Ground voltage and ground current cancellation by coaxial cable

The most frequent use of coaxial cable is to prevent electrical noise adding to signals passing between equipment. Ground voltage differences exist and appear in series with the signal. The screen of a coaxial cable grounded at both ends induces a voltage in the inner conductor which starts cancelling ground voltages at about 2.5 kHz and can produce -60 dB cancellation at 10 MHz. The signal current also produces a flux external to the screen, which induces a loop current through the screen and ground path. Above 2.5 kHz, the loop current starts opposing the returning ground current, redirecting it from the lower impedance ground path to the screen, thereby reducing the external magnetic field. An equivalent circuit explains the phenomena and the extreme sensitivity of coax and oscilloscope probes to short pigtail leads. Experiments and data are described which justify the theory. Remarks are made on pigtails, cable trays, and feeding power via coax.