Very high frequency

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Antenna

Rabbit-ears dipole antenna with UHF loop 20090204

Superturnstile Tx Muehlacker

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VHF Usage

Very High Frequency (VHF) refers to the radio frequency range from 30 to 300 megahertz (MHz). Frequencies in this band are used for many critical services, including radio broadcasting, television broadcasting, land mobile stations for emergency services, air traffic control communications, and marine communications. VHF is also used for long-distance data transmission through the ionosphere in a process known as "skywave" or "skip" propagation, allowing communication over several hundred kilometers.

Characteristics

VHF propagates mainly by line-of-sight; therefore, its range is limited by the curvature of the Earth, typically to about 100 kilometers (62 miles), depending on antenna height. However, under certain atmospheric conditions, VHF signals can travel much further via tropospheric ducting or ionospheric reflection.

Advantages

• Line-of-sight propagation makes VHF useful for short to medium distance communication, with relatively stable and predictable signal quality.
• Less susceptibility to interference and noise compared to lower frequencies.
• Suitable for both voice and data transmission, supporting a wide range of applications.

Disadvantages

• Limited range compared to MF and LF bands, especially for ground-wave propagation.
• Requires larger antennas than higher frequency bands, such as UHF.

Applications

Radio Broadcasting

VHF includes the FM radio band, which ranges from 88 to 108 MHz. This band offers high-fidelity sound over short to medium distances, making it ideal for music and entertainment broadcasting.

Television Broadcasting

The VHF band was historically used for analog television broadcasting in many countries before the transition to digital broadcasting. It includes two separate bands: Band I (channels 2-6) and Band III (channels 7-13).

Emergency Services

Many emergency service providers, including police, fire, and ambulance services, use the VHF band for communication due to its reliability and the ability to penetrate buildings and rugged terrain.

Marine Communications

VHF is extensively used in maritime communications, especially for near-shore communications. It is used for ship-to-ship, ship-to-shore, and distress communication systems.

Air Traffic Control

Air traffic control communications between ground controllers and aircraft pilots are conducted over VHF to ensure clear and reliable communication over short to medium distances.

Propagation Modes

• Line-of-Sight: The primary mode of propagation, limited by the horizon.
• Tropospheric Ducting: Occurs when layers of the atmosphere bend VHF signals, extending their range.
• Ionospheric Reflection: At certain times, especially during high solar activity, VHF signals can bounce off the ionosphere, allowing for long-distance communication.

Regulation and Allocation

The allocation of VHF frequencies is regulated by national and international bodies, including the International Telecommunication Union (ITU). These organizations ensure that the spectrum is used efficiently and that services do not interfere with each other.

See Also

• Ultra High Frequency (UHF)
• Super High Frequency (SHF)
• Electromagnetic Spectrum
• Radio Spectrum

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