Posted by
Don Wilhelm-3 on
Mar 13, 2005; 11:09pm
URL: http://elecraft.85.s1.nabble.com/Power-Measurements-with-question-tp375733p375734.html
Fran,
I emphasize with your situation, one must know which instrument to believe -
and that is what calibration is all about. Here are some suggestions.
If your oscilloscope vertical input is properly calibrated and the 'scope
was not being used above it's frequency limit, I can only surmise that you
may have had some problem with the test setup - are you using X10 probes
that have been properly compensated? If not, loading by the probe on the
circuit that you were measuring may account for your confusing results. A
length of coax used as a 'scope probe may be OK at audio frequencies, but
will cause 'strange' readings at HF.
If you cannot trust your 'scope, I would suggest using an RF Probe instead
these are quite accurate at voltage reading above 1 volt or so (the exact
level depends on the diode and its voltage drop). You must know the
accuracy of your dummy load very closely (within 0.1 ohms should be
sufficient) if you expect reasonable accuracy in your readings.
Since you had the KAT2 installed, did you put it into CAL S or CAL P setting
before doing your test? If not, your results may not be meaningful.
You may have to re-calibrate the wattmeter pots on the KAT2 for proper power
measurement. If your initial calibration of the wattmeter was done using an
external wattmeter, I must ask if you knew how accurate it was - many will
be off as much as 20% of full scale (that is a error of up to 4 watts on a
20 watt scale) - measurements done with an RF Probe and a known non-reactive
50 ohm dummy load will be much more accurate. The dummy load can be easily
checked with an antenna analyzer.
73,
Don W3FPR
> -----Original Message-----
>
> I have completed my attempts to measure the power output from my
> K2 using an
> o'scope. I must say that my attempts were not successful. For
> some reason
> when set to 10 watts I get about twice the voltage as when set
> for 5 watts.
> At 15.2 I get about 3 times. This does not make sense because of the V*V
> term would indicate 9 times the 5 watt output when set for 15
> watts. I know
> that this cannot be true!
>
> Now I do have a KAT2 installed and as I understand it this should improve
> the accuracy of the power output because of the more accurate power
> measurement. I set the K2 to display output power and got the following
> results:
>
> MHz Req=5.0 Req=10.0 Req=15.2
> 3.6 6.4 12.6 16.1
> 7.1 6.2 10.7 15.6
> 10.1 5.5 11.5 15.5
> 14.1 5.6 11.0 16.6
> 18.1 5.2 11.1 14.8
> 21.1 5.7 10.8 14.1
> 24.9 5.7 11.0 14.4
> 28.1 5.7 11.0 14.1
>
>
> It certainly makes sense that when requesting 15.2 watts I get
> only 14 watts
> at higher frequencies, but why would I ever get readings that are
> significantly above the requested output power?
> I thought that this reading is what tells the controller how much power to
> output.
>
> This is certainly a puzzle to me.
>
> Fran
>
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