A microwave interferometer for the measurement of small phase angles

A microwave interferometer has been developed to measure small phase changes produced in a microwave beam traversing a fully ionized gas. The interferometer can detect phase changes of 1.2° with a repeatability of 0.1°. It has been calibrated for phase angles in the range 5° to 40°, with an accuracy to 6% over the frequency range 8080-11 510 Mc/s. The interferometer is insensitive to absorption of energy from the microwave beam by the ionized gas, and used a balanced output to reject electrical interference. The interferometer has been used to estimate the electron density of a tenuous plasma (4 × 10 10 cm -3 ) over a 10 cm path length, with a time resolution of 10 -6 sec.