doi.org/10.1088/0022-3735/1/2/423
J. Phys. E

Vermeer, A., & Ravenstein, A. A. (1968). A method for the measurement of small shifts of the resonance frequency of a microwave cavity. J. Phys. E, 1, 152–154. doi:10.1088/0022-3735/1/2/423