[Elecraft] Re: W1 wattmeter accuracy

[wayne burdick]

Hi Vic,

Vic K2VCO wrote:

> ...does the +/- 0.5 db accuracy of the W1 mean 0.5 db of full scale, 
> or 0.5 db of the reading?
>
> So if I am measuring a 5 watt signal, is that 0.5 db of 5 watts or 0.5 
> db of 14 watts?

The former. Here's the long answer (from the designer of the W1, Bob, 
N6CM):

The accuracy of the W1 is determined by two factors: the underlying 
accuracy of the directional coupler and detector system, and the 
resolution of the display. When used with an attached computer and the 
supplied software, the W1 accuracy is typically within 0.5 dB of the 
displayed (not full scale) power. This is more accurate than most 
available power meters. If a more accurate power meter is available, 
then even this error can be reduced by adjustment of the CAL control.

When used with the built in bar graph display, the accuracy can degrade 
due the resolution possible with the 10 LEDs that make up the bar 
graph.

The bar graph is calibrated so that a LED just lights at its marked 
power.  So, when setting transmit power, if the operator increases 
power until the desired power is first displayed, the 0.5 dB 
specification applies.  However, as the power is further increased 
additional error is introduced until the next LED lights.  For example, 
if we consider a case where the error in the accuracy of the 
coupler-detector system is zero and we are adjusting to 5 watts, the 5 
watt LED will light when the power reaches 5 watts and the error is 
zero. If the power is further increased to 5.9 watts, the 6 watt LED 
will not yet be lit and the error introduced by the resolution of the 
display is 0.72 dB.  As the power is increased to 6 watts, the 6 watt 
LED will light and the error is again zero.

73,
Wayne
N6KR

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