Basic Radio is a free introductory textbook on electronics based on tubes. See the editorial for more information....

U.H.F. Transmitters And Receivers

Author: J.B. Hoag

Now we are to consider the transmitters and receivers for use at frequencies higher than 30 MHz, corresponding to wave-lengths shorter than 10 meters. The division at 30 MHz is entirely arbitrary and must not be considered as a sharp boundary line.

The circuits used at the lower "communications" frequencies and at the ultra-high frequencies are very much the same, but the constructional features change progressively as the frequency becomes higher and higher. For example, the physical size of a grid condenser in a receiver is of minor importance at low frequency, despite the small capacitance which exists between it and its surroundings (the grounded chassis or shields). As the frequency increases, the reactance (1/2πfC) of this shunting capacitance becomes smaller and smaller. Very high frequency currents will be shunted to the ground in appreciable amount. Then, the voltage on the grid of the tube will be small and the grid-tuning circuit will be heavily loaded, have a low Q, and poor selectivity. In other words, the set will not operate properly unless the condenser is made physically small and is kept well away from the ground.

At the higher frequencies, the size and relative location of every part of a transmitter or receiver becomes of great importance.

Furthermore, as the frequency becomes higher, the capacitances and inductances needed to tune the circuits become smaller and smaller. A single, short, straight wire instead of a coil and condenser will be sufficient if the frequency is very high because it has the necessary distributed inductance and capacitance. There is capacitance between every point on the wire and all of the objects which surround it.

It is to be remembered that, as the frequency increases, the currents travel more and more on the skin of the conductors. The resistance can only be kept down by using conductors with large surface areas. Large copper tubes, isolated from other parts of the circuit as much as possible, are used both to maintain high Q and to give freedom from mechanical vibration. Quarter-wave concentric lines prove to be particularly desirable as tuning circuits at the very high frequencies.

To build a transmitter for the very high frequencies (say 300 MHz) one needs to be as much a mechanic as an electrician; and the physical appearance of units, with their large metal tubes and concentric lines, is very different from that of the units used for communication frequencies where lumped inductors, capacitors, and resistors are connected to each other by small wires.

In the ultra-high frequency region, the physical dimensions of the circuits become comparable with the wave-lengths to be handled. Standing-waves are common, with the result that the currents and voltages are not of the same magnitude at one point in a wire as they are at another (see the chapter on Short-Lines).

As the frequency is increased to the order of 100 MHz, the time for the electrons to travel from the filament to the plate in a vacuum tube becomes an appreciable part of one cycle. A voltage on the control grid may change the number of electrons flowing to the plate, but this change will not affect the plate current until some time later. Thus the transit time of the electrons can be thought of as the equivalent of an inductive lag in an ordinary circuit. If the time delay exceeds one-quarter of a cycle, it will act capacitatively. The tube capacitances and the transit time effects can be reduced by building tubes with smaller electrodes and by bringing out their leads so that grid and plate wires are distant from the cathode and from each other. The "acorn" and "doorknob" tubes (954, 955, 316A, 368A) are of this class.

Last Update: 2010-12-01