Hi I was wondering if someone can help me with an issue I don't understand.  On the output drain of an RF FET transmitting at a VHF frequency under exceptional circumstances, i.e. an open circuit I have measured with an oscilloscope 6 X the FET supply voltage pk-pk, it does not die down and is a steady RF oscillation.  I understand that poor mismatches can cause a standing reflected wave which can cause higher voltages but I don't understand how it is possible to achieve 6X the FET's supply voltage.  Any ideas, thanks in advance

If the Fet is oscillating  usually it  will start to draw more current then normal operation this is one factor you should be aware of since depending on the Q of the load the bias current can run up.  Also note that the circulating current in a resonant load (typically called a tank circuit) fed by a transistor in oscillation can be significantly larger than the transistor bias current, especially if the Q is high.   It is this circulating current multiplied with the equivalent parallel Resistance that sets the voltage.   So the higher the Q the higher the voltage due to higher current and higher parallel equivalent resistance.  

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