A pulsed GTEM system for UHF sensor calibration

A system for measuring the sensitivity of electric field sensors at UHF using a novel gigahertz transverse electromagnetic (GTEM) cell is described. The system was developed to enable calibration of UHF couplers that are used to detect insulation defects in gas insulated substations. The frequency response of a sensor is determined using fast Fourier transform (FFT) processing of its output voltage when it is subjected to a step electric field with a rise time of less than 300 ps. Provided the step response of the sensor is relatively short, the absorbing termination of the GTEM cell can be dispensed with, simplifying its construction. The need for impedance matching at the cell input can be eliminated by using a signal source that absorbs the reflected energy. In addition, the use of a wire septum offers the potential for improving field uniformity. A description of the measurement system is given, and some typical results are presented.