A Reactance Theorem for Antennas

A rigorous expression for the frequency derivative of the input reactance or susceptance of an arbitrary antenna is derived by integration of Maxwell´s equations. This expression is shown to depend on the polarization properties of the far field of the antenna and on a quantity which may be interpreted as the electromagnetic energy stored in its near field. The theorem gives a precise theoretical foundation to an intuitive analysis, based on network theorems, which has been used with some success to predict the bandwidth properties of antennas.