RF WATTMETER

Cost estimate: $1

This useful project is so simple I didn't waste film on it! With a few components, a voltmeter and a little math you can measure the approximate output of your QRP transmitter.

Resistor R is the load (a dummy load: 50 ohms). RF voltage is sampled across the load and rectified; DC voltage appears at the meter terminals. Make sure the total power rating of R can handle your transmitter's anticipated output. For the "Michigan Mighty Mite" use two 1/2-watt 100-ohm resistors in parallel, which yields a 1-watt power rating: plenty enough for that transmitter. A ceramic disc type capacitor is fine; use a germanium (1N34A) diode. 
To determine approximate peak power output in watts: 

               Power = (ExE)/R

              where

               E =  Meter reading in volts 
 
              and
               
               R =  Load Resistance in ohms (50 ohms)
When powered by 14 volts my version of the Mighty Mite makes the meter read 4.5 volts, so 
                     Power= (4.5 x 4.5) / 50
                          = 20.25 / 50 
                          = 0.405 W 
                                 ...is the approximate peak power output.
I wouldn't bet on the absolute accuracy of this, but it does give an in-the-ballpark indication of what's being transmitted.

(Thanks to Bruce Barley KB0PZD for his technical advice revising this page!)


..need parts?...

Source:
  Sessions, Kendall Webster (ed.), 
       Master Handbook of 100 Practical Electronic Circuits, Solid State Edition, 
           (Blue Ridge Summit, PA: TAB Books Inc, 1988) p 315.
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