[Elecraft] Where to put the wattmeter

[Fred Jensen]

and I'd buy one if it would do a trapezoidal pattern.  Judging TX signal 
quality looking at the time-domain envelope is pretty hard, but I can 
judge straight sides on the T-pattern

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

Jack Brindle via Elecraft wrote on 4/19/2023 1:06 PM:
> The original poster was using a sensor connected to a P3. For me, the big use for this is to view the modulation pattern of my transmission. I have other devices (like the KPA500 itself) that tells me the power. And if I really want to see the match on the antenna feed (which for my tribander should be close to 50 ohms), I will use my W2 and sensor between the KAT500 and the antenna feed line.
> I doubt there will be much distortion in the modulation pattern with a mismatch, though, Just don’t want to surpass the KAT500's 10:1 maximum rating.
>
> OK, so truth be told, I use it to make sure my signal is going out. And to see the nice pictures it produces…
>
> 73,
> Jack, W6FB
>
>
>> On Apr 19, 2023, at 2:42 PM, Al Lorona <alorona at sbcglobal.net> wrote:
>>
>> 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
>>
>> ____________________________________________________
>> 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
>>
>>
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