[Amps] Measuring RF Power

[David Kirkby]

R. Measures wrote:

>
> On Mar 25, 2005, at 2:00 PM, David Kirkby wrote:
>
>> R.Measures wrote:
>>
>>>
>>>>
>>>> You can not find RMS power by multiplying RMS voltage by RMS current.
>>>>
>>>
>>> I disagree. .
>>>
>>
>> Rich, I think you are wrong.  Here's  my reasoning - I may well be 
>> wrong, as maths never was a good point of mine.
>>
>> You need to understand calculus to properly work out the RMS of an 
>> arbitrary waveform, as you probably know.
>> Hence this explanation will really only be of use to someone who 
>> knows integration. But I can't see a simple way of computing the RMS 
>> value of the power, without it. People will just argue, but if you go 
>> back to first principles, you should get the right answer.
>
>
> The right answer is that RMS current x RMS potential = AC P

Any chance of you substantiating what you have said?

You stated you disagreed with Gary (K4FMX) who said

 > You can not find RMS power by multiplying RMS voltage by RMS current."

Now I think you are wrong to disagree with  K4FMX. I don;'t believe a 
simple multiple of the RMS values of voltage and current gives the rms 
power.

If you have 1V peak across 1 Ohm you have an RMS voltage of 0.707 V, and 
RMS current of 0.707A. If you multiply the RMS voltage by the RMS 
current  you get 0.5W. (0.707*0.707=0.5). But I computed, and have fully 
justified, that the actual RMS power is 0.612273 Watts for a 1V peak.  I 
don't think the term RMS power is very useful, but if you want to use 
the term, then at least use it correctly.

I stuck a few notes at

http://www.g8wrb.org/useful-stuff/index.html

where you will find an html page on it

http://www.g8wrb.org/useful-stuff/rmspower.html

but its best to look at http://www.g8wrb.org/useful-stuff/index.html
, as you can find other file formats, that are more useful.

You have rather a habbit Rich of saying things without justification. 
Tell me where my analysis is wrong? Where is my mistake? I'm not saying 
there is no error (I admitted maths is not a strong point of mine) - but 
at least I don't dismiss other peoples comments and just think mine is 
right all the time. I try to substantiate what I say, rather than come 
back with one-liners that you expect everyone to take as gospel.

-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/