> 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
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