1.
Super high frequency
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Super high frequency is the ITU designation for radio frequencies in the range between 3 GHz and 30 GHz. This band of frequencies is known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the band, so radio waves with these frequencies are called microwaves. This frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave relay links. Wireless USB technology is anticipated to use approximately one-third of this spectrum, frequencies in the SHF range are often referred to by their IEEE radar band designations, S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations. Microwaves propagate entirely by line of sight, groundwave and ionospheric reflection do not occur, although in some cases they can penetrate building walls enough for useful reception, unobstructed rights of way cleared to the first Fresnel zone are usually required. Wavelengths are small enough at microwave frequencies that the antenna can be larger than a wavelength. Therefore, they are used in point-to-point terrestrial communications links limited by the visual horizon, such high gain antennas allow frequency reuse by nearby transmitters. The size of SHF waves allows strong reflections from objects the size of automobiles, aircraft, and ships. Thus, the narrow beamwidths possible with high gain antennas and the low atmospheric attenuation as compared with higher frequencies make SHF the main frequencies used in radar. Attenuation and scattering by moisture in the increase with frequency. Small amounts of energy are randomly scattered by water vapor molecules in the troposphere. This is used in communications systems, operating at a few GHz. A powerful microwave beam is aimed just above the horizon, as it passes through the some of the microwaves are scattered back to Earth to a receiver beyond the horizon. Distances of 300 km can be achieved and these are mainly used for military communication. The wavelengths of SHF waves are small enough that they can be focused into narrow beams by high gain antennas from a meter to five meters in diameter. Directive antennas at SHF frequencies are mostly aperture antennas, such as antennas, dielectric lens, slot. Large parabolic antennas can produce very narrow beams of a few degrees or less, for omnidirectional applications like wireless devices and cellphones, small dipoles or monopoles are used
2.
Extremely low frequency
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Extremely low frequency is the ITU designation for electromagnetic radiation with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, a definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency oscillations are considered to lie in the ULF range. ELF radio waves are generated by lightning and natural disturbances in Earths magnetic field, because of the difficulty of building antennas that can radiate such long waves, ELF frequencies have been used in only a very few human-made communication systems. ELF waves can penetrate seawater, which makes them useful in communication with submarines, the US, Russia, and India are the only nations known to have constructed ELF communication facilities. The U. S. facilities were used between 1985 and 2004 but are now decommissioned, ELF waves can also penetrate significant distances into earth or rock, and through-the-earth underground mine communication systems use frequencies of 300 to 3000 Hz. The frequency of alternating current flowing in electric grids,50 or 60 Hz, also falls within the ELF band. Some medical peer reviewed journal articles refer to ELF in the context of low frequency magnetic fields with frequencies of 50 Hz. United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz. Due to their long wavelength, ELF waves can diffract around large obstacles. ELF and VLF waves propagate long distances by an Earth-ionosphere waveguide mechanism, the Earth is surrounded by a layer of charged particles in the atmosphere at an altitude of about 60 km at the bottom of the ionosphere, called the D layer which reflects ELF waves. ELF waves have extremely low attenuation of 1 –2 dB per 1000 km. giving a single transmitter the potential to communicate worldwide, ELF waves can also travel considerable distances through lossy media like earth and seawater, which would absorb or reflect higher frequency radio waves. At certain frequencies these oppositely directed waves are in phase and add, in other words, the closed spherical Earth-ionosphere cavity acts as a huge cavity resonator, enhancing ELF radiation at its resonant frequencies. These are called Schumann resonances after German physicist Winfried Otto Schumann who predicted them in 1952, the United States Navy utilized extremely low frequencies as radio band and radio communications. The Submarine Integrated Antenna System was a research and development effort to communicate with submerged submarines, the Soviet/Russian Navy also utilized ELFs for submarine communications system, ZEVS. The Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class, because of its electrical conductivity, seawater shields submarines from most higher frequency radio waves, making radio communication with submerged submarines at ordinary frequencies impossible. Signals in the ELF frequency range, however, can penetrate much deeper, generally, ELF signals were used to order a submarine to rise to a shallow depth where it could receive some other form of communication. One of the difficulties posed when broadcasting in the ELF frequency range is antenna size, simply put, a 3 Hz signal would have a wavelength equal to the distance EM waves travel through a given medium in one third of a second
3.
