Evidence and effects of higher order modes on the response of guided wave radar-based level sensors

In this work we investigate the effects of the pulse duration and geometry of the tank on the time response of the guided wave radar (GWR) level transmitter. By 1COMSOL® modelling and simulation we show that for shorter pulses below 0.5ns and larger diameters of the nozzle (above 150 mm) parasitic higher order modes, like TE11 are generated, affecting the accuracy of level measurement. This TE11 mode interferes with TEM mode creating standing waves evidenced as “ringing” in the time domain response. The experimental results shown here for pulse duration of 0.27 ns and a diameter of the nozzle of 152 mm are in very good agreement with the model.