Power Measurement Question

Hi Everybody;

 Don just reminded me to check the math in the DL1 "Manual" and update it if
necessary. Good Idea!
 I also got out the W1 and checked my K2 power output Vs the W1 and my
DVM voltage reading at the DL1 test points. They all read about the
same *within
10% but*
which reading would be the most accurate? I suspect it would be the DL1 but
it would
be nice to know for sure.
 Ok RF Engineers, all thoughts are welcome!

Best Regards, Bob K7HBG
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[Don Wilhelm-4]

Bob,

The answer is - "It all depends ..." -- read on if you want "the rest of
the story".

I constantly rely on the RF voltage produced across a known accurate 50
ohm resistor for my power measurement calculations, so that is the
method I depend on.

The W1 is quite accurate, but there is one "gotcha" - the power display
is granular.  The 4 watt level is indicated when the 4 LED *just*
illuminates, and there is no additional indication on the LEDs until you
get up to 5 watts.
Yes, the W1 software provided by AB3AP's software can indicate the 4 to
5 watt level in increments down to .01 watt,  but there is no assurance
of exactness at that level of refinement.  The W1 manual claims the
normal (manual method) for calibration results in an accuracy of 0.5 dB,
and I have no reason to disbelieve it - BUT at the 4 watt level,  that
0.5 dB translates to a potential difference of up to 0.488 watts. - One
must conclude from that fact that at 4 watts indicated by the W1, the
error can be almost a half watt.  So be careful about relying on the
number of significant digits that can be displayed unless your
calibration technique is adequate to justify the number of digits that
you see.  Note well that at 4 watts indicated by the W1, the 0.5 dB
accuracy says that the actual power can be anywhere between 3.52 watts
and 4.48 watts (note that I dropped the last significant digit from the
0.488 watt = 0.5 dB value).  If you have not calibrated your W1 to
greater precision than that 0.5 dB accuracy, you will just have to
accept that fact.  This is in no way to be construed as a deficiency of
the W1, for it is more accurate than the majority of wattmeters commonly
available to the amateur community.

The DL1 accuracy does depend on the exact value of the resistors used
for the dummy load.   The DL1 states a power measurement accuracy of
10%, so again, the result is about the same - at the 4 watt level, an
error of up to 0.4 watts is possible - so if it indicates 4 watts, it is
somewhere between 3.6 watts and 4.4 watts.

Actually an error of 0.5 dB or 10% is very good in the world of power
measurement where many (even most) commonly available amateur grade
wattmeters can only claim 20% of full scale.  That means that on a 100
watt scale, the error can be as much as 20 watts!  Even the well
respected Bird meters are speced for 5% (but only after calibration) of
full scale - and that means with a 100 watt slug, a recently calibrated
Bird can have an error of 5 watts.  How many Bird wattmeters in the
hands of amateurs have just come out of a calibration lab and have the
sticker to prove it?  Both the Elecraft W1 and DL1 will provide better
accuracy than a recently calibrated Bird wattmeter, but like everything
else, they have their limitations (based on the specs).  All I am asking
is that these limitations be understood and that you have some idea
about what that means.  "Trust no tool until it has proven its worth".

I personally use 1% precision resistors for dummy loads when testing
power accuracy, and even at that, I will claim no accuracy better than
5% just due to the granularity in my methods of determining the RF
voltage across these precision resistors.  All sources of error must be
taken into account.  RF power measurement is no trivial matter when
accuracy is desired - zero error is unattainable with any measurement
method that I know about.

73,
Don W3FPR

k7hbg @dslextreme.com wrote:

> Hi Everybody;
>
>  Don just reminded me to check the math in the DL1 "Manual" and update it if
> necessary. Good Idea!
>  I also got out the W1 and checked my K2 power output Vs the W1 and my
> DVM voltage reading at the DL1 test points. They all read about the
> same *within
> 10% but*
> which reading would be the most accurate? I suspect it would be the DL1 but
> it would
> be nice to know for sure.
>  Ok RF Engineers, all thoughts are welcome!
>
> Best Regards, Bob K7HBG
>  
>

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