INVENTED IN 1901
The point at which wireless messages are sent and received is called
a "station". There are no wires connecting the transmitting station
with the distant receiving station, even though they may be hundreds or
even thousands of miles apart. One station could even be on a ship
or airplane. If communication both ways is desired, each station
would be equipped with its own transmitter and a receiver. The signals
from transmitter to receiver are carried by invisible electromagnetic waves
which travel through space at the speed of light. The signals consist
simply of turning the waves on and off using a code to represent letters
and numbers. The purpose of each instrument in this wireless telegraphy
system is explained below.
Key. A key is nothing more than a simple electrical switch. This switch has a pair of contacts that makes and breaks the circuit to the induction coil. The key is ergonomically designed to allow the operator to rapidly make and break the contacts in order to transmit symbols of the Radio code. This code is not exactly the same as the Morse code, but an improved version that is less susceptible to errors when the receiving operator must listen through static, noise and interference conditions which one frequently encounters in wireless radio.
Induction coil. The induction coil is a device that raises the voltage or force of an electric current. It consists of two separate coils of wire wound on an iron core, with a circuit interrupter positioned at one end of the core. One coil of wire, with relatively few turns of wire, is connected to the interrupter contacts, the key and the battery. The other coil of wire contains very many turns of fine wire and is connected to the spark gap. Whenever the key is closed, electric current flows through the small coil of wire and is interrupted several hundred times per second by the interrupter. At each interruption, a voltage is INDUCED into the second coil. Since the second coil contains many more turns of wire, the voltage induced is very high, typically 10,000 volts or more. The induction coil with its inefficient interrupter was soon replaced by an AC transformer where AC power was available, or with a high voltage alternating current dynamo.
Battery. This is a battery made of several electrochemical cells connected in series. The usual voltage for powering an induction coil is 6 volts.
Spark gap. This is the heart of the transmitter. The spark gap is nothing more than two conductors separated by an air gap. The high voltage of the induction coil charges up the antenna with electricity. When the voltage is high enough, the air in the gap breaks down with a spark, and all the electricity put into the antenna OSCILLATES freely back and forth between the antenna wire and ground at an extremely high frequency rate. This alternating current frequency can be as high as hundreds of thousands of times per second, or even millions of times per second. The size of the antenna determines this frequency - the larger the antenna, the lower its resonant frequency. When all the electric charge is used up, perhaps a thousandth of a second later, the induction coil again charges up the antenna with electricity until the spark gap fires again. This happens over and over again as long as the key is held down. In the 1920's, electron tubes replaced the induction transformer/spark gap method of generating high frequency radio currents. Today miniature silicon transistors are used. These later methods use much lower voltages and generate extremely pure waves continuously when the key is held down with no pulsing or interrupting of the wave as is the case with the induction coil/spark gap method.
Antenna. The antenna is a wire or set of wires. Electromagnetic waves are created that travel away from the antenna whenever high frequency alternating current electricity is made to flow in it. The frequency of the outgoing waves is the same as that of the current in the wire. A high antenna, clear of obstructions such as trees and buildings, is a very efficient radiator of electromagnetic waves. The waves travels outwards in ever increasing spheres, dissipating the energy over a huge volume of space hundreds or even thousands of miles away. Antennas were made very large in the early days of radio because low frequencies were desirable for the long distance communications needed by ships. Small radios and walkie-talkies may have very small antennas and they do not effectively transmit a signal as far away.
Ground. Connection to earth or ground. The high frequency
alternating currents oscillate between the antenna and ground. In
modern radios, the ground connection is replaced by another antenna wire
positioned close to the real antenna wire.
Antenna. The antenna is a wire or set of wires. The antenna intercepts a tiny portion of the passing electromagnetic waves from a distant transmitter, causing a feeble high frequency alternating electric current to flow in it. The frequency of the current in the wire is the same as that of the incoming waves. A high antenna, clear of obstructions such as trees and buildings, is a very efficient receptor of electromagnetic waves. Small radios and walkie-talkies may have very small antennas and they do not effectively receive a signal from as far away.
Ground. Connection to earth or ground. The high frequency alternating currents oscillate between the antenna and ground. In modern radios, the ground connection is replaced by another antenna wire positioned close to the real antenna wire.
Crystal detector. The purpose of the detector is to change the high frequency alternating currents from the antenna into pulses of direct current electricity. Current which only flows in one direction can perform a useful task such as operate a headphone set. The high frequencies used by radio cannot operate headphones directly. Various chemical solutions, crystals of galena and other minerals have the property of this function to varying extent and efficiency, as do electron tubes. Today a tiny germanium or silicon crystal is used as a detector.
Telephone receiver, or headphones. The headphones change
the pulses of direct current into audible sounds. These sounds correspond
to the key presses at the distant transmitter, and so the operator can
receive the transmitted message. Since there is no battery or power
source in this type of receiver, the strength of the signal heard is due
only to the fraction of energy extracted from the spread out transmitted
waves. The sounds are therefore extremely faint for distant stations,
if even heard at all. Here you may listen to what an early spark
wireless station sounds like through a crystal receiver: CLICK
There is one serious disadvantage to the simple radio receiver described here. It will receive signals from more than one transmitting station at the same time. It responds to ANY electromagnetic wave. This would cause bad interference in the receiver. By international agreement, all radio transmitters are licenced, and create one particular frequency of electromagnetic waves by having their antenna/ground circuits tuned using several additional parts. For example, WABC in NYC sends out a wave having a frequency of 770,000 oscillations per second, or 770 kilohertz. By adding a TUNING circuit to the simple radio receiver, it can be made to respond to only ONE frequency of waves, such as WABC's, and not respond to any other. In this way the receiver is tuned to select the desired transmitting station.
To make the radio receiver more responsive to weak signals at further distances from a transmitter, battery operated amplifiers may be added between the detector and the headphones. If the signals are amplified enough, over one million times, a loudspeaker may be used. All the complexities and electronic parts used in modern radio circuits have to do with tuning and amplification of high frequency alternating currents, and to allow voice and music, picture (television sets), and data (wireless radio-computer connections) to be received. However, the fundamental principles of how radio receivers work are covered entirely by the simple radio receiver above.
Such a simple pair of transmitter and receiver as shown above does indeed
work, and was installed on the ship Titanic in 1912. Ironically,
the SOS distress signals were heard all the way into central Canada and
into the US, but the nearest ship to the Titanic only a few miles away
which could have saved over a thousand lives had its radio receiver turned
off. This was before the days of mandatory monitoring of wireless
CQD CQD SOS SOS CQD DE MGY MGY M? ...silence...