Radio Transmitters

The typical way to produce radio waves for communication is to set an electrical current oscillating in a straight conductor. If you send an electrical current (a moving mass of electrons) into a wire, that current travels down the wire until it reaches the end. There, with nowhere to go, the electrons bunch up. Whenever you force electrons together, you get an increase in voltage (electrical pressure). This pushes the electrons back in the opposite direction. This is like surface waves on water hitting a wall and reflecting back. The electrical current now travels down the wire in the opposite direction until it hits the other end. Here the electrons bunch up again and reflect back in the original direction. The following illustration shows electrons moving back and forth in a wire.

This mass of electrons travels back and forth along the wire repeatedly^[1]. This creates a high electrical current in the middle of the wire where the electrons are in motion. There is a high voltage at the ends of the wire where the electrons bunch up.

With the electrical current traveling back and forth in the wire, there is an oscillating magnetic field around the wire. This oscillating magnetic field is strongest at the center of the wire where the current is the highest. This oscillating magnetic field radiates electromagnetic waves.

The frequency of the radio waves produced by the wire is equal to the number of round trips the electrons make per second. With a longer wire, it takes a longer time for the electrons to complete the trip from end-to-end in the wire. Therefore, the longer the wire, the lower the frequency of the radio waves.

Early radio transmitters

Guglielmo Marconi is usually credited with inventing the radio. He did some early experiments on the practical use of radio but, in the end, he was more of a financier than an engineer. Like others at his time, Marconi used the spark gap transmitter.

Spark gap transmitters worked by charging a capacitor until it contained enough voltage to ionize the air between two electrodes and create a spark. One electrode was connected to one end of the antenna wire. The surge of current through the gap would cause a surge of current in the wire. This current would travel back and forth in the wire and produce a burst of radio waves.

Damped wave

Some of the energy of the spark is dissipated as heat as the current travels through the resistance of the wire. However, most of the energy is converted into radio waves and is radiated into space. This loss of energy causes the oscillations to die very quickly. Only a short burst of four or five cycles of radio waves are produced with each spark. This is called a damped wave. The spark is repeated many times per second to produce a continuous stream of radio waves.

The burst of energy caused by a spark is not clean. It contains a wide range of frequencies, much like white light. This means that the there is not a clean wave of energy in the antenna wire. Frequencies near the resonant frequency^[2] of the antenna are strongest. Those frequencies that are distant from the resonant frequency are weaker. There are also strong bands of frequencies at the harmonics of the resonant frequency. The result is that the signal from a spark gap transmitter is essentially like white light. A single transmitter used the entire available radio frequency spectrum. The signals from two transmitters operating at the same time would mix together and there was no method to separate the signals.

Early radio communication consisted of turning a transmitter on and off with a telegraph key and sending messages by Morse code. Since early radio transmitters produced a broad spectrum of energy and sent it in all directions, this was much like sending a message in a room by flashing the lights in Morse code. Everyone in the room would receive the same message. There was no way to send individual messages except to send them in turn.

After a while, radio experimenters learned to produce radio waves at different ranges of frequencies by using different antenna lengths. Receivers used tuned circuits to limit the range of frequencies they would receive. Each range of frequencies was like a different color of light. Take another look at the analogy of sending messages by flashing the room lights in Morse code. One person could send a message using red light while another could send a message using green light. The flashes would still compete with each other for attention. However, the person for whom the message in red was intended could look through a red filter. He or she would not see the green light at all. The person for whom the green message was intended could use a green filter in the same way. Two messages could be sent at the same time. Today we call this frequency division multiplexing or wavelength division multiplexing (FDM or WDM).

Reginald Fessenden wanted to use radio to send voice rather than Morse code. He saw that a spark gap transmitter would not be able to do this because its signal consisted of nothing but noise. He tried to develop a transmitter that sent a continuous sine wave instead. Around 1906 he developed an alternator-based transmitter that produced continuous waves at a frequency of 75 kHz. He placed a carbon microphone between the transmitter and the antenna and successfully transmitted his voice over radio waves.

It was not long before the vacuum tube-based oscillator was invented. This could produce stable sine waves at a single frequency. Radio signals with extremely narrow ranges of frequencies could be produced with oscillator-based transmitters. During the 1920s, spark gap transmitters were outlawed by international treaty.

A modern radio transmitter uses an oscillator circuit to produce a sine wave at a single frequency. This sine wave is amplified until is strong enough to drive a power amplifier. This can send a large current into the antenna to produce radio waves. These radio waves are at a single frequency instead of the broadband of frequencies developed by spark gap transmitters. Oscillators can also produce frequencies that are orders of magnitude above those produced by early transmitters. Many transmitters can operate at the same time because each one uses only a tiny part of the available frequency spectrum. Take another look at the analogy of flashing lights in a room. Modern transmitters are like each sender filtering his light through a high-quality filter that only allows a single wavelength (color) to pass. His receiver would look through an identical filter. Hundreds of separate messages could be sent at a time using such narrow bandwidths.