Interesting topic on the amps reflector about
the definition of the FCC's 1500 watt PEP
limit for amateurs.
Have a look at:
http://www.pldi.net/~fisher/
Note the equation for PEP, as apparently understood
by the great minds and the FCC; there is a .707 squared
in the numerator.
Or, PEP is the product of the peak envelope voltage,
multiplied by 0.707 and that number then squared and
divided by the impedance, or in our case, 50 ohms.
RF Applications, Inc. builds the VFD RF power/VSWR
meter. This meter reads, per their unit's manual, page 7
second paragraph:
"For the purpose of this discussion, RF power refers to the
peak voltage measured on the line, multiplied by itself,
and divided by 50 (voltage squared divided by the impedance
of the load, which is watts)."
Note: no .707 in their algorithm.
Of course, this is the same as E squared divided by R,
but I think this formula must really apply to "heating power"
as it is derived from the famous I-squared-R formula for
power dissipated in a resistor, which is generated by the
effective voltage, not the peak voltage.
Therefore, it would seem the Fisher-White equation is
correct, and the RF Applications formula not quite correct??
Or, perhaps the "peak" voltage sensed and held by the VFD
circuit is not the "peak envelope" voltage, per Fisher-
White.
Or, the Fisher-White equation calculates peak instantaneous
power, and this is not PEP ??
At least the VFD is not reading PEP power but rather RMS
power, as the VFD and Bird 43, which reads RMS power
agree within their tolerances, at least in my measurements.
So what is the PEP power?
Well, per the Fisher-White equation, the correct peak
envelope voltage required to generate a PEP of
1500 watts on a 50 ohm line becomes the square root
of the product of 1500 times 50 divided by .707 squared,
or the square root of 150,000, which becomes
about 387 volts peak envelope voltage on the line.
Now what number would be read out on the VFD
because of the circuit and algorithm used with
387 volts peak on the line? Well, it is 387 squared
divided by 50, or 3000 !!! Unless, in fact, the
VFD circuit peak detector is not storing the actual
peak envelope voltage, but the RMS voltage, in
which case, the 0.707 term would not be in the VFD
algorithm, and it is not, so maybe the VFD would
read 1500 watts with 387 volts peak envelope
voltage on the line, don't know!
Now, what is wrong here? If I run SSB power such that
I can see the VFD reading 1500 watts occasionally on
voice peaks, am I really putting out an SSB PEP
power well in excess of the 1500 watt PEP legal limit?
Maybe not, lets look at Part 97's definitions again:
"97.3 Definitions.
(b) The definitions of technical symbols used in this Part are:
(6) PEP (peak envelope power). The average power supplied
to the antenna transmission line by a transmitter during one RF
cycle at the crest of the modulation envelope taken under normal
operating conditions."
Or, PEP is the "AVERAGE power supplied......"!!??
Talk about getting confused, nuts! Sounds as if the
REAL rule is, no more than 1500 watts output as
measured on an RMS reading watt meter; which
is what both the VFD and Bird 43 would read, at
least the Bird would were it fast enough, and the Bird
would read the same as the VFD during steady CW
key down.
73, Jim, KH7M
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