Bob,
Do the math for the percentage accuracy for each wattmeter and I think
you will have a reasonable explanation.
Then you can decide which one that you would rather believe.
The accuracy percentage for any instrument is an important parameter,
and understanding the conditions under which the instrument is specified
for will tell you how much you can trust whatever its indication may
be. In general, wattmeters are woefully inaccurate unless you have
precision instruments, and the Bird 43 wattmeter, while well regarded,
is not as accurate as many hams give it credit.
A newly calibrated Bird 43 wattmeter is supposed to have a potential
error of 5% of full scale - note that the slug must conform to the
frequency range you are trying to measure. So if you are using a 250
watt slug, the error can be as much as 12.5 watts on either side of the
indicated power - at 100 watts indicated by the Bird, the actual power
could be anywhere between 87.5 watts and 112.5 watts. If the Bird and
slug have not been recently calibrated, allow more like 10% for the
possible error unless you have made reliable measurements to justify
otherwise.
The K3 wattmeter can be calibrated following the instructions in the
manual, and if the calibration has not been done, it could be off by as
much as 15%. Note that the percentages for the K3 are my estimates
based on my own experience and judgement - I have no information from
Aptos on this matter other than what is in the K3 specification - the K3
wattmeter is not specified.
If you have previously calibrated your K3 wattmeter, the potential error
will be as large as the potential error of the instrument you used for
calibration plus some additional percentage for a reading error of the
instrument used, plus a small percentage (up to 5%) for differences in
the K3 detector at different frequencies. If you have a factory built
K3, the wattmeter was calibrated at the factory and you can assume a 5%
error at the actual indicated power - if the K3 indicates 100 watts, the
actual power can be between 95 and 105 watts.
Ron Eau Claire had a good explanation of wattmeter accuracy on this
reflector yesterday under the subject line of "K3 Watt-meter question!"
73,
Don W3FPR
Robert P. Ward wrote:
> Hi List
>
>
>
> I am wondering how to correct the power output on 6 meters.
>
> The K3 claims 120 watts and the Bird wattmeter into a 50 ohm load
>
> shows 90 watts. Also with the Bird showing 50 watts, the K3 shows
>
> 68 watts. Any suggestions?
>
>
>
> Bob NQ3N
>
>
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May 29, 2009; 6:32pm
