Application of impedance measurements in an investigation of a waveguide thermistor mount

The concept of waveguide impedance is used to develop a simple equivalent circuit for a thermistor bead mounted in the centre of a rectangular waveguide. Impedance measurements on such a mount at a frequency of 3GHz show that the self capacitance of the bead is 0.2¿0.3pF. This result affords a possible explanation of the errors experienced with some thermistors when measuring microwave power in terms of a d.c. calibration.