Harmonics

Author: J.B. Hoag

The power stages of the r.f. amplifier of a transmitter are usually operated Class C. In this method, the plate current consists of a succession of pulses equivalent to d.c. plus a large number of strong harmonics. All but the fundamental or first harmonic are suppressed by tuning the plate circuit and hence do not pass to the next stage. In order that this be essentially true in practice, the plate tank circuit of each amplifier stage should have a Q of 12 or more when the circuit is loaded, i.e., when the next stage is connected and is in operation.

The harmonic feed from the final tank circuit to the antenna can be kept to a small value by use of capacitive coupling. With magnetic coupling, an electrostatic shield may be used, as in Fig. 31 G.

Fig. 31 G. The use of a Faraday shield to reduce the harmonics from a transmitter

The shield is constructed as follows : Insulated wire is wound in a single layer on a suitable form and is scraped bare along a single straight line parallel to the axis. A stiff straight wire is then soldered along this line, making contact with every turn. The wires are then cut along one side of the soldered line and the coil is flattened out. Insulating " dope " is used to keep the wires in position. The net result is a comb-shaped Faraday shield. The wire which was soldered on, and forms the back of the comb, is connected to the ground.