Ground voltage and current cancellation by co-axial cable

The most frequent use of co-axial cable is to prevent electrical noise entering signals being passed between equipment. Ground voltage differences are common and add in series with the signal voltage. The screen of co-axial cable grounded at both ends induces a voltage in the inner conductor which cancels higher frequency ground voltage differences, starting from a break point of typically 2 kHz. The signal current also produces a flux external to the screen, which induces a loop current through the screen and earth path which opposes the earth current. Signal currents above 2 kHz thus increasingly return through the screen, in spite of a lower impedance ground path. An equivalent circuit explains the phenomenon and the extreme sensitivity of co-axial and oscilloscope probes to short pigtail leads. Experiments are described and data presented which justify the theory. Remarks are made on end connections, cable trays, and feeding power lines via co-axial cable.