R: [AMPS] Watt meters and PEP

[measures]

>
>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. 

€  amen

>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.
>
For PEP measurement?
>
>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.
>
This is essentially how the FCC measures PEP.  .Are we to ignore the FCC 
inspectors' wattmeter, Ian?    

>An RF waveform with the heating ability of 100 DC watts has a power of
>100W - by definition.
>
And if the waveform is a sinewave, the max instantaneous power is 200w.  
Sure, it can not be measured with a calorimeter, but instantaneious power 
is the standard for radars.  

cheers

-  Rich..., 805.386.3734, www.vcnet.com/measures.  


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