[Elecraft] KPA1500 SWR Tolerance

[Bob McGraw K4TAX]

SWR only tells a small part of the story.   Remember, most all systems 
measure and display only the ratio of forward voltage to reflected 
voltage on the line.   Thus it has little to do with actual impedance, 
specially in a more common reactive environment.

But for most folks, it is a good indicator of match as long as the 
system isn't too far away from 50 ohms, one way or the other, and there 
isn't too much common mode current flowing.

73

Bob, K4TAX


On 3/16/2020 9:43 PM, K9MA wrote:
> 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
>