Radio wave

Radio waves, in physics, are electromagnetic radiations, belonging to the electromagnetic spectrum, in the frequency band between 3 kHz and 300 GHz, i.e. with a wavelength greater than 1 mm. At 300 GHz, the corresponding wavelength is 1 mm (shorter than a grain of rice); at 30 Hz the corresponding wavelength is 10,000 km (longer than the radius of the Earth).

Radio waves travel from a radio transmitter to a radio receiver. The information to be transmitted (conversations in radiotelephony, words or music in radio broadcasting, images and sounds in television broadcasting, etc.) are first converted by a transducer into electrical signals of variable amplitude. Then these signals act on a wave, called carrier, of constant amplitude and frequency, generated in the transmitter through a process called modulation. The modulation consists in varying, instant by instant, one of the characteristic quantities of the periodic signal (amplitude or frequency) used as a “carrier” for transmission (the carrier), in accordance with the variations of the signal that contains the information to be transmitted, called modulant. The modified carrier signal is called modulated. In amplitude modulation (AM) the information is transmitted in the circuit by modulating the amplitude of the carrier wave, while in frequency modulation (FM) the information is transmitted by modulating the frequency of the carrier.

The signal, amplified, is sent to the antenna, which radiates it into space in the form of electromagnetic waves. The radio receivers pick up the electromagnetic waves through another antenna and, after a process of amplification and demodulation, obtain the output information emitted in transmission. Because of their long wavelength, radio waves are not stopped in their path by medium-sized obstacles, such as houses or trees (however, they are blocked by mountains, which constitute shadow zones), and can be transmitted at a distance because they are reflected by the ionized layers of the atmosphere. Repeaters intercept the waves and re-radiate them after having amplified them again, in order to send the signal with an effective power over long distances.

The considerable increase in radio traffic has led to the adoption of repeaters, positioned on geostationary satellites, which rotate in space with the same speed of the Earth and therefore “see” always the same geographical area. The frequencies used for radio communications are between 3 kHz and 300 GHz and within this spectrum are further divided into bands: the low frequency waves (Low Frequency, LF), range from 30 to 300 kHz and are mainly used for applications such as air or naval navigation; the medium waves (Medium Frequency, MF), from 300 kHz to 3 MHz are used for amplitude modulation broadcasting; short waves (High Frequency, HF), from 3 MHz to 30 MHz, are used for satellite telecommunications; Very High Frequency (VHF), from 30 MHz to 300 MHz, and microwaves (Ultra High Frequency, UHF) are used for radio broadcasting, television, mobile broadcasting and radar.

BandFrequencyWavelengthMain applications
< 3 Hz> 100.000 km
ELF
(Extremely low frequency)
3–30 Hz100.000 km – 10.000 kmRadio communication with submarines, pipe inspection, study of the earth’s magnetic field.
SLF
(Super low frequency)
30–300 Hz10.000 km – 1.000 kmCommunication with submarines, e.g., Russian ZEVS radio.
ULF
(Ultra low frequency)
300–3000 Hz1.000 km – 100 kmUsed for mine communications.
VLF
(Very low frequency)
3–30 kHz100 km – 10 kmNavy communication with surfacing submarines.
LF
(Low frequency)
30–300 kHz10 km – 1 kmIntercontinental AM radio transmissions, transmission of the standard time signal for radio controlled clocks.
MF
(Medium frequency)
300–3000 kHz1 km – 100 mAM radio transmissions.
HF
(High frequency)
3–30 MHz100 m – 10 m
(Onde corte)
Amateur Radio, City Band, intercontinental transmissions in Morse code.
VHF
(Very high frequency)
30–300 MHz10 m – 1 mCommercial FM Radio, Aviation, Navy, Law Enforcement, Television, Amateur Radio, Radio beacons.
UHF
(Ultra high frequency)
300–3000 MHz1 m – 100 mmPMR radio, Television, Cellular telephony, WLAN.
SHF
(Super high frequency)
3–30 GHz100 mm – 10 mmRadar, Satellite, WLAN.
EHF
(Extremely high frequency)
30–300 GHz10 mm – 1 mmSatellite transmissions and ham radio.
THF
(Tremendously high frequency)
300-3000 GHz1 mm – 100 micrometroSatellite transmissions (submillimeter waves or submillimeter band 300 GHz 3 THz) and ham radio.

The working frequencies for the various radio services are chosen within frequency ranges (or bands) that are defined at international level, in order to avoid interference or overlapping as much as possible.

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