We shall prove a general form of the electromagnetic reciprocity theorem due to Lorenz which states that fields E 1 , H 1 and E 2 , H 2 generated by two different sinusoidal current densities respectively J 1 and J 2 of the same frequency, satisfy the condition ∫ ∫ V [ J 1 ⋅ ⋅ E 2 − − E 1 ⋅ ⋅ J 2 ] d V = ∮ ∮ S [ E 1 × × H 2 − − E 2 × × H 1 ] ⋅ ⋅ d S .