Measurement of surface impedance anomalies at radio frequencies

Oscillator and twin-T bridge methods for measuring small variations of radio-frequency surface impedance are described and evaluated for the investigation of Doppler-shifted cyclotron resonance of helicons in metals. Oscillator methods are found to be more sensitive; bridge methods give better separation of surface resistance and reactance, provided one arm of the bridge is stabilized or phase sensitive detection is used. The work is illustrated by the detection of Doppler-shifted cyclotron resonance of helicons in a number of metals in the frequency range 0.2-12 Mc/s.