Reverberation chambers and the measurement of antenna characteristics

Over the past ten years, Reverberation Chambers (RC) have emerged as a promising facility for the measurement of antenna characteristics for the wireless communications industry. The RC has begun to diverge from its initial purpose of performing Electromagnetic Compatibility (EMC) measurements, as conceived by H. A. Mendes back in 1968. Unlike the Anechoic Chamber (AC) however, the RC facility and measurement protocols are relatively in their infancy whose standardisation is yet to be finalised. The purpose of this thesis is to present a further study into reverberation chambers and their associated measurement procedures, aimed at smoothing the transition into a fully standardised and accepted facility within the measurement industry. This thesis is comprised of three main investigation areas. The first area under investigation concerns single port textile antennas designed for use in the on-body communications role. The purpose of this investigation is twofold: (1) to chart the efficiency and impedance matching performance of the antennas in both free space and on-body roles to completely characterise their performance, and (2) to devise and configure operational procedures for the measurement of antenna characteristics on human subjects using the RC. Two separate varieties of textile antenna are subject to investigation; the first consists of two antennas with an equally small ground plane designed for use in the Industrial Scientific and Medical (ISM) 2.45 GHz band. The second variety also consists of two separate antennas that have a larger ground plane size and are designed to offer a dual band characteristic; to operate at 2.45 GHz and 5.2 GHz respectively. The results for the smaller ground plane sized antennas show that in free space conditions, the textile antenna constructed from the higher conductivity textile material exhibits a greater level of efficiency which is expected. However, when placed on-body, the antenna with the lower conductivity textile material remarkably outperforms the antenna with the higher conductivity material which is contrary to expectations; this represents new and important knowledge. The results for the larger ground plane sized textile antennas conform completely to expectations. That is, the higher conductivity material outperformed the lower conductivity material in both free space and on-body roles. Comparing both cases, further new knowledge can be concluded in the fact that in addition to the conductivity of the textile material, the ground plane dimensions is also of crucial importance. The second area under investigation concerns multiport (array) antennas. This area is sub-divided into two sections to chart a distinction between multiport antennas designed for Multiple Input Multiple Output (MIMO) applications and more conventional array antennas that are not. The first section concerns the complete practical verification of two new dual feed Planar Inverted F Antennas (PIFAs). Results show that both antennas yield a high level of diversity gain and channel capacity (close to the theoretical maximums) and very low correlation between the two feeds despite the antennas small size. Furthermore, the antennas are also proved to be highly efficient at the desired frequency of operation. Comparing all performance results, it is possible to conclude that due to the small size and excellent performance of the new designs, they could be useful in more practical and commercial applications than larger sized elements that currently exist. The second section focuses upon more conventional larger sized array antennas used for radio astronomy applications. In this thesis, a series of power dividers is used to emulate a realistic ‘all - excited’ scenario, but the power divider approach has a consequence in that it will give rise to an external power loss that is not attributed to the antenna array. A new equation is developed in this work that allows for the accurate efficiency determination of the array and the de-embedding of the power divider in one. It is shown that the new equation can make this whole process simpler and straightforward to accomplish whilst maintaining accuracy. The final area under investigation concerns the design of reverberation chambers. The most common of the mode stirring techniques used in reverberation chambers is via the rotation or movement of electrically large metallic paddles inside the chamber known as ‘Mechanical Stirring’. In this thesis, a technique based upon a meanderline principle is used to cut slots into the mechanical stirring paddles to increase the current path length (induced when a wave hits the metallic surface) and thereby increase the electrical size of the paddle. New paddle designs for reverberation chambers are designed and verified. It is shown that the overall paddle dimensions do not need to be increased in size, meaning that the working volume of the chamber can remain as large as possible. The results show that the new designs exhibit enhanced performance over and above conventional paddle designs at lower modal numbers, meaning that any chamber will be able to better perform at frequencies where fewer modes exist. Results also show that at higher frequencies, the slot cuts do not adversely affect the chambers higher frequency performance. This work therefore has the potential to forge a new way of thinking when it comes to the design of mechanical stirrers in RC’s