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. A small-signal silicon "glass" diode such as the 1N914 will work,
but add a little less than avolt to the readout before computing to compensate for its voltage drop.
(The voltage drop for a germanium diode is about 0.3V; for a silicon diode about 0.7V)
To determine approximate peak power output in watts:
Power = (ExE)/R
E = Meter reading in volts
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!)
Sessions, Kendall Webster (ed.),
Master Handbook of 100 Practical Electronic Circuits, Solid State Edition,
(Blue Ridge Summit, PA: TAB Books Inc, 1988) p 315.
***3W 2-BAND TRANSMITTER***
***15 METER CONVERTER***
***15 METER VXO-TUNED TRANSMITTER***
***FIELD STRENGTH METER***