RE: Re: K2 Power Callibration

Posted by Don Wilhelm-3 on Feb 23, 2006; 12:42am
URL: http://elecraft.85.s1.nabble.com/Re-K2-Power-Callibration-tp386863p386864.html

John,

One must be careful not to zap the diode in the RF Probe - so measuring
directly across the dummy load is good only up to 20 watts or so.  A 'scope
with a proper probe will handle the full 100 watts of the KPA100 easily.
Builders may not have a known good 'scope, and the RF Probe method must be
used with some intelligence - many folks do not know the accuracy of their
dummy load at the frequency of operation - and both the 'scope and RF Probe
methods are highly dependent on knowing the accuracy of the dummy load - it
must be entirely resistive for accurate results.

Yes, I feel that my 'scope and 1% 50 ohm dummy load (checked for both R and
X on several antenna analyzers) will provide better than 5% accuracy for any
wattmeter that I calibrate by that method.  Of course, I can do a more
accurate job at 30 watts or lower because I can read the voltage differences
on my 'scope face with greater precision (the 10 volts per division scale)
and at higher power, I end up at the 20 or 50 volts per division scale and
the interpolation becomes more coarse.

73,
Don W3FPR

>
> I think this speaks somewhat to assumptions about what the k2 builder
> will already have or purchase for the project.  I already had a good
> DMM  which agrees in the areas we checked with a really good Fluke - no
> fluke apparently.  The manual for the KPA100 suggest you may have
> problems if you callibrate with a bad wattmeter.  Soooooo, if you can
> use the rf probe which you build with the k2 kit and a DMM, why not
> suggest this as an alternative method to anyone who suspects his
> wattmeter is not good or (gasp) doesn't have one?  Or am I all confused.
>
> Incidently 5087 works so well, I can't believe the range I get both in
> and out on ssb 20, 40 & 80- meters with just the 23 foot whip.  73,
> John KI4NGH
>
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