Superdirective spherical radiator Available to Purchase

The condition for the maximum directivity from a spherical radiator is presented and it is shown that this directivity index is 20 log N, where N is the number of consecutive modes excited. For superdirectivity, the relationship among the expansion coefficients is particularly simple and a closed‐form expression for the beam‐pattern function results. Using this method, a high directivity may be obtained no matter how small the sphere is, provided that the prescribed velocity distribution is maintained. Examples of the resulting beam patterns are given.