Jim Reid wrote:
>
>
>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??
>
That's right. I'm not familiar with the way that instrument actually
measures RF power, but it seems likely that it is *calibrated* using CW
and a standard wattmeter. The statement about "peak voltage measured on
the line" is probably a bit of incorrect theory - they got it wrong in
the manual, but it doesn't affect the calibration of the instrument in
real life.
>Or, perhaps the "peak" voltage sensed and held by the VFD
>circuit is not the "peak envelope" voltage, per Fisher-
>White.
>
The voltage that is "sensed and held" is the rectified RF at the peak of
the MODULATION. Remember that "peak envelope" means the peak of the
modulation waveform, NOT the RF waveform.
>Or, the Fisher-White equation calculates peak instantaneous
>power, and this is not PEP ??
>
No - precisely the opposite.
>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.
>
There's no such thing as "RMS power" in this discussion - see below.
>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.
>
Peak RF voltage, at the peak of the modulation waveform. These two
things occur on very different timescales - see the illustration on Lu's
web page.
>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?
>
As I said, I don't believe there is anything wrong with the calibration
of the instrument - only with the manual.
>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!
Sorry that the penny hasn't dropped yet.
RF power is always averaged over one RF cycle (or, in practice, over
many almost identical cycles). It is not called "RMS power" but simply
"power".
If you measure the peak RF voltage, then the factor 0.707 is used to
give the root-mean-square average of the voltage over the whole sine-
wave RF cycle. Then you use V^2/R to calculate the power.
OK, so now you know how to calculate RF power in any one RF cycle.
Next, go and find the biggest RF cycle in your whole transmission - the
one at the peak of MODULATION (that's what the peak-hold circuit in the
VFD does). Calculate the RF power in that cycle. That is your peak
envelope power.
>Sounds as if the
>REAL rule is,
You'll never get anywhere by re-writing the rule to fit your own
(mis)perceptions. The definition of PEP in your license is already
correct. It's your own perceptions that need to change, until you can
see *why* it is correct.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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