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.[6] 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.
[Illustration: 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.[7]
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.
[Illustration: 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.
[Illustration: 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).
[Illustration: 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?