Limitations on efficiency, size and bandwidth of small antennas

Licentiatavhandling, 2008

The allocated space in a mobile phone for the antennas has become smaller and smaller following the size reduction of the mobile phones themselves and the integration of more functions in them. At the same time new communication systems are being planned where such small antennas also need to be wideband (UWB systems) or have multiple ports (MIMO systems). Therefore, this thesis takes a new and more practical look upon the fundamental size limitations of small antennas. The thesis aims explaining what the limits in radiation Q means in terms of practical bandwidth limitations of three different types of small antennas: single-resonance-type, gradual-transition type and cascaded-resonances-type. Small antennas are often resonant, so when the theoretical bandwidth limitations were formulated it was natural to reuse the formulations and terminology which was used to characterize resonators. The frequency selectivity of resonators is normally expressed by the quality factor Q. That is why the fundamental limitations on antennas are formulated as limitations on radiation Q. The inverse of this quantity represents the relative bandwidth, which is the parameter of interest for antennas. This thesis explains how this radiation Q is related to: 1. The bandwidth-efficiency product of small single-resonance-type antennas. 2. The gradual cut-off of spherical waves for wideband gradual-transition-type small antennas. 3. The number of resonances needed to cover a certain frequency band for wideband cascaded-resonances-type small antennas. The thesis also introduces one radiation Q for basic single TE and TM spherical mode sources, and another for combined TE and TM sources The thesis also includes some measurements of performance of small MIMO antennas in reverberation chamber.