Very low frequency
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Very low frequency or VLF is the ITU designation for radio frequencies in the range of 3 kHz to 30 kHz and corresponding wavelengths from 100 to 10 kilometres, respectively. The band is known as the myriametre band or myriametre wave as the wavelengths range from one to ten myriametres. Due to its bandwidth, audio transmission is highly impractical in this band. The VLF band is used for a few radio navigation services, government time radio stations, since VLF waves can penetrate at least 40 meters into saltwater, they are used for military communication with submarines. The main mode of long distance propagation is an Earth-ionosphere waveguide mechanism, the Earth is surrounded by a conductive layer of electrons and ions in the upper atmosphere, the ionosphere D layer at 60 km altitude, which reflects VLF radio waves. The conductive ionosphere and the conductive Earth, form a horizontal duct a few VLF wavelengths high, the waves travel in a zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM mode. Therefore, VLF transmissions are very stable and reliable, and are used for distance communication. Propagation distances of 5000 to 20000 km have been realized, however, atmospheric noise is high in the band, including such phenomena as whistlers, caused by lightning. VLF waves can penetrate seawater to a depth of at least 10 to 40 meters, depending on the frequency employed, VLF waves at certain frequencies have been found to cause electron precipitation. A major practical drawback to this band is that because of the length of the waves, vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30 kHz would be 2.5 kilometers high. So practical transmitting antennas are short, a small fraction of a wavelength long. Due to their low radiation resistance they are inefficient, radiating only 10% to 50% of the power at most. With the rest of the power dissipated in the antenna/ground system resistances, Very high power transmitters are required to radiate enough power for long distance communication. Transmitting antennas for VLF frequencies are very large antennas, up to a mile across. They consist of a series of radio masts, linked at the top with a network of cables. Either the towers themselves or vertical wires serve as monopole radiators, high power stations use variations on the umbrella antenna such as the delta and trideco antennas, or multiply-tuned flattop antennas. For low power transmitters, inverted-L and T antennas are used, a large loading coil is required at the antenna feed point to cancel the capacitive reactance of the antenna to make it resonant. To minimize power dissipated in the ground, these antennas require extremely low resistance ground systems, the high capacitance and inductance and low resistance of the antenna-loading coil combination, makes it act electrically like a high Q tuned circuit
4.
Low frequency
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Low frequency or LF is the ITU designation for radio frequencies in the range of 30 kHz–300 kHz. As its wavelengths range from ten kilometres to one kilometre, respectively, LF radio waves exhibit low signal attenuation, making them suitable for long-distance communications. In Europe and areas of Northern Africa and Asia, part of the LF spectrum is used for AM broadcasting as the longwave band, in the western hemisphere, its main use is for aircraft beacon, navigation, information, and weather systems. A number of time signal broadcasts are also broadcast in this band, because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ground wave, is the mode in the LF band. Ground waves must be polarized, so monopole antennas are used for transmitting. The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies, low frequency ground waves can be received up to 2,000 kilometres from the transmitting antenna. Low frequency waves can also travel long distances by reflecting from the ionosphere, although this method. Reflection occurs at the ionospheric E layer or F layers, skywave signals can be detected at distances exceeding 300 kilometres from the transmitting antenna. In Europe and Japan, many low-cost consumer devices have since the late 1980s contained radio clocks with an LF receiver for these signals. Since these frequencies propagate by ground wave only, the precision of time signals is not affected by varying propagation paths between the transmitter, the ionosphere, and the receiver. In the United States, such devices became feasible for the market only after the output power of WWVB was increased in 1997 and 1999. Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 metres, the longer the wavelength, the British, German, Indian, Russian, Swedish, United States and possibly other navies communicate with submarines on these frequencies. In addition, Royal Navy nuclear submarines carrying ballistic missiles are allegedly under standing orders to monitor the BBC Radio 4 transmission on 198 kHz in waters near the UK. In the US, the Ground Wave Emergency Network or GWEN operated between 150 and 175 kHz, until replaced by satellite systems in 1999. GWEN was a land based military radio communications system which could survive, the 2007 World Radiocommunication Conference made this band a worldwide amateur radio allocation. An international 2.1 kHz allocation, the 2200 meter band, is available to amateur operators in several countries in Europe, New Zealand, Canada. The world record distance for a contact is over 10,000 km from near Vladivostok to New Zealand
5.
Medium frequency
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Medium frequency is the ITU designation for radio frequencies in the range of 300 kHz to 3 MHz. Part of this band is the medium wave AM broadcast band, the MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten to one hectometer. Frequencies immediately below MF are denoted low frequency, while the first band of frequencies is known as high frequency. MF is mostly used for AM radio broadcasting, navigational beacons, maritime ship-to-shore communication. A major use of frequencies is AM broadcasting, AM radio stations are allocated frequencies in the medium wave broadcast band from 526.5 kHz to 1606. Although these are medium frequencies,120 meters is generally treated as one of the shortwave bands, there are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz. These include, as examples, the French MRCC on 1696 kHz and 2677 kHz, Stornoway Coastguard on 1743 kHz,2182 kHz is the international calling and distress frequency for SSB maritime voice communication. It is analogous to Channel 16 on the marine VHF band,500 kHz was for many years the maritime distress and emergency frequency, and there are more NDBs between 510 and 530 kHz. Navtex, which is part of the current Global Maritime Distress Safety System occupies 518 kHz and 490 kHz for important digital text broadcasts. Lastly, there are aeronautical and other mobile SSB bands from 2850 kHz to 3500 kHz, an amateur radio band known as 160 meters or top-band is between 1800 and 2000 kHz. Amateur operators transmit CW morse code, digital signals and SSB voice signals on this band, in recent years, some limited amateur radio operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden. Propagation at MF wavelengths is via ground waves and reflection from the ionosphere, ground waves follow the contour of the Earth. MF broadcasting stations use ground waves to cover their listening areas, however at certain times the D layer can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer and this can be very useful for long-distance communication, but can also interfere with local stations. Due to the number of available channels in the MW broadcast band. On nights of good skywave propagation, the signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency. These channels are called clear channels, and the stations, called stations, are required to broadcast at higher powers of 10 to 50 kW. Transmitting antennas commonly used on this band include monopole mast radiators, top-loaded wire monopole antennas such as the inverted-L and T antennas, ground wave propagation, the most widely used type at these frequencies, requires vertically polarized antennas like monopoles
