RADIO WAVES

Lasers by Hal Hellman, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. RADIO WAVES

RADIO WAVES

One of the first of the newly discovered electromagnetic radiations to be put to work was the radio wave, which is characterized by long wavelength and low frequency. The low frequency makes it relatively easy to produce a wave having virtually all its power concentrated at one frequency.

The advantage of this capability becomes obvious after a moment’s thought. Think for example of a group of people lost in a forest. If they hear sounds of a search party off in the distance, all likely will begin to shout in various ways for help. Not a very efficient process, is it? But suppose all the energy going into the production of this noise could be concentrated in a single shout or whistle. Clearly, their chances of being found would be much improved.

Figure 5 (a) Temporally coherent radiation. (b) Temporally incoherent radiation.

The single frequency capability of radio waves has been given the name temporal coherence (or coherence in time) and is illustrated in Figure 5. Part a shows a single sine wave, the common way of representing electromagnetic radiation, and particularly temporally coherent radiation. In b we see what temporally incoherent radiation (such as the mixed sounds of the stranded party) would look like.

It was on Christmas Eve 1906 that music and speech came out of a radio receiver for the first time. Today the sight of someone walking, riding, or studying with an earpiece plugged into a transistor radio is common. But the early radio enthusiasts had to wear earphones because it takes considerable power to activate a loudspeaker and the received signal was quite weak. Some means of increasing, or amplifying, the signal was needed if the process was to advance beyond this primitive stage.

The use of vacuum tube or electron tube amplifiers is so widespread that it is unnecessary to explain their operations here in any detail. It is important that the principle of amplification be understood, however. The input or information wave causes the grid to act as a sort of faucet as shown in Figure 6. That is, it controls the flow of electrons (the current in the circuit) from cathode to anode. A weak signal can therefore cause a similar, but much stronger, signal to appear in the circuit. The larger signal is subsequently used to power a loudspeaker in the radio set.

Figure 6 Amplification by a three-element vacuum tube.

Power source

Cathode

Grid

Input wave

Anode

Output wave

The amplification principle can be applied in another equally important way. Once a signal gets started in the circuit, part of it can be fed back into the input of the circuit. Thus the signal is made to go “round and round”, continuously regenerating itself. The device has become an oscillator, that is, a frequency generator that produces a steady and temporally coherent wave. The frequency of the wave can be rigidly controlled by suitable circuitry.

The oscillator plays a vital part in radio transmission, for a transmitter beams energy continuously, not just when sound is being carried. The oscillator generates what is called a “carrier wave”. Information, such as speech or music, is carried in the form of audio (detectable-by-ear) frequencies, which ride “piggyback” on the carrier wave. In other words, the carrier wave is modulated, or varied, in such a way that it can carry meaningful information. The familiar expressions AM and FM, for example, stand for Amplitude Modulation and Frequency Modulation—two different ways of impressing information on the carrier wave. Figure 7 shows a basic and an amplitude- (or height-) modulated wave.

Figure 7 (a) Unmodulated radio wave. (b) Amplitude-modulated wave carries information.

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.

At this point in our story we have a situation where low temporally coherent radio waves and microwaves can be generated, but nothing of higher frequency. Communications engineers have gazed wistfully, but almost hopelessly, at light waves, whose frequencies are millions of times higher than radio waves. Thus, just by way of example, some 15 million separate TV channels could operate in the frequency range between red and orange in the visible band.

What, then, is the problem?

Why is light so much more difficult to handle?

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