Very high frequency

VHF (Very high frequency) is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF). The frequency allocation is done by ITU.

These names referring to high-end frequency usage originate from mid-20th century, when regular radio service used MF, Medium Frequencies, better known as "AM" in USA, below the HF. Currently VHF is at the low-end of practical frequency usage, new systems tending to use frequencies in SHF and EHF above the UHF range. See Radio spectrum for full picture.

Common uses for VHF are FM radio broadcast, television broadcast, land mobile stations (emergency, business, and military), long range data communication with radio modems, Amateur Radio, marine communications, air traffic control communications and air navigation systems (e.g. VOR, DME & ILS).

Propagation characteristics

VHF propagation characteristics are ideal for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter (see formula below). Unlike high frequencies (HF), the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and don't interfere with transmissions thousands of kilometres away). VHF is also less affected by atmospheric noise and interference from electrical equipment than lower frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less affected by buildings and other less substantial objects than UHF frequencies.

Two unusual propagation conditions can allow much farther range than normal. The first, tropospheric ducting, can occur in front of and parallel to an advancing cold weather front, especially if there is a marked difference in humidities between the cold and warm air masses. A duct can form approximately 250 km (155 mi) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of kilometers. For example, a 50 watt Amateur FM transmitter at 146 MHz can talk from Chicago, to Joplin, Missouri, directly, and to Austin, Texas, through a repeater. In a July 2006 incident, a NOAA Weather Radio transmitter in north central Wisconsin was blocking out local transmitters in west central Michigan, quite far out of its normal range. In midsummer 2006, central Iowa stations were heard in Columbus, Nebraska and blocked out Omaha radio and TV stations for several days, while WBNX-TV in Akron, Ohio, a television station on Channel 55 in the analog age, was noted for bleeding over other Channel 55 stations in Wausau and Kenosha, Wisconsin as far west as the Wisconsin River valley for hours at a time. Similar propagation effects can affect land-mobile stations in this band, rarely causing interference well beyond the usual coverage area. The second type, much more rare, is called Sporadic E, referring to the E-layer of the ionosphere. Phenomena still not completely understood (as of 2010) may allow the formation of ionized "patches" in the ionosphere, dense enough to reflect back VHF frequencies the same way HF frequencies are usually reflected (skywave). For example, KMID (TV Channel 2; 54–60 MHz) from Midland, Texas was seen around Chicago, pushing out Chicago's WBBM-TV. These patches may last for seconds, or extend into hours. FM stations from Miami, Florida; New Orleans, Louisiana; Houston, Texas and even Mexico were heard for hours in central Illinois during one such event.

Line-of-sight calculation

For analog TV, 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 miles = $\backslash sqrt\{1.5\; \backslash times\; A\_f\}$ where $A\_f$ is the height of the antenna in feet

distance in kilometres = $\backslash sqrt\{12.746\; \backslash times\; A\_m\}$ where $A\_m$ is the height of the antenna in metres.

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.

The accuracy of these calculations for digital TV signals is being debated.

Universal use

Certain subparts of the VHF band have the same use around the world. Some national uses are detailed below.

108–118 MHz: Air navigation beacons VOR and Instrument Landing System localiser.

118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency

By country

, FM radio, amateur radio, marine radio and aviation.]]

Australia

The VHF TV band in Australia was originally allocated channels 1 to 10 - with channels 2, 7 and 9 assigned for the initial services in Sydney and Melbourne, and later the same channels were assigned in Brisbane, Adelaide and Perth. Other capital cities and regional areas used a combination of these and other frequencies as available. For some strange reason, the initial commercial services in Hobart and Darwin were respectively allocated channels 6 and 8 rather than 7 or 9.

By the early 1960s it became apparent that the 10 VHF channels were insufficient to support the growth of television services. This was rectified by the addition of three additional frequencies - channels 0, 5A and 11. Older television sets using rotary dial tuners required adjustment to receive the new channels.

Several TV stations were allocated to VHF channels 3, 4 and 5A, which were within the FM radio bands although not yet used for that purpose. A couple of notable examples were NBN Newcastle, WIN-4 Wollongong and ABC Illawarra on channel 5A. Most TVs of that era were not equipped to receive these broadcasts, and so were modified at the owners' expense to be able to tune into these bands; otherwise the owner had to buy a new TV. Beginning in the 1990s, the Australian Broadcasting Authority began a process to move these stations to UHF bands to free up valuable VHF spectrum for its original purpose of FM radio. In addition, by 1985 the federal government decided new TV stations are to be broadcast on the UHF band.

Two new VHF frequencies, 9A and 12, have since been made available and are being used primarily for digital services (e.g. ABC in capital cities) but also for some new analogue services in regional areas. Because channel 9A is not used for television services in or near Sydney, Melbourne, Brisbane, Adelaide or Perth, digital radio in those cities are broadcast on DAB frequencies blocks 9A, 9B and 9C.

