With the longest wavelength in the electromagnetic spectrum (>10cm), radio waves are known to mankind since the 19th century. These waves can be longer than a football field to as short as a football. Italian physicist and inventor G. Marconi, in 1901, became the first person to send a radio message across the Atlantic Ocean, from England to New Foundland in Canada.
Radio waves are used for wireless transmission of information, for communication and for maritime and aircraft navigation. The information is imposed on the electromagnetic carrier wave as amplitude modulation (AM) or as frequency modulation (FM) or in digital form (pulse modulation). Transmission therefore involves not a single-frequency electromagnetic wave but rather a frequency band whose width is proportional to the information density.
There being a lower frequency limit to for the radio waves travelling near the earth's surface near 10000Hz, scientists questioned the efforts of Marconi to develop long-range radio. However, his success in transmitting messages over more than 2000km led to the discovery of the Ionosphere (or the Kennelly-Heaviside layer), an approximately 300-kilometre-thick layer above the Earth's surface in which the atmosphere is partially ionized, which enables it to affect radio waves. Due to the Sun's involvement, the height, width, and degree of ionization of the stratified ionosphere keeps varyimg. Radio waves transmitted by antennas are reflected back to Earth by the ionosphere. They may bounce off the Earth and be reflected by the ionosphere repeatedly, making radio transmission around the globe possible.
Long-distance communication is further facilitated by the ground wave, which closely follows the ground due to its interaction with the terrestrial surface. The range of the ground wave (up to 1,600 km) and the bending and reflection of the sky wave by the ionosphere depends on the frequency. Under normal conditions the highest frequency of a wave that can be reflected from the Ionosphere is 40 MHz.
In order to accomodate the large band width of the transmitted signals, television transmitters need to be placed on high towers or on hilltops.
Radio waves can penetrate nonconducting materials such as wood, bricks, and concrete fairly well, but cannot pass through electrical conductors. Above 40 MHz, radio waves from deep space can penetrate the Earth's atmosphere, which makes radio astronomy observations with ground-based telescopes possible.
The wavelengths of radio waves being very large, an array of radio telescopes are used to study the radio waves originating from comets, stars and galaxies, by which astronomers can learn about their composition, structure, and motion.
Radio waves are highly useful for wireless transmissions, where the receivers are unspecified, spread out and numerous, except in the case of cable television, where the electromagnetic radiations are transmitted by co-axial cable systems, for high-quality signals.
A radio station transmits both FM Signals and AM Signals, the later one being more noisy, but more convinient to receive.
Use of radio waves in the medical field includes the use of their combination with strong magnetic fields by Magnetic Resonance Imaging (MRI) to produce diagnostic pictures of parts of the human body and brain. Extremely low-frequency (ELF) radio waves are of interest for communications systems for submarines. The weak absorption by seawater of ELF waves and prominent resonances in water makes the range suitable for this reflection from the Ionosphere.
Although radio waves are practically harmful, ELF waves and oscillating magnetic fields that occur near electric power transmission lines can have adverse effects on human health and the electrochemical balance of the brain. Prolonged exposure can even lead to diseases like leukemia, lymphoma, and brain cancer in children.
