While a high frequency source is used for measuring the ground impedance, there are several factors having an effect on the measured value. A primary factor of the measurement error is the ac mutual coupling between current and potential test leads. The mutual coupling causes the test current to induce a voltage into the potential test lead that adds to the actual ground potential rise and produces a significant measurement error as the length of the test leads paralleled is prolonged. In order to avoid the mutual coupling, it is recommended that the ground impedance be measured by angled arrangement of test leads. The mutual impedance due to the inductive coupling with an angle of $90^{circ}$ was calculated at $0^{circ}$ by Campbell/Foster Method. With an angle of $180^{circ}$, the mutual impedance was calculated large value enough to introduce a fairly large margin of error, however, the measured value of ground impedance was close to the value at $90^{circ}$.