The radiation by an annular slot in a perfectly conducting prolate spheroidal antenna is studied, when the antenna is coated by a confocal layer made of either isorefractive material or anti-isorefractive DNG metamaterial. The field components are expressed as infinite series of prolate spheroidal wavefunctions, whose modal expansion coefficients are determined analytically. Numerical results are presented for the radiated far field, the field inside the coating layer, and the input admittance of the antenna.
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