A method of finding a cavity coupling factorQxfrom the separation in wavelength between branches of tuning curves obtained by moving a short circuit in the feeder is described. This is complementary to a derivation from slopes proposed earlier. The alternative method is suitable over a large range ofQx(from about 30, where the formulas obtained forQxare least accurate, to 105or more). The highest value ofQxthat can be resolved is limited by losses in the cavity, feeder or short circuit.Experimental results obtained for a magnetron coupled to a waveguide are given, andQxfound by the above method is compared with that derived from standing-wave measurements. Good agreement in the values ofQxwas obtained.