Hi Guys:
While watching the PEP discussion, I had to add some input.
How RF Applications, Inc. measures the power you see displayed on the VFD
(and all our other meters of this type):
We use a directional coupler that senses forward and reflected power and
rectifies those quantities for measurement by a small computing device in
the wattmeter itself. All of the peak and hold technology (which is really
good, in my humble opinion, for on-the-air operating measurements) is done
by that small computing device in the wattmeter.
ALMOST all of the terms of the math needed for power/VSWR display are done
by that computing device as well. Our manual (which is about to get more
technically detailed as a result of this discussion) starts with "For the
purpose of this discussion...". What is left out is that there is a
coupling factor associated with the directional coupler, and there is a
CALIBRATION pot that acts as a voltage divider between the coupler and the
input circuit of our wattmeter.
So, Ian, you have it exactly right - we calibrate with CW (using the
venerable (calibrated) Bird 43 as a reference). During this calibration,
any other factors (like the square root of 0.5) required in the equation
are taken in to account at that time.
For the record (all measurements approximate): if you take an RF probe and
look at the voltage on a steady RF sine wave at 10 watts (as displayed on a
Bird 43), you'll see about 31 volts (which is the peak voltage of the 62
volt peak-to-peak sine wave). If you multiply 31 by 0.707 (coupling factor
and calibration pot), you get 22. If you square 22, you get 484, and
divided by 50 you get about 10. So, I think you can see that Bird and the
rest of us are using the popularly recognized formula.
I think a lot of the confusion comes from discussion of "modulation peaks"
and such - I don't want to get in to that any further as I think that
subject has been beat to death and is as clear as it's going to be.
The bottom line is that we base our readings on what would happen if a
steady CW signal is measured - on SSB, you'll get peaks as you speak, but
if you whistle, you'll get that sine wave back and the readings will all
agree with what's expected.
And this, my friends, is what our meters do so well. We find the peak and
display it while you're talking. Sorry for the plug of our products -
couldn't help myself.
Happy heating,
Bruce R. Knox W8GN
RF Applications, Inc.
KH7M wrote:
>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??
---------
To which G3SEK replied:
>
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.
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