[Elecraft] New K3/K3S Beta Firmware, rev. 5.57: Faster response to high reflected power

[Bill Leonard N0CU]

I frequently wonder just how much protection a typical SWR protection circuit
can provide against an instantaneous discontinuity if it relies totally on a
feedback approach. If the response time of the feedback circuit isn’t faster
than the risetime of the voltage at the output transistors that results from
a sudden failure, then it isn’t apparent to me how this approach can provide
reliable protection. A well-designed feedback circuit can provide adequate
protection during tune up into a high SWR. It can also provide adequate
protection during CW or voice modulation since the bandwidth, and therefore
the risetime, of the transmitted signal is limited by the transmitter
circuitry. However, the voltage rise due to a sudden failure in an antenna
or transmission line connection could conceivably result in a risetime in
the 100 nsec range at the drains/collectors of the output transistors. I
have never seen any specs on the response time of any SWR protection
circuit, but I would be surprised if any of them can even approach the low
nsec range. If the failure occurs when the transmitter is at full output,
then I would expect the voltage to quickly exceed the max rating on the
output transistors. This is clearly a worst-case scenario that probably
doesn’t occur very often. However, I wonder how many (if any) manufacturers
of ham transmitters have disconnected the antenna when their transmitter was
putting out full power as part of their evaluation of their SWR protection
circuit.

Bill  N0CU



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