Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF). VHF radio waves propagate mainly by line-of-sight, so they are blocked by hills and mountains, although due to refraction they can travel somewhat beyond the visual horizon out to about 160 km (100 miles). Common uses for radio waves in the VHF band are Digital Audio Broadcasting (DAB) and FM radio broadcasting, television broadcasting, two-way land mobile radio systems (emergency, business, private use and military), long range data communication up to several tens of kilometers with radio modems, amateur radio, and marine communications. Air traffic control communications and air navigation systems (e.g. VOR and ILS) work at distances of 100 kilometres (62 miles) or more to aircraft at cruising altitude. In the Americas and many other parts of the world, VHF Band I was used for the transmission of analog television. As part of the worldwide transition to digital terrestrial television most countries require broadcasters to air television in the VHF range using digital, rather than analog encoding.
Propagation characteristics Radio waves in the VHF band propagate mainly by line-of-sight and ground-bounce paths; unlike in the HF band there is only some reflection at lower frequencies from the ionosphere (skywave propagation). They do not follow the contour of the Earth as ground waves and so are blocked by hills and mountains, although because they are weakly refracted (bent) by the atmosphere they can travel somewhat beyond the visual horizon out to about 160 km (100 miles). They can penetrate building walls and be received indoors, although in urban areas reflections from buildings cause multipath propagation, which can interfere with television reception. Atmospheric radio noise and interference (RFI) from electrical equipment is less of a problem in this and higher frequency bands than at lower frequencies. The VHF band is the first band at which efficient transmitting antennas are small enough that they can be mounted on vehicles and portable devices, so the band is used for two-way land mobile radio systems, such as walkie-talkies, and two way radio communication with aircraft (Airband) and ships (marine radio). Occasionally, when conditions are right, VHF waves can travel long distances by tropospheric ducting due to refraction by temperature gradients in the atmosphere.
Line-of-sight calculation
VHF transmission range is a function of transmitter power, receiver sensitivity, and distance to the horizon, since VHF signals propagate under normal conditions as a near line-of-sight phenomenon. The distance to the radio horizon is slightly extended over the geometric line of sight to the horizon, as radio waves are weakly bent back toward the Earth by the atmosphere. An approximation to calculate the line-of-sight horizon distance (on Earth) is:
distance in nautical miles = 1.23 × A ft {\displaystyle 1.23\times {\sqrt {A_{\textrm {ft}}}}} where A ft {\displaystyle A_{\textrm {ft}}} is the height of the antenna in feet distance in kilometers = 12.746 × A m {\displaystyle {\sqrt {12.746\times A_{\textrm {m}}}}} where A m {\displaystyle A_{\textrm {m}}} is the height of the antenna in meters. These approximations are only valid for antennas at heights that are small compared to the radius of the Earth. They may not necessarily be accurate in mountainous areas, since the landscape may not be transparent enough for radio waves. In engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station.
Antennas
VHF is the first band at which wavelengths are small enough that efficient transmitting antennas are short enough to mount on vehicles and handheld devices, a quarter wave whip antenna at VHF frequencies is 25 cm to 2.5 meter (10 inches to 8 feet) long. So the VHF and UHF wavelengths are used for two-way radios in vehicles, aircraft, and handheld transceivers and walkie-talkies. Portable radios usually use whips or rubber ducky antennas, while base stations usually use larger fiberglass whips or collinear arrays of vertical dipoles. For directional antennas, the Yagi antenna is the most widely used as a high gain or "beam" antenna. For television reception, the Yagi is used, as well as the log-periodic antenna due to its wider bandwidth. Helical and turnstile antennas are used for satellite communication since they employ circular polarization. For even higher gain, multiple Yagis or helicals can be mounted together to make array antennas. Vertical collinear arrays of dipoles can be used to make high gain omnidirectional antennas, in which more of the antenna's power is radiated in horizontal directions. Television and FM broadcasting stations use collinear arrays of specialized dipole antennas such as batwing antennas.
Universal use Certain subparts of the VHF band have the same use around the world. Some national uses are detailed below.
50–54 MHz: amateur radio 6-meter band. 108–118 MHz: Air navigation beacons VOR and Instrument Landing System localizer. 118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency 144–146 MHz: amateur radio 2-meter band (Extends up to 148 MHz in some Regions). 156–174 MHz: VHF maritime mobile band for maritime two-way radio on ships.
By country
Australia
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