Gary Schafer wrote:
>Since there seems to still be some confusion on the subject here is a
>nice article written by Roy Lewallen W7EL, the author of eznec software.
>
>He explains "RMS" power in layman's terms. A very good read.
>http://www.eznec.com/Amateur/RMS_Power.pdf
>
>73
>Gary K4FMX
>
>
I must agree with your Gary, that is well written, in a way that is
pretty easy for anyone with a very modest knowledge to understand. You
don't need calculus knowledge as you do to fully understand my explanation
http://www.g8wrb.org/useful-stuff/rmspower.html
although hopefully the graphs I drew, showing the instantaneous power
are usable by anyone.
The article by Roy Lewallen W7EL, shows the product of RMS current and
RMS voltage is indeed mean power, and that this is different from RMS
power. This is done with a 50% duty cycle square wave, which make it
easier to do.
Interestingly the RMS power for his square waves is sqrt(2)=1.414 x the
average power, whereas I have shown it is sqrt(3/2)=1.225 x higher for
sine waves.
One thing can cheer hams up, is that their 1 kW (average power) linears
can now produce in excess of 1200 W RMS. However, as the author of that
article above says
*
"In fact, the RMS value of power does not represent anything useful". *
so perhaps they should not get too excited about the fact their linears
are now producing 22.5% more power.
I doubt Rich is convinced, but hopefully some others will have learnt
something.
I have learnt something from all this, as up until a couple of days ago
I'd not even thought about RMS power, although one soon realises it is
not worth worrying about anyway.
The confusion reminds me of the Nyquist Sampling Theorem. 99% of
professional engineers think you have to sample a signal at 2 or more
times the highest frequency component to recover it without aliasing.
That is not true, but many engineers think it is true, and many books
say it is true.
--
Dr. David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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