[Elecraft] KPA1500 SWR Tolerance

[K9MA]

I've long been aware that my KPA1500 would not run at full power (1.5 
kW) unless the SWR was very low. (This is SWR at the input to the ATU, 
if used.) The current ATU firmware does a pretty good job of keeping the 
SWR below about 1.3:1, which usually is low enough.

In any case, I just ran a simple test on 160, where I have an antenna I 
can easily tune to 1:1, or deliberately higher. The ATU was engaged, but 
it's settings were not changed during the test. The SWR indicated was 
that at the ATU input (amplifier output).  I found that the output power 
remained nearly constant up to 1.4:1, but the current increased from 58 
to 62 A. (This with no adjustment of the K3 power, though the indicated 
input power varied slightly.) At 1.5:1, the current exceeded 65 A and 
the amplifier faulted. By reducing drive, I was able to run it at 1.4 kW 
output and 60 A.

The above is typical of my experience on other bands, etc. The effect of 
SWR on the amplifier, of course, depends on the actual impedance at the 
amplifier, so in some cases it may be more or less sensitive to SWR.

Do others see similar effects?

73,

Scott K9MA

-- 
Scott  K9MA

k9ma at sdellington.us