Output-Coupling Devices

Author: J.B. Hoag

As will be explained later, antennas are very frequently adjusted to resonate with the carrier frequency. When so doing, like lumped circuits, they act as pure resistances. These pure resistances are connected across a transmission line (which is used to feed power to it from the transmitter). By properly designing the transmission line, essentially all of the power leaving the transmitter, passing through the transmission line and arriving at the antenna, will be absorbed by the antenna and radiated out into space. Our immediate problem, therefore, is properly to couple the transmission line to the output stage of the transmitter. There are a wide variety of methods for doing this. In the first place, we may place a few turns of wire near the grounded end of the tank circuit and connect their ends to the transmission lines as in Fig. 31 K.

Fig. 31 K. Coupling the transmitter to a transmission line

A little care in the adjustment of the spacing between these turns and the coil in the tank circuit will suitably load the plate circuit of the tube so that its current will have the correct value, as specified by the manufacturer of the tube. It is also possible to connect to the transmitting line through series condensers. In addition, it is possible to use a coupling unit consisting of a coil and a condenser. A third method, that of link coupling between the tank and the transmission line, is sometimes used and has great flexibility.

Fig. 31 L. A pi network for coupling the transmitter to the antenna. A commonly-used circuit in commercial practice

Figures 31 L and M show two coupling methods commonly used in commercial practice.

Fig. 31 M. A capacitively-coupled antenna circuit