Very high frequency

Very high frequency (VHF) 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, private use 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.

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 = where is the height of the antenna in feet
• distance in kilometres = where 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.^[1]

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
• 144–146 MHz: Amateur radio. In some countries 144–148 MHz.

By country

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 these new channels. 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.

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. 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 to New Zealand households. Other stations, including a variety of pay and regional free-to-air stations, are forced to broadcast 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 FOUR), is unavailable in some smaller towns.

However, by the end of 2013, there will be no television channels broadcasting on the VHF bands.^[2]

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, and VHF band II is used for FM radio, as it is in most of the world.

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.^[3]
• 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; some 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 an emergency frequency
• 137-138 Space research, space operations, meteorological satellite ^[4]
• 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," public safety, 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.
• 159.81-161.565 MHz Railways ^[5] 159-81-160.2 are railroads in Canada only and is used by trucking companies in the U.S.
• 160.6-162 Wireless microphones and TV/FM broadcast remote pickup
• 162.40–162.55: NOAA Weather Stations, narrowband FM
• 174-216 MHz television channels 7 - 13 (VHF-Hi), known as "Band III" internationally. A number of DTV channels have begun broadcasting here, especially many of the stations which were assigned to these channels for previous analog operation.
• 174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
• 216–222 MHz: land mobile, fixed, maritime mobile,^[6]
• 222–225 MHz: 1.25 meters (US) (Canada 219-220, 222-225 MHz) amateur radio
• 225 MHz and above: Military aircraft radio, 243 Mhz is an emergency frequency(225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104

• 329-335 Mhz: Air Traffic Control VHF Glide slope

VHF television

It is considered that one of the most significant events in the history of broadcast television regulation was the creation of an artificial scarcity of VHF licenses. The FCC's decision to locate television service on the limited VHF band changed the ways of television service and network competition in the industry. The rationale of this policy was to create a situation of increased competition and viewer choice. Television was added to the VHF band in 1941 on channels one through six. During World War II, channel one was removed and used only for war purposes. Later, in 1945, channels seven through thirteen were added.^[7]

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 analog 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, have operated 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 could receive the audio for analog-mode 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.^[8]

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