Amps
[Top] [All Lists]

Re: [Amps] PEP power..

To: "Gary Schafer" <garyschafer@comcast.net>,"'Ian White GM3SEK'" <gm3sek@ifwtech.co.uk>, <amps@contesting.com>
Subject: Re: [Amps] PEP power..
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Mon, 16 Oct 2006 14:02:24 -0400
List-post: <mailto:amps@contesting.com>
> To add a little more clarification to Ian's nice 
> explanation;
> The AVERAGE power that Ian is describing is what many 
> (incorrectly) refer to
> as RMS power of the RF.



ALL the power is the effective heating or work power, and 
since the RF cycle is a sine wave it is derived from the RMS 
voltage and/or current. If we take the peak voltage of the 
RF sinewave (it is always a sinewave) and multiply it times 
.707 and use E*2/R, we have the power. Roy and others may 
not like it and indeed it is technically incorrect, but it 
is so well established and makes so much sense to the layman 
RMS will always be the words people use. This is true even 
if average or mean is technically more accurate.



As a matter of fact using the word "average" creates more 
problems than it solves.



The problem comes in because Hams often call "average power" 
indicated by a power meter when looking at a varying 
envelope RMS power. We read average and peak envelope power, 
not RMS and peak envelope power.  The average and the peak 
are based on the work power of the sine waves, one being the 
highest effective power of the envelope and the other being 
the average power of the envelope.



To a lesser extent Hams assume peak envelope power is where 
peak RF voltage is multiplied by peak current, or where 
instantaneous peak current or peak RF cycle voltage is used 
with resistance to find some fictitious power that really 
isn't related to work.



When we say peak envelope power is "based on the average 
power", we would have to say the "average power" is the 
average of the average power of each RF cycle over a period 
of time. Either way is confusing.



It would be accurate and less confusing to say peak envelope 
or average power is the peak or average of  the effective 
power, work power, or mean power when dealing with the RF 
cycle power, even though it is indeed based on the RMS 
voltage or RMS current of each RF cycle.



> Peak envelope power has nothing to do with "peak power". 
> It is all to do
> with average rf power.

Peak envelope power is the highest envelope value of power 
that does work. It is all about the RMS voltage of the 
individual RF cycles times the RMS current of the RF cycles.



Look at this example. If I pick a capacitor I have to choose 
the rating based on the peak RF voltage. That is the voltage 
seen on a scope at the crest of the highest sinewave, or on 
a meter that charges to the highest crest of the RF sine 
wave cycle. That voltage is not the same as the effective 
voltage that heats a resistor or does some other form of 
work. We'd have to use the RMS voltage to calculate power.



This is why people use the term RMS power, not because the 
power is RMS but because the power is derived from the RMS 
voltage and/or current rather than peak voltage and/or 
current.



The reason people get confused is because many engineering 
terms are ambiguous. The important thing is people get the 
correct picture in their minds.



73 Tom


_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>