Gain manipulation of a slotted sphere using general bi-isotropic coating

The gain characteristics of a general bi-isotropic (BI) coated slotted sphere have been studied using numerical calculations. An eigenfunction expansion method has been used to derive analytical expressions for the radiated electric and magnetic fields. The effects of various BI and Tellegen coatings upon the gain pattern are investigated. It is proven that the chirality and Tellegen parameters of the coating can significantly modify the gain. It is found that a lossless BI coating has the highest gain in the forward direction as compared to lossless dielectric, Tellegen, and chiral coatings. Such type of enhanced gains in the forward direction are desirable for point-to-point communication. It is further shown that the Tellegen coating guides most of the radiated field from the slot toward polar directions and reduces the gain in the forward direction provided that the Tellegen parameter is not close to unity. For a Tellegen coating whose Tellegen parameter is close to unity, the maximum gain in the forward direction is found. The proposed theory is also extended to realistic BI and low-loss dielectric coatings. It is investigated that by using a proper selection of coating thickness, the forward direction gain of a realistic BI coating can be enhanced or diminished as compared to a low-loss dielectric coating.