Research Article

The directivity of a compact antenna: an unforgettable figure of merit

¹ University of Technology Sydney (UTS), Global Big Data Technologies Centre, Ultimo, NSW 2000, Australia
² The University of Arizona, Department of Electrical and Computer Engineering, Tucson, AZ 85721, USA

^* e-mail: Richard.Ziolkowski@uts.edu.au

Received: 23 February 2017
Received in final form: 28 July 2017
Accepted: 25 August 2017
Published online: 11 October 2017

Abstract

When an electrically small antenna is conceived, designed, simulated, and tested, the main emphasis is usually placed immediately on its impedance bandwidth and radiation efficiency. All too often it is assumed that its directivity will only be that of a Hertzian dipole and, hence, its directivity becomes a minor consideration. This is particularly true if such a compact antenna radiates in the presence of a large ground plane. Attention is typically focused on the radiator and its size, while the ground plane is forgotten. This has become a too frequent occurrence when antennas, such as patch antennas that have been augmented with metamaterial structures, are explored. In this paper, it is demonstrated that while the ground plane has little impact on the resonance frequency and impedance bandwidth of patch antennas or metamaterial-inspired three-dimensional magnetic EZ antennas, it has a huge impact on their directivity performance. Moreover, it is demonstrated that with both a metamaterial-inspired two-element array and a related Huygens dipole antenna, one can achieve broadside-radiating electrically small systems that have high directivities. Several common and original designs are used to highlight these issues and to emphasize why a fundamental figure of merit such as directivity should never be overlooked.