A radio system consists of a “transmitter” that produces radio waves and one or more “receivers” that pick them up, with both transmitter and receiver(s) fitted with antennas. The very earliest “wireless telegraphy” radio systems used a transmitter that simply generated a burst of radio energy by opening an electric circuit containing an inductive coil with a telegraph key, causing a spark. The radio waves propagated through space and set up an electric current in a receiving antenna, which in turn closed a relay switch. Messages were sent using Morse code.

The problem with this simple scheme was that the transmitter generated waves over a wide and indiscriminate range of frequencies, with a single receiver picking up and mixing up transmissions from every transmitter in the line of sight. This problem was solved by fitting each transmitter with a “variable oscillator” — an electronic circuit that generated electrical signals at different frequencies, as set by a knob turned by the transmitter operator. A receiver picked up this signal with its antenna, with the signal run through a “variable filter” — an electric circuit consisting of an inductor coil and a variable capacitor that could be set by a knob to block out all frequencies except one.

This scheme allowed multiple transmitters to operate in a given area without interference. The transmitter operator set the transmitter oscillator to a given frequency or “channel”, and then used a telegraph key to gate the oscillator output on and off into an amplifier circuit, which drove a high-power signal out the antenna. The receiver operator set the receiver filter to the same channel. The receiver picked up radio waves on all frequencies and amplified them. The amplified received signal was run through the variable filter, and then into a “detector” circuit to convert high-frequency signals into a direct-current signal to activate the relay switch.

The detector included a “rectifier”, a one-way valve for electricity, that eliminated half the waveform, with this rectified signal then passed through a “low pass filter”, consisting in the simplest case of a resistor and a capacitor, that smoothed the received signals into pulses. The block diagram below gives a simple representation of such a system, For simplicity, the transmitter and receiver in the diagram can only be set to one of seven specific frequencies — an unrealistically small range.

* This is obviously the same concept that is used in tuning a modern voice radio to different channels, though a voice radio works somewhat differently from a radio telegraph. In a simple “amplitude modulated” voice radio, the voice of a user is converted into an electrical waveform that controls or “modulates” the amplitudes or “envelope” of a variable oscillator signal. The modulated signal is then amplified and transmitted over an antenna. The oscillator frequency is known as the “carrier” frequency, since it “carries” the audio signal.

A receiver picks up the signal with its antenna, using a variable filter to isolate the desired channel. The signal is then amplified and passed through a detector circuit to extract the original audio signal. The audio signal is amplified and driven to a loudspeaker.

A radio channel is actually not a single frequency but range of frequencies. Although the details are beyond the scope of this simple document, the frequency range or “bandwidth” of a channel is roughly proportional to the amount of information carried by the channel. In the case of an audio broadcast, the bandwidth is a factor of the range of audio frequencies transmitted. That means that as a rule, a hi-fidelity audio transmission will take up more bandwidth than a low-fidelity transmission.

* The audio receiver described above will work, but in practice it doesn’t work well for producing channels at high frequencies. In practice, most modern radio receivers use a “superheterodyne” scheme, in which the received signal picked up by the antenna and amplified is then run through a “mixer” or “heterodyne” circuit that combines the received signal with a lower fixed “intermediate frequency (IF)” signal produced by an oscillator. The mixer effectively subtracts the IF signal from the input signal, resulting in a low frequency modulated signal that is then detected and driven out the loudspeaker.

* The main problem with amplitude modulation is that most sources of environmental noise cause changes in the amplitude of a signal, introducing static and distortion. In addition, multiple signals on the same band will simply add to each other, with the receiver picking up both at the same time. This is actually an advantage in some circumstances, for example allowing multiple police cars to monitor each others’ conversations, but it can be a nuisance in others.

The solution is what is known as “frequency modulation (FM)”, in which the audio signal is mixed with the modulating signal to cause changes in frequency, not in amplitude. The rest of the stages of the transmitter are not conceptually different from those of an AM radio, and the receiver isn’t much different either — except that the simple detector circuit is replaced by some class of frequency-to-voltage converter circuit to extract the signal. FM is more noise-immune because there are relatively few forms of FM noise; and it is less prone to interference from other signals, because the strongest FM signal completely masks out weaker ones.

* An old-fashioned analog television signal was conceptually much the same as a voice radio. The TV sound track was transmitted with FM, while the basic visual signal, which was always in black and white, was transmitted by AM. A separate color signal was sent that “painted” colors on top of the basic black and white signal, with various schemes being used in various countries. Incidentally, this division of labor between the black and white signal and the color signal was the result of the fact that early on, broadcast TV was black and white, and when color was introduced there was a need to ensure that the broadcast signals could still be used by black and white sets.

Of course, since a moving picture generally contained more information than an audio signal, a TV signal required more bandwidth than an ordinary audio signal. Analog TV is now being quickly replaced by digital broadcast systems, in which the signals are transmitted in the form of a stream of binary number values encoding the video and audio signals.

* Transmitter output power is measured in watts, or (as far as radar is concerned) more usually kilowatts (kW, thousands of watts) and megawatts (MW, millions of watts). Receiver “sensitivity”, or the ability of the receiver to amplify received signals, is determined in terms of “decibels”, defined as:

decibels = 10 * LOG10( output_power / input_power )

The amplification factor is commonly referred to as “gain”.

A radio receiver, particularly one that is built into an automobile and is moving around, may be picking up a transmitter signal that varies in strength. That means that the volume of the radio output will tend to fade or grow continuously, requiring the listener to keep adjusting the volume control. A circuit known as an “automatic gain control (AGC)” helps correct this problem by measuring the average received power of the signal and adjusting the receiver gain to ensure that it stays as constant as possible.