RF WATTMETER

Cost estimate: $2 (excluding meter)

This useful project is so simple I didn't include a photo! 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 or Schottky diode such as a 1N5711.

To determine approximate peak power output in watts:When powered by 14 volts my version of the Mighty Mite makes the meter read 4.5 volts, soPower = (ExE)/RwhereE = Meter reading in voltsandR = Load Resistance in ohms (50 ohms)

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.

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.