[Elecraft] wattage on k3 vs kpa500

[Rick Bates]

Here's my take:

Most if not all RF power meters will only be accurate at a 1:1 SWR (when
there is resonance).  The actual power can more accurately determined by
reading the voltage and current and then only if the amp efficiency is
known.

It's likely that neither the K3 or KPA500 meter is entirely accurate; the K3
would sense the 'bad' SWR and start a power foldback and the amp is a random
location on the feedline away with the mismatch causing a sensing error.
(Adding another random feed length would likely give a different reading.)

>From memory, at 3:1 you have approximately a 25% 'reflection'.  I didn't do
the math to verify, but in this case happens to be the same as shown as
added power in the KPA500.  Perhaps someone else can add more.  

The K3 is producing the power and should be the more accurate reading.
However, an improvement of 3:1 should be obtained before either the K3 or
the more importantly the KPA500 applies any significant power (in fact the
amp will 'complain' and switch to standby at ~2.1:1).  That higher SWR in
the real world comes back as heat, which is bad and shortens the life
expectancy of the final(s).

Make sense?

Rick WA6NHC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Richard
Sent: Wednesday, November 23, 2011 6:40 PM
To: elecraft at mailman.qth.net
Subject: [Elecraft] wattage on k3 vs kpa500

Hi
  Maybe some one can answer this.   I set my k3 to 60 watts and measure 60
watts on my kpa500 meter with the amp in standby trnsmitting into a dummy
load.  When I switch to an antenna with a 3:1 swr the k3 still says 60 watts
but the kpa500 now reads  80 watts  with the amp still in standby.   So
which is correct  or neither ?
Thanks Richard
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