenna systems for wireless communication with hearing aids.

Recent years' tremendous development of wireless communication involving ever smaller terminals has caused a significant research interest in antenna miniaturization. The challenge is not just to develop geometrically small antennas but electrically small antennas that are less than half a wavelength of the RF signal - the traditional minimum length of resonant antennas. This recent research on antenna miniaturization has spawned many new results on fundamental properties and new design technologies involving high-permittivity ceramics, structural optimization, and novel materials. Indeed, while an antenna size of the order of one-tenth of the wavelength is realistic at present, the sub-wavelength resonance properties of the so-called double-negative metamaterials, reported among others by DTU researchers, may hold the potential for far smaller antennas.

This Ph.D. project comprises 3 major areas of investigation for the antennas of wireless communication systems with hearing aids.

1. Complex Environment
The antennas are to function in very complex environments that will significantly influence the performance of the antennas. An antenna radiating a primary RF field will induce currents in all nearby objects and these currents will then radiate a secondary RF field that interferes - in a potentially destructive manner - with the primary RF field. It is thus necessary in the design of the antennas to take into account the influence of this complex environment. For an antenna integrated in a hearing aid, this means that the influence of the hearing aid itself, the ear, the head, and a part of the body must be included in the analysis. Mobile phone communication faces a similar challenge and much of the experience gained in this field over the past 10 years can be exploited in the present work. However, while mobile phone communication is always between the phone and a distant base station, the communication paths for the functionalities envisaged in this project could be more complex; e.g. communication between the hearing aid and a mobile phone located on the body of the person, or communication between two hearing aids on either side of the person's head.

2. Antenna Miniaturization
Since the units into which the antennas will be integrated are already small, the antennas must be geometrically very small, the maximum dimension of which is no larger than 1cm. The operating frequency of the wireless communication is chosen to be 405.5 MHz and the wavelength is thus 74cm. This means that the geometrically small antennas are also electrically small and this project thus calls for highly miniaturized antennas. Miniaturized antennas composed of traditional materials may prove inadequate; e.g. it is difficult to tune such small antennas for an effective power transfer to and from the electronics of the radio connected to the antenna. In recent years, there has been a tremendous technical interest in the use of so-called metamaterials for antenna miniaturization. The well-documented sub-wavelength resonances of these materials can potentially be exploited for the effective tuning of miniaturized antennas.

3. Small Antenna Measurements
Accurate experimental characterization of small antennas in complex environments requires development of new measurement techniques. The measurements will be performed at the DTU-ESA Spherical Near-Field Antenna Test Facility operated by the Technical University of Denmark (DTU) as an external reference laboratory for the European Space Agency (ESA). This is a state-of-the-art facility that for several years has served Danish and European industry with high-accuracy measurements and calibrations of antennas. Furthermore, for the measurement of the small antennas of the present project it will be necessary to design and implement new measurement set-ups and techniques for two reasons: First, since the antennas will eventually operate in a complex environment comprising a hearing aid, a person's head and body, it is necessary that the measurements are also conducted in a model of this environment. Hence, a so-called phantom must be designed and implemented. To this end, computational simulations will be performed to determine how accurate a model this phantom must be of the real environment in order for the measurement results to be representative. Second, since the antennas are electrically small the measurements will be strongly influenced by the electrical and mechanical support equipment used during the measurement and a calibration/correction for this is required. This will necessitate the development of other measurement techniques than those used for traditional antennas; e.g., to determine the radiation efficiency and gain of the small antennas.


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