New Zealand

44–51, 54–68 MHz: Band I Television (channels 1–3)

87.5–108 MHz: Band II Radio

174–230 MHz: Band III Television (channels 4–11)

In New Zealand, the four main Free-to-Air TV stations still use the VHF Television bands (Band I and Band III) to transmit their programmes to New Zealand households. Other stations, including a variety of pay and regional free-to-air stations, are forced to broadcast their programmes in the UHF band, since the VHF band is very overloaded with four stations sharing a very small frequency band, which can be so overcrowded that one or more channels, more often than not one of the MediaWorks-owned channels (TV3 and C4), is unavailable in some smaller towns.

United Kingdom

British television originally used VHF band I and band III. Television on VHF was in black and white with 405-line format (although there were experiments with all three colour systems—NTSC, PAL, and SECAM—adapted for the 405-line system in the late 1950s and early 60s).

British colour television was broadcast on UHF (channels 21–69), beginning in the late 1960s. From then on, TV was broadcast on both VHF and UHF (VHF being a monochromatic downconversion from the 625-line colour signal), with the exception of BBC2 (which had always broadcast solely on UHF). The last British VHF TV transmitters closed down on January 3, 1985. VHF band III is now used in the UK for digital audio broadcasting.

Unusually, the UK has an amateur radio allocation at 4 metres, 70-70.5 MHz.

United States and Canada

Frequency assignments between US and Canadian users are closely coordinated since much of the Canadian population is within VHF radio range of the US border. Certain discrete frequencies are reserved for radio astronomy. The general services in the VHF band are: 30–46 MHz: Licensed 2-way land mobile communication.

30–88 MHz: Military VHF-FM, including SINCGARS

43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way mobile communication. The FM broadcast band originally operated here (42-50 MHz) before moving to 88-108 MHz.

50–54 MHz: Amateur radio 6 meter band

54-72 and 76-88 MHz TV channels 2 through 6 (VHF-Lo), known as "Band I" internationally; a tiny number of DTV stations will appear here. See North American broadcast television frequencies

72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz in the USA and Canada, air navigation beacons 74.8-75.2 MHz.

88–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial in the United States) (Known as "Band II" internationally)

108–118 MHz: Air navigation beacons VOR

118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency

137-138 Space research, space operations, meteorological satellite

138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services

144–148 MHz: Amateur radio 2 Meters band

148-150 Land mobile, fixed, satellite

150–156 MHz: "VHF Business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM

156–158 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency.

160-161 MHz Railways

162.40–162.55: NOAA Weather Stations, narrowband FM

174-216 MHz television channels 7 - 13 (VHF-Hi), known as "Band III" internationally. A minority of DTV channels may appear here.

174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)

216–222 MHz: land mobile, fixed, maritime mobile,

222–225 MHz: 1.25 meters (US) (Canada 219-220, 222-225 MHz) Amateur radio

225 MHz and above: Military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104

The large technically and commercially valuable slice of the VHF spectrum taken up by television broadcasting has attracted the attention of many companies and governments recently, with the development of more efficient digital television broadcasting standards. In some countries much of this spectrum will likely become available (probably for sale) within the next decade or so (June 12, 2009, in the United States).

87.5-87.9 MHz

87.5-87.9 MHz is a radio frequency which, in most of the world, is used for FM broadcasting. In North America, however, this bandwidth is allocated to VHF television channel 6 (82-88 MHz). The audio for TV channel 6 is broadcast at 87.75 MHz (adjustable down to 87.74). Several stations, most notably those joining the Pulse 87 franchise, operate on this frequency as radio stations, though they use television licenses. As a result, FM radio receivers such as those found in automobiles which are designed to tune into this frequency range can receive the audio for programming on the local TV channel 6 while in North America.

87.9 MHz is normally off-limits for FM audio broadcasting except for displaced class D stations which have no other frequencies in the normal 88.1-107.9 MHz subband on which to move. So far, only 2 stations have qualified to operate on 87.9 MHz: 10-watt KSFH in Mountain View, California and 34-watt translator K200AA in Sun Valley, Nevada.

Unlicensed operation

In some countries, particularly the United States and Canada, limited low-power license-free operation is available in the FM broadcast band for purposes such as micro-broadcasting and sending output from CD or digital media players to radios without auxiliary-in jacks, though this is illegal in some other countries. This practice was legalised in the United Kingdom on 8 December 2006.

See also

Marine VHF radio

Oldest radio station

Apex (radio band)

FM broadcast band

Moving image formats

Polar mesosphere summer echoes

Television channel frequencies

Knife-edge effect

Instrument Landing System

VHF omnidirectional range

References

Category:Radio spectrum