A reaction theorem and its application to antenna impedance calculations

The reaction theorem is generalized to allow the fields of an antenna in one environment to be employed in calculations of mutual impedance in another environment. Several expressions for self-impedance and mutual impedance are presented. These are in the form of surface integrals or volume integrals of the field intensities or the current density. It is shown how the fields of an antenna in free space can be useful in calculating the impedance in the presence of scatterers.