[Elecraft] Maximum SWR for KPA-500 Amplifier without Tuner

[Jack Brindle]

Byron;

The KPA500 uses reflected power for its fault determination. It will protect itself at
60 watts reflected. As explained in the manual, the protection inserts a 3db pad in
the RF circuit, effectively dropping the input power in half. If the amplifier continues
to see 60 watts reflected it will go into the fault condition. Also, if it sees 100 watts
or more of reflected power for a short time, it will immediately go into the fault mode.
The only hard SWR limit is 18:1, which is purely a protective scenario for extremely
bad loads (short- or open-connections).

What all this means is that the protections depend on the transmit power. If you are
running 540 watts, an SWR of 2:1 will give you 60 watts reflected, which is the
threshold to cause the KPA to start to protect itself. At 240 watts the threshold will be
seen at 3:1 SWR, at 135 watts it is 5:1. You can calculate them for yourself with
the SWR / power formulas available in the ARRL handbook.

So the short answer is that the SWR value for fault depends entirely on how much
power you are getting from the amplifier.

Jack Brindle, W6FB



> On Sep 13, 2015, at 11:44 AM, Byron Peebles <NZ3O at arrl.net> wrote:
> 
> The external amplifier that I am planning to replace handles an SWR of 3:1 without a tuner.
> What is the recommended SWR maximum without tuner for the Elecraft KPA-500?
> 
> The only mentions of SWR in the manual are "connect to load of 1.5SWR or less" and the hard fault 9.
> Does this imply the amp will fault at an SWR of 2.0-3.0 without an external tuner?
> 
> 73, Byron
> 
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