Because electromagnetic radiation travels in spare quad in square lines, late 19th-century scientists questioned the efforts of the italian physicist and inventor Guglielmo Marconi to develop long-range radio. Earth ’ second curvature limits the line-of-sight distance from the clear of a 100-metre ( 330-foot ) tugboat to about 30 km ( 19 miles ). Marconi ’ second unexpected achiever in transmitting messages over more than 2,000 kilometer ( 1,200 miles ) led to the discovery of the Kennelly-Heaviside layer, more normally known as the ionosphere. This region is an approximately 300-km- ( 190-mile- ) thick layer starting about 100 km ( 60 miles ) above Earth ’ s coat in which the air is partially ionized by ultraviolet light from the Sun, giving rise to enough electrons and ions to affect radio receiver waves. Because of the Sun ’ sulfur affair, the stature, width, and degree of ionization of the stratified ionosphere vary from day to night and from summer to winter. radio waves transmitted by antennas in certain directions are flex or even reflected back to land by the ionosphere, as illustrated in Figure 5. They may bounce off earth and be reflected by the ionosphere repeatedly, making radio transmittance around the earth possible. long-distance communication is promote facilitated by the alleged ground wave. This form of electromagnetic wave closely follows Earth ’ s surface, peculiarly over water, as a result of the wave ’ s interaction with the planetary surface. The range of the grate roll ( up to 1,600 km [ 1,000 miles ] ) and the deflect and reflection of the flip brandish by the ionosphere depend on the frequency of the waves. Under normal ionospheric conditions 40 MHz is the highest-frequency radio wave that can be reflected from the ionosphere. In club to accommodate the large band width of air signals, television frequencies are necessarily higher than 40 MHz. television receiver transmitters must therefore be placed on high towers or on hilltops .ionospheric reflection figure 5 : Radio-wave transmission reaching beyond line of sight by means of the sky curl reflected by the ionosphere and by means of the ground curl ( see textbook ) .Encyclopædia Britannica, Inc. As a radio wave travels from the transmitting to the receive antenna, it may be disturbed by reflections from buildings and other big obstacles. Disturbances get up when several such reflected parts of the wave reach the receive antenna and interfere with the reception of the wave. Radio waves can penetrate nonconductive materials, such as forest, bricks, and concrete, fairly well. They can not pass through electric conductors, such as body of water or metals. Above ν = 40 MHz, radio waves from thick space can penetrate Earth ’ s atmosphere. This makes radio-astronomy observations with ground-based telescopes possible.
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Whenever transmission of electromagnetic energy from one placement to another is required with minimal energy passing and disturbance, the waves are confined to a limited region by means of wires, coaxial cables, and, in the microwave region, waveguides. Unguided or radio receiver infection is naturally preferred when the locations of receivers are unspecified or besides numerous, as in the case of radio and television communications. cable television, as the name implies, is an exception. In this case electromagnetic radiation is transmitted by a coaxial cable system to users either from a community antenna or directly from broadcasting stations. The shielding of this guide transmission from disturbances provides high-quality signals. calculate 6 shows the electric field E ( solid lines ) and the magnetic field B ( crash lines ) of an electromagnetic wave guided by a coaxial cable. There is a potential dispute between the inner and outer conductors and so electric airfield lines E extend from one conductor to the early, represented here in cross section. The conductors carry opposition currents that produce the magnetic field lines B. The electric and magnetic fields are perpendicular to each other and vertical to the direction of propagation, as is characteristic of the electromagnetic waves illustrated in Figure 2. At any cross incision viewed, the directions of the E and B airfield lines change to their opposite with the frequency ν of the radiation. This direction reversal of the fields does not change the direction of propagation along the conductors. The speed of propagation is again the universal joint amphetamine of light if the region between the conductors consists of tune or release space .cross section of a coaxial cable carrying high-frequency current calculate 6 : Electric field lines E ( upstanding ) and magnetic field lines B ( dashed ) are mutually plumb line and plumb line to the electromagnetic beckon propagation, which is toward the viewer .Encyclopædia Britannica, Inc. A combination of radio waves and firm magnetic fields is used by charismatic resonance visualize ( MRI ) to produce diagnostic pictures of parts of the human body and genius without apparent harmful effects. This imaging proficiency has thus found increasingly wide application in medicate ( see besides radiation ). extremely low-frequency ( ELF ) waves are of matter to for communications systems for submarines. The relatively fallible absorption by seawater of electromagnetic radiation sickness at low frequencies and the being of big resonances of the natural cavity formed by Earth and the ionosphere make the range between 5 and 100 Hz attractive for this application.