Open resonators for the measurement of permittivity

A small modification to Yu's model leads to more consistent values of permittivity. The difference is insignificant for confocal resonators but is appreciable if the resonator is very short or very long.

[1]  Alexander Lamb Cullen,et al.  The accurate measurement of permittivity by means of an open resonator , 1971, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.