[Elecraft] Measuring power with a scope...

[John Clifford]

Greetings fellow Elecrafters,

I received a very nice LDG DWM-4 wattmeter from another list member, and am
trying to set it up to use with my equipment.  For those who aren't familiar
with this meter, it allows for up to 4 sensors to be connected.  Sensors are
designed for HF (1.0-54 mHz), VHF, or UHF.  The HF sensors (the topic of
this email) work by measuring forward and reverse voltages as
derived/obtained from a toroidal bridge (this will range from 0-4.5v), and
the DWM-4 lets each sensor position be adjusted to HF Medium power (0-150w)
or HF QRP (0-15w) via two potentiometers that limit the voltage being
returned from the sensors, e.g., one can adjust the forward/reverse pots so
that 4.5v is reached either at 15w or 150w, and the DWM-4 lets the user tell
it whether a particular sensor is measuring to 150w (HF Medium) or 15w (HF
QRP).  All in all, a pretty slick idea.  Here's a link to a PDF of the QST
article describing it:  http://www.ldgelectronics.com/dwm_qst.pdf

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.

What am I doing wrong?

Thanks,

 - jgc

John Clifford KD7KGX

Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1

email: kd7kgx@arrl.net