Mutual impedance between an overhead and a buried line with earth return

A simple approximated formula for the evaluation of the mutual impedance between an overhead line and a buried line with earth return is proposed. The paper presents the deduction of this formula starting from the exact one with the relevant approximations involved. Then, some tests with the exact formula have been done showing a good agreement for engineering applications. Moreover, the calculations also show that this formula, due to its higher range of applicability, seems to be preferable to another approximate formula appearing in the CCITT's 1989 Directives.