Power Output Specs

[k6dgw]

(Possibly) stupid questions:

1.  Does my thru-line wattmeter indicate RMS or P-P power?

2.  Are TX/Amp power output specs given in RMS or P-P?

I thought I understood this, but recent reflector traffic leads me to
question that belief.

Fred K6DGW
Auburn CA CM98lw

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[KK7P]

> 1.  Does my thru-line wattmeter indicate RMS or P-P power?

Typically RMS, but there is a time constant involved in the meter
movement, and some versions are/can be peak reading.  Keep in mind that
most wattmeters, thru-line included, don't actually measure power but
voltage induced in a directional coupler, and are only at their rated
accuracy when they are terminated in the load for which thy are
designed, usually 50 ohms non-reactive.

> 2.  Are TX/Amp power output specs given in RMS or P-P?

Usually RMS, but might be peak envelope power (PEP).  CW ratings are
usually RMS; SSB ratings are usually PEP.

The KPA100, for example, is rated at 100 watts output power into a
50-ohm non-reactive load.  If you are in CW, this is the carrier (RMS)
power.  If you are in SSB, this is the PEP rating.

Enjoy!

Lyle KK7P

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[Don Wilhelm-3]

Fred and all,

Your power meter will try to show either average or peak power - no
relationship to RMS for either of these.

Extended 'TECH TALK' follows:

Power is always computed using RMS values.  For power measurements, there is
some sense in referring to either INSTANTANEOUS power or AVERAGE power.
Whether a meter can really read true instantaneous power depends on the
meter itself, but some will show the peak power in a particular increment of
time - most meters simply provide an "average" power reading - I put the
'average' term in quotes because a meaningful average must also specify the
time over which the average is taken, and most meters I have encountered do
not do that, they simply depend on the latency of the meter movement to
average things out - that is usually OK because the real number we are
usually looking for is the number displayed when we hold the key down for a
bit - this is what we refer to as PEP (OK, PEP with speech is more
complicated - look in the reference books on the subject for the nuances).

Most common amateur grade power meters that provide 'peak' readings are just
charging up a capacitor to slow the meter response down so the meter retains
the higher reading for a bit - just how long and how accurate it is depends
on the speech pattern and the judgement of the meter designer for 'how long
is long enough?'.

Just so there is no mis-understanding, power in a wave cycle is computed
with RMS values of voltage (or current), but zero to peak voltages or peak
to peak voltages can be used if the proper math is performed to obtain the
RMS value - if the waveform is a sinewave, the RMS value is V(0-p)*(sqrt
2)/2 or Vp-p*(sqrt 2)/4.  Sorry, but these quanties are difficult to show in
text mode (no subscripts, etc. available).

73,
Don W3FPR

> -----Original Message-----
>
> (Possibly) stupid questions:
>
> 1.  Does my thru-line wattmeter indicate RMS or P-P power?
>
> 2.  Are TX/Amp power output specs given in RMS or P-P?
>
> I thought I understood this, but recent reflector traffic leads me to
> question that belief.
>

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