Re: KPA-1500 ATU not working well

Posted by David Woolley (E.L) on Jun 23, 2018; 3:58pm
URL: http://elecraft.85.s1.nabble.com/KPA-1500-ATU-not-working-well-tp7642239p7642282.html

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.


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