Measurement of Power Output

Author: J.B. Hoag

In order to measure the power output of the transmitter, a "dummy antenna" is used instead of a regular antenna. The dummy is supposed to have the same electrical characteristics as those of the real antenna, except that it does not radiate energy. There are certain "standard" dummy antennas, whose design can be found in more advanced books. However, a simple dummy suitable for comparison purposes can be built of a non-inductive resistor R, in series with an r.f. ammeter M and a few turns of wire. This circuit is coupled to the tank circuit as in Fig. 31 F.

Fig. 31F. A simple "dummy antenna" circuit used to measure the power output and to adjust the transmitter. LC is the tank circuit of the last stage of the transmitter

The coupling is adjusted until the amplifier draws its rated plate current when tuned to resonance. Then the power output P, in watts, is given by P = I²R, where I is the current in amperes as read on the r.f. ammeter, and R is the resistance in ohms of the non-inductive resistor. Special resistors for this purpose, with resistances ranging from 73 to 600 ohms, and for power ratings up to 100 watts, can be purchased on the market. Higher power requirements can be met by the use of series-parallel combinations of these units. For crude work, incandescent lamp bulbs may be used in place of the resistor and meter.