Effects of a non-matching impedance between the transmission line and the antenna

  • Thread starter Ionito
  • Start date
14
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Besides the obvious inefficiency (waste of energy) of a non-matching impedance between the transmission line and the antenna for a RF transmitter circuit, what are the additional effects?

Is it safe to say the reflected portion of the signal returning to the source will also cause distortion in the signal? If this is the case, what is the bibliography which I can use that explain in details the amount of distortion caused by the non-matching impedance?

Thanks.
 
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The energy reflected from an unmatched load returns to the output transistor and heats it up. If the amount of energy is sufficient such as from an open or short on the output it can destroy the output transistor. In higher power circuits the amount of reflection is detected and used to lower the output power in order not to damage the circuit.

Apart from that, even a small amount of refection can change the output impedance and move it into an unstable region and cause the output stage to oscillate. I once designed an exciter with a good 50 ohm output match that was unconditionally stable in band, yet when attached to a power amp with a 50 ohm input impedance it oscillated. Why, because outside of band the power amp input appeared open and the small amount of out of band energy escaping from the exciter was reflected and caused it to oscillate.
 

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