[Elecraft] Power measurement with a 'scope

[Ron D' Eau Claire]

> I have a 100 MHz 'scope (Tektronix 465, Ebay, about $60). How is
> QRPp measured using a scope and a dummy load...
> Dan / WG4S / K2 #2456

The oscilloscope accuracy is very important.

Power = E^2/R. That is Power is equal to the voltage squared divided by the
resistance.

Any error in the voltage reading is squared! So if your scope is 5% off, the
resulting calculated power will be 20% off!

It's real easy to have a 5% or 10% error in the voltage reading if that
scope has not been properly calibrated recently, or if you aren't careful
about how you line up the trace on the graticule. Frankly I'd suggest that
you can do better with a simple peak-reading voltmeter.

Also, you are dependent upon the resistance of the dummy load. Don't assume
that it is 50 ohms, as John observed. However, it won't hurt it it's not too
far off - the K2 output filters tolerate a fair variation in the output
load.

Of course, you want to read RMS volts to calculate RMS power, which is what
you are looking for if you want what most Hams refer to as the "output
power".

Ron AC7AC
K2 # 1289