Level of pattern amplitude and phase ripple due to a multipath (stray) signal due to chamber wall or extraneous signal reflected as the antenna is rotated.  This is also the level of a ripple measured across a frequency range in a transmission line due to a second mismatch.  Since a Smith chart repeats at half wavelength intervals, the frequency difference between two nulls corresponds to the half wavelength distance between the two mismatches in the dielectric constant of the coax.