Small antennas as wireless system components

Abstract

Small antennas, or more specifically, electrically small antennas, are passive radiating elements whose size is much smaller than the operating wavelength. The current wireless application systems usually have size constraints that, for the most common operating frequencies except the highest, also implies small electrical size. This applies to most of the wireless systems in a wide variety of applications such as consumer and medical devices, sensor networks and military. The design of small antennas brings forward some challenges that are not obvious when considering large antennas radiating into free-space for line-of-sight communications, the realm of classical radio systems. Small antennas are nowadays deployed in very different conditions, usually integrated into device packaging and/or operating near dispersive media such as the human body. The typical antenna characterization was devised for the classical scenario, an ideal case that bares low resemblance to the highly variable deployments, might not be very useful in predicting the antenna performance. This should be taken into account in the design cycle as small antenna performance is heavily dependent on the close surroundings. This thesis proposes some novel tools and insights towards the objective of a more system integrated antenna design cycle. It begins by studying efficiency, an antenna parameter specially relevant for small antennas. It is so because small antennas are usually omnidirectional and in this case the simple figure of merit can grasp most of the antenna spacial radiation characteristics. The result of this study is the proposal of a novel technique to measure antenna radiation efficiency using small overmoded metallic cavities in a very large band. It is based on simple input reflection coefficient measurements and the suggested post-processing can successfully decrease the effect of cavity resonances. This was the main limiting factor in measuring antenna efficiency in small metallic cavities. The technique requires only readily available laboratory equipment. It was compared to existing alternatives that are also subjected to improvements. Three different antenna prototypes, with significant and different antenna efficiency characteristics were used for the comparison. The approach provides a controlled environment providing the possibility to experimentally validate simulation models that can be confidently used in more complex situations, such as close to the body. The second part of this thesis reports on several small antennas where the wireless system as a whole was considered in the design cycle. It begins by considering Body Area Network (BAN) systems were a small antenna is expected to operate near the human body or subjected to deformations when built in flexible material for integration into garments. In this context, the development of simplified computational models for a limb compared favorably to other more complex solutions when predicting antenna performance near the body. A simple dielectric loading technique was refined and confirmed to reduce antenna-human body interactions, increasing the antenna matching performance without major impacts on other antenna parameters. A flexible antenna was built in plastic material and characterized when subjected to deformations. In the field of Wireless Sensor Networks (WSN) two design workflow examples are given. One antenna was built and tested for usage in the base-stations [...]