[Elecraft] Power Measurement - Where am I wrong?

[W3FPR - Don Wilhelm]

Fran,

You have the calculations correct - I use Vp-p^2/400 all the time for a 50
ohm load.  I use the 'scope probe connected directly across the dummy load
(the 'scope probe has a short grounding lead).

It is quite unlikely that the power output really climbs with power, so
there must be something strange going on with your 'scope and its probes.
My Tek 465B with 150 MHz 10x probes (with the probe compensators corrected
to the 'scope's internal square wave signal) can easily be trusted to 30 MHz
or maybe even 50 MHz, but that is just what I have here - I don't know what
you are using.

Are you certain you are using a good 50 ohm resistive dummy load?  If you
have any doubts, sweep it with an antenna analyzer to be certain - don't
have an analyzer?, then take your dummy load to someone who has one and do a
quick sweep on it.

A quick sanity check on your 'scope is to use an RF Probe and a DMM to
measure the RF Voltage at the dummy load - compare that voltage reading to
what you are seeing on the 'scope.  Due to it's simplicity a properly
constructed RF Probe is usually quite accurate up to 30 MHz or more if the
tip to crystal path length and the ground leads are short.  Actually, most
RF Probes are more accurate than many wattmeters provided you have a good 50
ohm resistive dummy load.

73,
Don W3FPR

> -----Original Message-----
>
> I do not have a power meter so I figured that I could compute power from a
> peak to peak measurement using my scope.  This should be accurate
> enough for
> ballpark knowledge.
> However, the measured voltage increases as frequency increases with the 10
> meter voltage being nearly double the 80 meter voltage.  This would mean
> power increases with frequency unless the formula I was going to use is
> missing a frequency dependent component.
>
> I have a KAT2 installed and I used a good 50 ohm resistive load.
>
> My math follows:
>
> Vrms = Vp-p/(2*sqrt(2))
> Vrms = Irms*R assuming purely resistive load
> or Irms = Vrms / R
>
> if Prms = Irms*Vrms
> substituting yields
> Prms = (Vp-p * Vp-p) / (8 * R)
>
> I know that Pave = Prms / 2
> but that does not help much because I would still get more than
> 10 Watts out
> on 10 meters when set for 5 watts out.
>
> Where did I go wrong?
>
> Fran
>
>