[Elecraft] Measuring power with a scope...

[Don Wilhelm]

John,
Your calculations are correct.  You did not say if your observed voltages
were high or low.  I may be able to give a better guess if I knew that.

I think there are two possible reasons why you are not seeing the voltages
on your 'scope as you expected.

First is the possibility that the probes you are using are not responding
properly at the frequency of the RF.  I prefer to use X10 'scope probes
rated for at least 5 times the frequency of the RF I am measuring.
Calibrate the probes using the square wave output on the rear of the 'scope.
An X1 probe may work FB at DC or for low impedance audio, but may be
inaccurate at RF.  If you are simply using test leads, your results may be
even more misleading at RF.

Secondly, even though you obtain a 1:1 SWR reading, this may not be a true
50 ohm resistive load.  Much better to see if you can find a good dummy load
to borrow (or purchase).

73,
Don W3FPR
----- Original Message -----
...
> Any way... I wanted to go over the calibration, but I ran into the
> chicken-and-egg syndrome... how do I calibrate a wattmeter if I don't have
a
> calibrated wattmeter to calibrate it by?  I do have an oscilloscope (but
no
> dummy load), so I found a clear frequency on 7 mHz, set my xmtr at very
low
> power (1 watt), tuned my LDG AT11MP antenna tuner until it showed an SWR
of
> 1:1 using my K2/KAT2 in Pout mode (no reflected power, which should imply
a
> 50 ohm load... right?), and then hooked up the scope inside the sensor to
> the center and ground connectors of a PL-259.
>
> Now, according to the ARRL Handbook, power can be computed as follows:
> P=((Ep/SQR(2))**2)/R ... with Ep being peak voltage [I derived this
formula
> from P=E**2/R and Ep=E*SQR(2)].  Moving a few things around and plugging
in
> constants for 5 watts and 50 ohms gives me 15.81v for vRMS (E) and 22.36v
> for vPEAK (Ep).  Therefore, I am transmitting 5 watts when my scope is
> showing me a peak positive voltage of 22.36v.  Is this right?
>
> The reason I'm asking is... when I hook everything up as shown, set the
K2's
> power control to 5 watts, and hit the TUNE button... I'm not getting these
> voltage figures on my scope.  I checked to ensure that my scope was
reading
> what I thought it was by measuring the deflection that occurs when I
hooked
> the probes up to a 12v gel cell and also measured the voltage with my
Fluke
> 87 meter... the scope and meter agreed.  BTW my scope is a Tektronix 466
and
> appears to work fine... I have a Heath ET-3100 breadboard and it measures
> the various voltages and waves the board puts out at the expected values.
>