Radio

Radio is the use of radio waves is to carry information, such as sound, by systematically modulating some property of electromagnetic energy waves transmitted through space, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.

Radio systems need a transmitter to modulate (change) some property of the energy produced to impress a signal on it, for example using amplitude modulation or angle modulation (which can be frequency modulation or phase modulation). Radio systems also need an antenna to convert electric currents into radio waves, and vice versa. An antenna can be used for both transmitting and receiving. The electrical resonance of tuned circuits in radios allow individual stations to be selected. The electromagnetic wave is intercepted by a tuned receiving antenna. A radio receiver receives its input from an antenna and converts it into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc. Radio frequencies occupy the range from a 3 kHz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum.

Canal

Canals and navigations are human-made channels for water. In the vernacular both are referred to as 'canals'. The main difference between them is that a navigation parallels a river and shares its drainage basin, while a canal cuts across a drainage divide.

Types of artificial waterways

A navigation is a series of channels that run roughly parallel to the valley and stream bed of an unimproved river. A navigation always shares the drainage basin of the river. A vessel uses the calm parts of the river itself as well as improvements, traversing the same changes in height.

A true canal is a channel that cuts across a drainage divide, making a navigable channel connecting two different drainage basins.

Most commercially important canals of the first half of the 19th-century were a little of each, using rivers in long stretches, and divide crossing canals in others. This is true for many canals still in use.

Structures used in artificial waterways

Both navigations and canals use engineered structures to improve navigation: