b. Radio Communication System. A radio communication system is similar in principle to the simple
flashlight analogy. However, since humans cannot sense the presence of radio waves, special devices that are
sensitive to radio energy must be used.
(1) In the lesson on telephone communication, we learned that direct current from a battery is modulated
by voice energy from the person speaking into the microphone. Since direct current serves as the
medium of intelligence transfer, a wire must be used to convey the current and its impressed
intelligence to the distant receiving earpiece.
(2) Radio communication is more complex than telephone communication because the connecting wire is
missing. Some means other than a wire must therefore be found to convey the intelligence. The
solution to this problem establishes the need for a radio transmission path.
(3) Suitable sending and receiving devices must be placed at the terminations of the radio transmission
path. A radio transmitter generates and modulates the radio signal, and a transmitting antenna
radiates the energy. The radio receiver at the distant end of the transmission path receives and
demodulates the signal picked up on its receiving antenna.
(4) A further complication lies in the fact that each radio signal must be assigned its distinct place in the
radio-frequency spectrum. Since there are literally thousands of radio stations, it follows that each
radio transmitting station must generate and radiate its signal at one specified frequency, or a
conglomeration of received intelligence will result. When this happens we have a situation analogous
to a telephone system without a switchboard; everyone hears everyone else simultaneously.
Moreover, since each radio transmitting station operates on its assigned frequency, each radio
receiving station must be suitably equipped to select the one radio signal it needs from the great
gamut of signals that impinge upon the receiving antenna.
c. Basic System. In the basic radio communication system shown in block form in figure 4-1, a radio
transmitter is used to develop and process the RF waves which are to be radiated into space. The simplest radio
comparatively weak output of the oscillator is applied to one or more amplifier stages which increase the
transmitter output power to a more desirable level.
d. Modulation. If the transmitter is to radiate energy containing intelligence, a device must be used to
control the transmitter output. A radio wave controlled by the intelligence signal is called a modulated radio
wave. A telegraph key is used to control the waves radiated from a radiotelegraph transmitter. When the key is
closed, an RF signal is radiated. When the key is open, the RF signal ceases. In this way, a message in a
telegraphic code consisting of dots and dashes is transmitted. If speech intelligence