The amount of leakage radiation, as well as the radiation pattern and its gain, produced by a complete-360 degrees circumferential-narrow gap cut in a coaxial cable carrying a TEM mode is determined. Leakage levels for such a typical cable of approximately 26% are predicted to occur for gaps as small as about 0.0074 lambda. The former is substantiated by measurement on a 7/8 '' diameter coaxial cable of 50-Omega characteristic impedance operating at 2 GHz. Estimates of the EMI levels produced by typical coaxial choked-rotary joints (where the chokes significantly decrease the gap leakage) operating at high power levels are then given.