The electron tube made its giant contribution to radio, television, and other electronic devices by making it possible to generate, detect, and amplify radio waves.
Because radio waves are easily controlled, something useful can be done with them. Suppose we set up five radio transmitters, all beaming at the same frequency. The waves might look like those shown in [Figure 8]. Although the waves are temporally (or time) coherent, they are out of step, and not spatially coherent. But since good control is possible in radio circuits, we can force each antenna to radiate in phase (that is, in step) with the others, thus producing fully coherent radiation ([Figure 8]).
Figure 8 (a) Spatially incoherent radiation. (b) Spatially coherent radiation.
Such a process can increase the radiation power to an almost unlimited degree. But it does nothing to solve the problem of the limited total carrying capacity of the radio spectrum.
The most obvious and best way out of this difficulty is to raise the operating frequencies into the higher frequency bands. There are two reasons for this. First, it is clear that the wider the frequency band (the number of frequencies available) with which we work, the greater the number of communication channels that can be created.
But second, and more important, the higher the frequency of the wave, the greater is its information-carrying capacity. In almost the same way that a large truck can carry a bigger load than a small one, the greater number of cycles per second in a high frequency wave permits it to carry more information than a low frequency wave.
However, high frequencies must be generated in different ways than low frequency waves are; they require special equipment to handle them. Radio waves are transmitted by causing masses of free electrons to oscillate or swing back and forth in the transmitting antenna. (Any time electrons are made to change their speed or direction they radiate electromagnetic energy.)
Each kind of oscillator has some limit to the frequencies at which it can operate. The three-element electron tube has been successfully developed to oscillate at frequencies up to, but not including, the vibration rate of the microwave region. Here ordinary tubes have trouble for the unexpected reason that free electrons are just too slow in their reactions to oscillate as rapidly as required in microwave transmission.
To overcome this obstacle, two new types of electron tubes were developed: the klystron in 1938 and the traveling-wave tube some 10 years later. These lifted operation well up into the microwave region; it was the klystron that made wartime radar possible. Today many communication links depend heavily upon microwave frequencies.