[Elecraft] KPA-1500 ATU not working well

[David Woolley]

I'm not clear whether you are talking about the SWR before or after the 
tuner, but efficiency is determined by the load seen by the PA, which 
has two degrees of freedom (reactance and resistance), both of which can 
vary either side of the ideal.  SWR reduces this to a single variable, 
that doesn't even respect the sign of the deviation from the ideal load. 
That means that efficiency may even be better than the nominal 
efficiency at a particular SWR, whereas that same SWR can also produce a 
lowered efficiency.  (Similarly, an SWR may destroy a PA or be safe, 
depending on the fine details.)

In particular, a higher load resistance may result in a higher 
efficiency, at the expense of a lower maximum output.  There is actually 
a K2 build option, that winds the output transformer to impose a higher 
load resistance, in order give better efficiencies at low powers.

Tuning across the band will change at least the reactance component of 
the load on the PA.

Also, if input SWR isn't 1:1, I think you will find that the measured 
forward power doesn't reflect what is actually available to go out 
through the antenna.  Whilst some of it will circulate back and forth 
and eventually make its way out in the right direction, some will also 
get absorbed by the PA.

On 22/06/18 20:36, Paul Baldock wrote:
> I have found that if the KPA1500 is feeding  a largish SWR (like 1.5:1 
> as you suggest) then the drive power required to maintain a constant 
> output will vary significantly as you tune across a band. This means you 
> have to keep adjusting the power control within a single band. This 
> appears not to occur if the SWR is 1.1:1 or better.
> 
> I don't know, but I would guess a solid state untuned amplifier 
> efficiency is better into a 1:1 SWR  than a 1:5:1. This could lead to 
> the dreaded more fan noise.