R: [AMPS] Watt meters and PEP

[Ian White, G3SEK]

Mauri wrote:
>
>
>Measuring a power through a voltage (or current) is an indirect way to
>estimate something we're not really measuring.

That's a very important point. No amateur "power" meter actually
MEASURES RF power. Instead, it detects voltages and currents on the
line, and INFERS what the power must be, often via a long string of
assumptions. The meter may be calibrated in watts, but that is only ink
on a meter scale.

The only way to truly measure RF power is to destroy it - to dissipate
it in a dummy load, and then measure the rate of heat release. That is
how NIST and other standard RF wattmeters work. All other "wattmeters"
use inferential/indirect techniques, and are calibrated against these
standards.

To find out more about RF power measurement, go to www.agilent.com
(formerly Hewlett-Packard) and order Application Note 64-1A
"Fundamentals of RF and Microwave Power Measurements". It's free.

>A conventional power meter is not measuring a power but the voltage (or
>current) of a complex signal. In a complex AC it's correct to talk about RMS
>as the closest equivalent.

RMS averaging of an AC/RF waveform is better than "closest" - it gives
the CORRECT answer for the energy content ( = heating effect). That is
why RMS is used in power calculations.

>Evaluating RMS as .707 of the peak would be true with pure carrier sine
>waves but its very confortable to utilize the diodes and a suitable RC time
>constant to rectify peak voltage as a DC level.
>Obviously, in a DC voltage the RMS and the AVG are coincident but this DC is
>not existing beeing only created in the Wattmeter itself.

It is very difficult to accurately measure the rectified peak voltage on
the line. Although many wattmeters do use diode detectors, they do not
calculate the power directly from the rectified voltage. Instead they
are calibrated from other wattmeters whose calibrations are ultimately
traceable to thermal standards.


Earlier, Rich Measures said:
>> ?  RMS is  equivalent DC heating ability.  For example, with a
>> sine-wave,
>> 200w-pk would have the heating ability of 100DC watts.  .

This "200w-pk" is a peak instantaneous power. Peak instantaneous power
is a notion with very little physical meaning, and no practical use.
Ignore it - its only power is to confuse.

An RF waveform with the heating ability of 100 DC watts has a power of
100W - by definition.

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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