[Amps] Dummy Loads & Wattmeters

[Dennis OConnor]

The simplest way for a ham to accurately measure to within 5% power is to measure the RF voltage impressed across a known impedence...
There are a number of ways to refine your voltage measurement to be within 5% deviation from NBS, but quick and dirty works just fine for me...
 
The very easiest is to get a dummy load... And get a 1% precision resistor that is near to 50 ohms -  Mouser, et. al. about a $1.70...  Use the resistor to measure your VOM and calculate a correction factor then measure the dummy load, apply the correction factor from the precision resistor and you know the DC resistance of your dummy with near 1% precision... (ya, I know DC and RF ain't the same - I said quick-n-dirty)
Once you know that, you apply the RF from the amp to the dummy load and measure the RF voltage...  That measurement can be direct, with an oscilloscope, or by rectifying the RF to DC and measuring that...  
If you are using a Fluke, or other quality VOM, you have a known precision factor for the DC measurement and can directly calculate your supposed precision value - just remember to add the 0.7 volt you lose across the diodes, to the measurement...
If you are using an oscilloscope you can take the RF voltage number at face value compared to the scope's built in calibrator  -   or there are other ways, just like the VOM + Precision resistor, to derive a correction factor... It would take a very long post to describe all the ways to do that - but i trust hams to come up with simple solutions...
 
So for most hams, a dummy load, precison resistor, a couple of diodes and a cap, and a VOM, will get you into the ballpark of laboratory precision for a few bucks...  Then this can be used to tweak your wattmeter to be right on (within your precison range) at the power output you normally run - or to make a correction card for the readings it currently gives (just like the compass on my boat)...

denny / k8do