Very high frequency

Wavelength: 10m to 1m

very high frequency (VHF)
Cycles per second: 30MHz to 300MHz

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

Common uses for VHF are FM radio broadcast at 88–108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestrial navigation systems (VOR in particular) and aircraft communications.

VHF frequencies' 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 low frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less bothered by buildings and other less substantial objects than higher 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 150 miles (240 km.) 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 miles. For example, a 50 watt Amateur FM transmitter at 146 MHz can talk from Chicago, Illinois, to Joplin, Missouri, directly, and to Austin, Texas, through a repeater. The second type, much more rare, is called Sporadic-E, referring to the E-layer of the ionosphere. A sunspot eruption can pelt the Earth's upper atmosphere with charged particles, which may allow the formation of an ionized "patch" dense enough to reflect back VHF frequencies the same way HF frequencies are usually reflected (skywave). For example, TV channel 2 (54–60 MHz) from Midland, Texas was seen in Chicagoland, pushing out Chicago's own TV channel 2. 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.

It was also easier to construct efficient transmitters, receivers, and antennas for it in the earlier days of radio, as compared to UHF. In most countries, the VHF spectrum is used for broadcast audio and television, as well as commercial two-way radios (such as those operated by taxis and police), marine two-way audio communications, and aircraft radios.

The large technically and commercially valuable slice of the VHF spectrum taken up by television transmission 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) in the next decade or so (currently scheduled for 2008 in the United States).

Line of Sight Formula

VHF range is a function of transmitter power, receiver sensitivity, and the distance to the horizon, since VHF signals propagate under normal conditions as a line-of-sight phenomenon.

An approximation to calculate the line-of-sight horizon distance is:

distance in miles = $\sqrt{1.5 \times A_f}$ where $A_f$ is the height of the antenna in feet
distance in kilometres = $\sqrt{12.7 \times A_m}$ where $A_m$ is the height of the antenna in metres

Australia

The VHF TV band in Australia was originally allocated channels 1 to 10 - with the 2, 7 and 9 frequencies assigned for the initial services in Sydney and Melbourne, and later the same frequencies would be assigned in Brisbane, Adelaide and Perth. Other capital cities and regional areas would utilise a combination of these and other frequencies as available.

By the early 1960s it was apparent that the 10-channel spectrum was not going to be sufficient 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 would require adjustment to enable tuning to the new frequencies.

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-3 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.

Two new VHF frequencies, 9A and 12, have since been made available and are being utilised primarily for digital services (eg. ABC in capital cities) but also for some new analogue services in regional areas.

New Zealand

44–68 MHz: Band I Television (channels 1–3)
88–108 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, broadcast their programmes using the UHF band since the VHF band is very overloaded with four stations sharing a very small frequency band.

United Kingdom

British television originally used VHF bands I and III. Television on VHF was in black and white with 405-line display. British colour television was broadcast on UHF (channels 21–69), beginning in the late 1960s. TV from then on was broadcast on both VHF and UHF, 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

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 mixed 2-way mobile communication
50–54 MHz: Amateur radio 6 meter band
54–72 MHz: TV channels 2-4 (to be auctioned for other uses once conversion to digital TV has been accomplished)
72–75.5 MHz: Remote Control devices
75.5–87.5 MHz: TV channels 5-6 (to be auctioned for other uses once conversion to digital TV has been accomplished)
87.5–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial)
108–118 MHz: Air navigation beacons VOR
118–132 MHz: Airband for Air Traffic Control, AM, 121.5 MHz is emergency frequency
132–144 MHz: Auxiliary civil services, satellite, space research, and other miscellaneous services
144–148 MHz: Amateur band 2 Meters
148–174 MHz: "VHF Business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
156–174 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency
162.40–162.55: NOAA Weather Stations, narrowband FM
174–216 MHz: TV channels 7 through 13, and professional wireless microphones (low power, certain exact frequencies only)
216–222 MHz: mixed services
222–225 MHz: Amateur "1¼ Meter" band (really closer to 1.33M)
above 225 MHz: Federal services, notably military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104

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 microbroadcasting and sending output from CD or digital media players to radios without auxiliary-in jacks, though this is illegal in some other countries, most notably the United Kingdom.

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 the United States, however, this bandwidth is allocated to VHF channel 6 (82-88MHz). The audio for TV channel 6 is broadcast at 87.7MHz, making it possible, to tune in Channel 6 audio on a standard FM radio in most countries using the NTSC TV broadcast system.

87.9MHz is normally is off-limits except for displaced class D stations which have no other frequencies in the normal 88.1-107.9MHz 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.

External articles

Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).

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