By Ian Darney
An experiment is described which enables transient radiation to be measured and analysed. It involves the application of a step pulse to one end a twin-conductor cable which is open at the far end and the use of a current transformer to monitor the differential-mode current. The waveform of this current is initially that of a square wave, but it quickly morphs into a sine wave. A circuit model of this configuration is created to replicate the observed waveform. Analysis of the model provides an insight into the mechanisms involved.
An interface box was first constructed in order to apply a step pulse to such a cable whilst maintaining isolation between the signal generator and the input terminals. This included a common-mode choke which provided high impedance to any antenna-mode current flow between the assembly under test and the ground terminals of the generator. The use of a current transformer to monitor the current ensured that there was complete isolation between the oscilloscope and the cable. Included in the Interface Box was a simple resistor network which provided a 50 ohm load to the Signal Generator and a low resistance signal source at the input to the cable.