Posted by
Don Wilhelm-4 on
Dec 10, 2009; 1:53am
URL: http://elecraft.85.s1.nabble.com/W2-Questions-tp4143062p4143252.html
Phil,
The W2 wattmeter is similar in its detector accuracy to the Tandem Match
(by John Grebenkemper KI6WX) in that its accuracy depends on matching
the detector diodes with the compensation diodes.
The "Gold Standard" of power measuring devices is the HP436A wattmeter,
and it has a stated accuracy of +/-0.05 dB. That amounts to a +/-1.16%
accuracy - you are not going to get much better than that lab instrument
in an amateur grade instrument.
The Tandem Match that KI6WX built tracks the HP436A within +/- 0.5 dB
over a range of 10 mW to 100W (your 11.2% error), and also tracks the
HP436A within +/-0.1 dB over a 1W to 100W range for a 2.33 % difference.
Power measurement is tough on accuracy as expressed in percentage. Most
ham grade wattmeters specify 20% of full scale. The Tandem Match and
the W2 wattmeter are percentages of actual readings.
Since power is normally best expressed in dB (because the apparent
signal strength is related in dB), a specification of 0.5 dB is not bad
at all.
BTW - I believe that is the accuracy of the power reported using the PC
link. The resolution of the LED scale is not adequate to indicate the
degree of precision available.
The directional coupler will have frequency dependencies as well as
accuracy implications.
73,
Don W3FPR
Phil Hystad wrote:
> A few questions about the W2 meter:
>
> 1. The posted accuracy is +- 0.5 dB which I calculate as about +- 12 %. Is this full scale accuracy and if so is half scale possibly more accurate or is this the most accurate the meter is likely to be. I am not even sure if it makes a difference for full scale or half scale for a digital meter so that part of my question may be moot.
>
> 2. What does it take to achieve an accuracy better then 5 % (for example), and is it possible to achieve an accuracy of 1 % or better? I am curious as to where the money needs to be spent in achieving such accuracy. Is it in the directional coupler?
>
> 3. I am thinking that the company that can build the best receiver on the market (the K3) can also build the best meter. So, would Elecraft principles consider a future super-accurate, best on the planet, amateur radio RF/SWR power meter? Oh, I think a current meter would be cool too.
>
> And, I know that having meter accuracy is of little importance in ham radio but there is some kind of deep seated quirk in me that wants accuracy just for the heck of it. Certainly 5 % is achievable, right?
>
> 73,
> phil, K7PEH
>
>
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