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If you have a good oscilloscope with a good probe, you may be able to measure the efficiency of your transmitter yourself. I say “may” be able to, because, if your transmitter is of the SSTRAN-type, the efficiency measurement is tricky, since the antenna loading coil forms part of the “L” network that performs the impedance transformation between resistive portion of the antenna circuit and the final stage. Measuring the efficiency of an SSTRAN-type transmitter has been discussed in other posts in this thread.

However, if your transmitter is of the Rangemaster-type, the efficiency measurement is appreciably easier, since all of the impedance transformation components are in the transmitter circuit.

First, you need to construct a dummy antenna made by connecting a resistor and capacitor in series. The capacitor represents the capacitive reactance of the antenna, and the resistance represents the ground resistance of the antenna. The difficulty here is that you can only guess at the values of the resistor and capacitor, because it is difficult to measure them with any accuracy. The dummy antenna I use is 30 pF in series with 46 ohms. The capacitor is a 30 pF +/- 5% mica capacitor. The resistor is a 47 ohm (nominal) 1/4 watt carbon composition (can be carbon film) resistor that measures to be actually 46 ohms. If you are optimistic, and think that your ground resistance is better represented by a lower standard resistor, like 27 ohms or 12 ohms, then use that value in your dummy antenna. As for the capacitor in the dummy antenna, if your antenna is 3 meters long, 30 pF is a reasonable approximation of the antenna capacitance.

Tune up your transmitter with your dummy antenna connected to the antenna terminals of your transmitter, your “real” antenna disconnected, and your oscilloscope probe connected across the resistor (R) in your dummy antenna. Your probe should be of the 10/1 type, with about 10 pF of capacitance. Don’t use a 1/1 probe! Set the transmitter to exactly 100 mW input and measure the peak-to-peak amplitude (Vp-p) of the unmodulated carrier waveform.The waveform should be a sine wave with very little distortion. The output power is P= [(Vp-p)^2]/(8R). Vp-p is in volts. R is in ohms, and P is in watts. Dividing P by the input power of .1 watts gives the efficiency.

For example, suppose R is 46 ohms and Vp-p measures to be 5 volts. In that case, the output power is P= 0.068 watts. Dividing by the input power of .1 watts gives an efficiency of .68, which is 68 %.