[Elecraft] Where to put the wattmeter

[Al Lorona]

Yes, if you're willing to do the math in your head, or can set up your fancy wattmeter to do the math for you, this'll work.

But the main reason why I don't like the idea of putting the wattmeter (or sensor, actually) in a non-50-ohm location is the additional uncertainty due to the mismatch(es). In the original poster's situation, there would be a mismatch at the interface between the input of the sensor and the KAT-500's output, and another mismatch at the interface between the output of the sensor and the input of the feedline. 

How large would the additional uncertainty be? It depends, but assuming that the wattmeter sensor has a (very good) match of 1.2:1, and the antenna has 4:1 -- which is not out of the question for many antennas-- the additional uncertainty is about ±1.0 dB.

This means that if the actual forward power were 500 W, the wattmeter may read (after doing the math mentioned above) as much as 1 dB lower, which is 397 W, or 1 dB higher, which is 630 W. That's a whole heck-of-a-lot of error there that many hams wouldn't tolerate.

Nobody had raised the issue of uncertainty and that's why I wanted to point it out. Y'all can have the last word on this.

R,

W6LX/4

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I'm fairly sure that (forward power) - (reverse power) gives the correct
nett output power, before cable and antenna losses.  I'd need to review 
the maths to be sure.  Most reflected power ends up re-re-reflected, as 
additional forward power.  A high SWR will giver reflected power almost 
as high as forward power.

-- 
David Wooley