[Elecraft] K3 6 meter power output

[Don Wilhelm]

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
>   
>