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Abstract
Directivity and radiation Q of dipole sources placed very close to a reactive impedance boundary are considered. By means of complex image theory, dominant low-frequency multipoles for fields radiated by point currents are evaluated. The multipole expansion is applied when the source region containing the currents and their images is reasonably compact and small in terms of wavelengths, which is when excitation of surface waves is unimportant. Such is the case when the surface reactance compared to the impedance of free space is very small and positive or very large and negative. Analytical expressions for directivity and Q are then written in order to study the influence of the surface reactance, and approximate expressions are derived for the surface reactance, which optimises either Q or directivity. The relation between directivity and Q is studied graphically.