The effect that the valid resonance frequency (the measured resonance frequency ) offsets fundamental resonance frequency which is used as reference, is attributed to the presence of higher-order transverse modes in the cavity. The effect,which is mainly due to the non-orthogonality of these modes that is caused by a limited aperture or an inhonogeneous surface of the detector, has been carefully studied. The results of calculation are discussed and the relationship between the resonance frequency offset and the parameters of the cavity is generalized. Methods to minimize the effect are suggested.