Lakhasly

The ionosphere is created by ionization of the upper atmosphere by ultraviolet and X-rays, and is divided in to D, E and F layers.The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar.This frequency range is use for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, and numerous short range terrestrial data links.Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz).Millimetre-length electromagnetic waves were first investigated in the 1890s by Bengali-Indian scientist Jagadish Chandra Bose.The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz(GHz).Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances.Since the ionization varies with altitude, signals are not reflected, but refracted by the ionosphere.4 Frequencies in the SHF range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band.Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere.However the short propagation range allows smaller frequency reuse distances than lower frequencies.

The ionosphere is created by ionization of the upper atmosphere by ultraviolet and X-rays, and is divided in to D, E and F layers. The F layer extends from 150 to 650 km and is responsible for most long-range HF propagation. Since the ionization varies with altitude, signals are not reflected, but refracted by the ionosphere. Signals of different frequencies are thus returned to Earth at different distances (or not at all). During the daytime, the “skip distance” for a 7-MHz signal might be 300 km, while that of 21-MHz signal could be 3000 km. With the lower electron densities at night, the skip at 7 MHz might increase to 2000 km, while 21 MHz would not be returned at all. Long-range communication can also be accomplished over multiple-hop paths that include a reflection from the earth or layer. Conditions vary with the time of day, latitude, season, and solar conditions [1].
Decades before the cellular technologies began to provide wireless access to the masses, two-way radio met the need for mobile communications. As early as 1946, AT&T obtained approval from FCC to operate the first commercial car-borne mobile-telephone service in St. Louis, MO. It had one base-station on high ground and was equipped with six channels. Within a year, the service had expanded to 25 other cities. The applications for modern mobile radio include land, marine, and airborne use by
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both civilian and military organizations. There are also telemetry and one-way paging services. Each service is assigned a specific set of frequencies in the UHF bands (primarily near 160 and 460 MHz) Conventional land-mobile radio (LMR) and private mobile radio (PMR) systems generally dedicate a single channel to a specific group of users (e.g., a local police department). Typically, a small number of base-stations with well-placed antenna systems communicate with a large number of mobile stations. Narrow-band FM (NBFM) with 25-kHz channel spacing is generally used, and most systems offer half-duplex or simplex communication. Selective signaling is accomplished by sub audible squelch tones or digital codes. Transmitter powers are typically on the order of 1–5 W for handheld, 25 W for mobile, and 100 W for base. Access times of a few tenths of a second make LMR systems very attractive for emergency services [1].
In a trunked radio system [also called specialized mobile radio (SMR)], a commercial radio operator provides land-mobile service to a number of different user groups. This allows more efficient use of the spectrum (since channels are shared), wider coverage, and a greater variety of services including interconnection to the Public Switched Telephone Network (PSTN). A single SMR base-station typically covers a 25-mi radius. Multiple sites are interconnected through the use of repeaters to extend coverage. Many systems also allow mobile-to-mobile “talk around” to improve local coverage. A wide variety of standards and protocols are in use. The original analog standards have been supplanted by digital standards within the last two decades [1]. Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz(GHz). This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar. This frequency range is use for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, and numerous short range terrestrial data links.
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Frequencies in the SHF range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band. Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band, and the far infrared band which is also referred to as the terahertz gap. Radio waves in this band have wave lengths from ten to one millimetre, giving it the name millimetre band or millimetre wave, sometimes abbreviated MMW or mmW. Millimetre-length electromagnetic waves were first investigated in the 1890s by Bengali-Indian scientist Jagadish Chandra Bose. Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Therefore, they have a short range and can only be used for terrestrial communication over about a kilometer. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances. However the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.