An equivalent nonuniform transmission line model for electromagnetic radiation in free space is developed. By properly defining a voltage and a current associated with the transverse component of the mode fields, a kind of Telegraphers’ equation is derived for each spherical harmonic mode. Based on the equivalent distribution inductance and capacitance, the local characteristic impedance and phase velocity are derived. For each spherical mode, a cutoff spherical surface and an associated cutoff radius are introduced to separate the space into an evanescent region and a propagating region. A spherical mode field will decay approximately exponentially in the evanescent region and experience local reflection in the propagating region. The proposed model may provide an intuitive illustration for the radiation process in free space.

This work is also part of my efforts in revisiting the issue of electromagnetic radiation and mutual coupling. Although the concepts I proposed, like the macroscopic Schott energy and this NTL model, may look strange and different from the conventional formulations, I believe they can provide intuitive and insightful interpretattion to the issue of EM radiation.