An improvement of the phase stabilization of a simple microwave interferometer

In order to obtain a good phase stabilization of the output signal of a simple microwave interferometer for plasma electron density measurements, it has been suggested that both paths of the interferometer should have the same electrical length. It can be shown, both theoretically and experimentally, that by deriving the output signal from the point where the first derivative of the phase to the frequency is equal to zero, a much better phase stabilization can be reached. In a 70 GHz device with an open path, having a length of 960 mm, a change of 100 MHz in the frequency and an uncertainty of 1 mm in the length of the path, cause a calculated phase alteration of 0·4°. In the point, given by the principle of equal electrical length, the same change in the frequency would cause a phase alteration of about 100°.