amplifier - Problems when loading an Ultrasonic transducer (reflected power)

I have a very simple setup that is giving me problems I cannot understand:

Apparently, the reflected power is the same as the power generated by the amplifier (both measured by the amplifier, although I double checked with a power meter). The load, a 50 ohms ultrasonic transducer in this case, seems to be completely mismatched. HOWEVER, this problem sometimes disappear after some minutes (5-10 min) without changing anything.

I have switched all the equipment, transducers and cables (+connections). Can someone rationalize this behavior? Could it be due to the driving frequency?

Some details if necessary:

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  The function generator is one from Agilent which I double checked by means of an oscilloscope.

  
  


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  The RF amplifier is from Electronics & Innovation

  
  



• The ultrasonic power transducers are from American Piezo. The behavior described is the same for everyone of them.

Note: Obviously, my background in electronics is very limited.

Answer

As @jms mentioned, the problem was the (very) narrow frequency range of the transducer.

I could find the freq. sweeping and monitoring the power consumed.