[Elecraft] K2 power out increase with higher SWR

[David Woolley]

Gary D Krause wrote:
> I've 
> noticed that with my K2 the power increases with highter swr.  While 
> adjusting the tuner as the swr goes up the power increases and sometimes 
> doubles and when going down the power goes down.  Just last night, I was 
...
> watts because, of the higher swr.  I've noticed this before.  Is this 
> normal?  It seems to make more sense for the opposite to happen.

Even if true power were being measured, I think you would be making the 
mistake of assuming that the output impedance of the PA is 50 ohms. 
Whilst my RF engineering knowledge isn't good enough, and I don't have 
SPICE tools, to work out the value in this case, it is almost certainly 
not true here.  In general PAs are not operated according to the maximum 
power transfer theorem, even though many simple text books may claim so.

Also, SWR is a rather crude measure.  It's a simple mathematical 
transformation of the ratio of forward and reverse power flows into a 
measure that was probably only directly meaningful with very old methods 
of measuring it.  As a figure of merit, the straight ratio would be just 
as good.

Neither really tell you what the threat to your PA is, because they 
ignore phase, because it is difficult to measure.  In particular, the 
same SWR value with different phases might cause both increases and 
decreases in net forward power.  Also the same SWR with different phases 
could result in either voltage or current breakdown.

Incidentally, for most combined SWR power meters, the power measurement 
is forward power, which exceeds the net forward power by the value of 
the reverse power.  As a result, they overestimate transmitted power 
when the SWR is not 1:1.  I don't know if the KAT2 firmware applies this 
correction.


-- 
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.