Re: New K3 built - can I reset the 5w calibration?

Posted by Don Wilhelm-4 on Dec 08, 2011; 11:09pm
URL: http://elecraft.85.s1.nabble.com/New-K3-built-can-I-reset-the-5w-calibration-tp7076112p7076282.html

Tom,

OK, it seems now is a good time for me to insert my periodic "wattmeter
accuracy" rant!  Sub-titled -- "Technician, know your tools but know
even better their limitations" OR - "No instrument can be relied on
until it has proven it is capable of telling the truth under the
conditions of use".

If you want quite good accuracy at the  5 watt level, I suggest you use
a 'scope with a 10X probe connected across the dummy load.  The peak to
peak RF voltage can be easily measured by the vertical deflection of the
'scope.  To convert that peak to peak voltage reading to power (when the
dummy load is 50 ohms), use your calculator - enter the peak to peak
voltage, square it, and divide by 400.

If you do not have an open dummy load that makes clipping the 'scope
probe across it easily, use a Tee adapter at the dummy load end so you
can gain access to the center conductor.

40 volts peak to peak is 4 watts - you might want to use that instead of
5 watts since it is even and easy to see on the 'scope face.  If you
really need 5 watts, adjust for 44.72 volts peak to peak.

In fact, you can use the 'scope probe across the dummy load up to any
level permissible by your 'scope and probe.  My probes will handle 100
volts peak easily, and that is the 100 watt level.  The accuracy is only
limited by the accuracy of your dummy load (mine is within 1%) and your
ability to interpolate the voltage deflection on the 'scope.  I might
suggest that you can come within 5% of the actual power level, which is
much better than analog power meters whose accuracy is expressed as a
percentage of full scale.  So a freshly calibrated Bird wattmeter with a
100 watt slug is spec'ed for 5% of full scale - OK, that is a 5 watt
potential error - and that applies anywhere on the scale - so trying to
read a 5 watt power level with a Bird and a 100 watt slug is an exercise
in futility - use the 'scope, it is a LOT more accurate.
OTOH, many recent digital wattmeters will be spec'ed for accuracy
anywhere within its range rather that as a percentage of full scale used
with the analog wattmeters.

The (Vp-p)exp2/400 formula (actually [Vp-p]exp2/8R) does not seem to be
well know.  I leave it "to the student" to derive it from standard
formulas for power, voltage and resistance.  Hint - use SQRT2 rather
than the 1.414 or 0.707 approximations when converting peak to peak
voltage to RMS - it is an exact formula, the squares and square roots
cancel.

73,
Don W3FPR

On 12/8/2011 5:12 PM, TJ Campie wrote:

> I finished building a K3 for the radio club at work and I thought I'd take
> the bull by the horns and attempt to improve the factory setting for the 5w
> calibration on the 100w Bird watt meter I had.  I couldn't get it to go up
> the the point that looked like 5w on the meter regardless of how much I
> turned the knob on the K3.  The setting ended up at 10 (the minimum) but I
> think it must have been more correct from the factory.  I *think* it was up
> at about 37 but I don't remember - is there a way to reset it to the
> factory setting?  I don't have access to a 5w accuracy meter.
>
> Otherwise, the build went great and was a blast :)
>
> TJ W0EA
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