I was wondering, assume you have a 144MHz FET based power amplifier and you drive it at full input power without a load connected what actually happens to the device? Do you have reflection of the expected output power?

My understanding is that reflections only occur when the length of a wire/trace is longer than lamda/10 (point at which you can model the wire as a transmission line), so I assume reflection of the output power will only take place if you connect a wire of this approximate length to the output?

  • \$\begingroup\$ In normal conditions use, a PA is "quasi" at the limit of power. "Line or no line", If you don't use a load for the RF power amplifier (which is unprotected by a "reflexive" system), it is quite clear that it should be overloaded at 2 times its nominal power. The problem is only the fact it can "support" the overload. Note also that the destructive effect can be very fast (some ms). \$\endgroup\$
    – Antonio51
    Jul 21 at 9:20
  • \$\begingroup\$ But you should also read this ... advancedenergy.com/globalassets/resources-root/white-papers/… \$\endgroup\$
    – Antonio51
    Jul 21 at 9:35
  • \$\begingroup\$ @Antonio51 I guess I'm struggling to understand why there would be reflection if there is effectively no transmission line due to the length of the line being less than lamda/10. For example does a 1kHz 1W signal reflect the same amount of power due to an impedance mismatch than a 1W 1GHz signal? \$\endgroup\$ Jul 21 at 10:10
  • \$\begingroup\$ @Antonio51 Thanks for the reference though! I appreciate it \$\endgroup\$ Jul 21 at 10:50

1 Answer 1


Reflections occur with no load, whether you have any line connected or not.

Whether the amplifier survives this abuse is a function of how it has been designed.

Some amplifiers are designed with sufficiently rugged output devices to tolerate the potentially 2x voltage and 2x current operation caused by using it into no load on an arbitrary length of line. Needless to say, these are more expensive than amplifiers designed for only the nominal voltage and current in the output devices, which require a well-matched load.

These better amplifiers will have been marketed as 'safe into any load'. You will also find intermediate amplifiers, sold as 'safe into VSWRs below x'.


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