Electrically short monopole antennas with dielectric or ferrite coatings

A comprehensive analysis of the electrically-small material-clad monopole is performed by representing the various fields by discrete and continuous eigenvalue spectrums. The cladding creates a dominant quarter-wavelength resonant-cavity effect whereby the leakage of radiation at the terminal plane is transformed into a low impedance at the annular feed. The mode coupling taking place at both feed and terminal plane is solved by variational methods to yield the antenna radiation pattern, bandwidth, efficiency and input impedance as a function of electrical size and frequency. The agreement between the computed and measured results establishes the usefulness of the analysis, and design curves are presented; other factors that influence the system performance of the antennas in practice are discussed. It is concluded that the optimisation of this type of antenna centres on the choice of suitable low-loss material with comparable relative permeability and permittivity values at the frequency of interest. In general, this calls for research into improved materials, but useful antennas can be made with commercially available materials.