[AMRadio] Re: What is PEP?

D. Chester k4kyv at charter.net
Fri Feb 15 15:30:06 EST 2008

> Don,
>   It seems that we have defined PEP power pretty well:
>>"Well, PEP is defined as the AVERAGE power over at least one RF cycle at
>> the most powerful point of the envelope."
> Now, since we do not speak with sine waves, the average "power point of 
> the envelope" is going to be less than 0.636 (sine wave average) of the 
> peak >value. With many voices the average might be 0.2 to 0.5 of the peak. 
> Doesn't this mean we can increase the peak power until the PEP as we have 
> defined >it hits 1500 watts? It seems that many of us confuse peak power 
> with Peak Envelope Power. The definitions are different. If I have this 
> correct, then >unprocessed voice peaks can be increased until the PEP 
> legal limit is achieved, and the carrier level might be a lot more than 
> 375 watts. Maybe the KW1, >or Johnson Desk KW at Hi-Tap are still legal?

Now, this makes me REALLY feel for all the Hammy Hambone types out  there. 
It has ME confused!

BTW, another term for average power is "mean" power.  There is no such thing 
as "rms" power.  MEAN power is calculated, using Ohm's law, by multiplying 
RMS voltage by RMS current, as they work into a resistive load.

Rms voltage and current are the "effective" values of an alternating current 
and voltage, meaning the equivalent measurement of DC that would produce the 
same effect.  With a sine waveform, the rms value is .707 times the peak 
value.  But the mean power into a  load is also dependent on power factor, 
that is, whether the voltage and current are perfectly in phase, as they 
would be into a resistor.  In a.c.circuits with a reactive component, 
voltage and current may not be exactly in phase, so that a direct 
calculation using measured values of voltage and current may give a figure 
that is greater than the actual power.  That's why transformers are rated in 
volt-amperes, not watts.

We don't talk in p.e.p. but we don't talk in sine waves, either.  The 
loudness and interference-causing capability of a signal over the air is a 
function of mean, or average power radiated from the antenna, regardless of 
where occasional voice peaks may reach.  The average amplitude of a typical 
human voice waveform is about 7 to 10 dB below the amplitude of the voice 
peaks.  In other words, unless we use clipping or severe peak limiting, when 
our voice modulates a  transmitter 100% on peaks, the average percentage of 
modulation is more like 30%.  If you ever visit a properly running broadcast 
studio that still uses analogue, electro-mechanical VU meters, you will 
notice the readings stay well over to the left side of the scale most of the 
time to avoid hitting the red zone on peaks.

Don k4kyv

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