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43-467: Terrestrial high-definition television is a form of broadcast high-definition television that is received via the terrestrial airwaves using either a VHF television aerial or a UHF television aerial. Depending on the country, the high definition television channels are broadcasts using either ATSC , ISDB-T , DVB-T or DVB-T2 . Some countries such as Germany , The Netherlands and Belgium may only broadcasts their channels in standard definition via

86-499: 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 channels, 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

129-666: A proposed transmitter station. 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,

172-505: 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. The practice largely ended with the DTV transition in 2009, although some still exist. The FM broadcast channel at 87.9 MHz is normally off-limits for FM audio broadcasting; it

215-436: Is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves can only travel in a direct visual path from the source to the receiver without obstacles. Electromagnetic transmission includes light emissions traveling in a straight line . The rays or waves may be diffracted , refracted , reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over

258-501: Is a consequence of a circular segment of earth profile that blocks off long-distance communications. Since the vacuum line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path. Assuming a perfect sphere with no terrain irregularity, the distance to the horizon from a high altitude transmitter (i.e., line of sight) can readily be calculated. Let R be

301-524: Is a radio band 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, known as Frankenstations , most notably those joining the Pulse 87 franchise, have operated on this frequency as radio stations, though they use television licenses. As

344-1049: 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. ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Radio horizon Line-of-sight propagation

387-457: Is composed of a conductor that completely surrounds an area on all sides, top, and bottom. Electromagnetic radiation is blocked where the wavelength is longer than any gaps. For example, mobile telephone signals are blocked in windowless metal enclosures that approximate a Faraday cage, such as elevator cabins, and parts of trains, cars, and ships. The same problem can affect signals in buildings with extensive steel reinforcement. The radio horizon

430-558: 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

473-463: 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. VHF radio is also used for marine Radio as per its long-distance reachability comparing UHF frequencies. Example allocation of VHF–UHF frequencies: Until 2013, the four main free-to-air TV stations in New Zealand used

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516-592: Is reserved for displaced class D stations which have no other frequencies in the normal 88.1–107.9 MHz subband to move to. So far, only two 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 . In some countries, particularly the United States and Canada, limited low-power license-free operation

559-433: Is the locus of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere without an atmosphere, the radio horizon would be a circle. The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Radio wave propagation is affected by atmospheric conditions, ionospheric absorption , and

602-454: Is usually chosen to be 4 ⁄ 3 . That means that the maximum service range increases by 15%. for h in metres and d in kilometres; or for h in feet and d in miles. But in stormy weather, k may decrease to cause fading in transmission. (In extreme cases k can be less than 1.) That is equivalent to a hypothetical decrease in Earth radius and an increase of Earth bulge. For example, in normal weather conditions,

645-687: 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

688-611: 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

731-428: The horizon or behind obstacles. In contrast to line-of-sight propagation, at low frequency (below approximately 3  MHz ) due to diffraction , radio waves can travel as ground waves , which follow the contour of the Earth. This enables AM radio stations to transmit beyond the horizon. Additionally, frequencies in the shortwave bands between approximately 1 and 30 MHz, can be refracted back to Earth by

774-418: The ionosphere , called skywave or "skip" propagation, thus giving radio transmissions in this range a potentially global reach. However, at frequencies above 30 MHz ( VHF and higher) and in lower levels of the atmosphere, neither of these effects are significant. Thus, any obstruction between the transmitting antenna ( transmitter ) and the receiving antenna ( receiver ) will block the signal, just like

817-404: The light that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic at these frequencies is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon". In practice,

860-597: 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

903-435: 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

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946-538: 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. Frequency assignments between US and Canadian users are closely coordinated since much of

989-453: 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. Radio waves in the VHF band propagate mainly by line-of-sight and ground-bounce paths; unlike in

1032-549: 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: Cable television , though not transmitted aerially, uses a spectrum of frequencies overlapping VHF. The U.S. FCC allocated television broadcasting to a channelized roster as early as 1938 with 19 channels. That changed three more times: in 1940 when Channel 19

1075-468: 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, were forced to broadcast in the UHF band, since the VHF band had been very overloaded with four stations sharing a very small frequency band, which was so overcrowded that one or more channels would not be available in some smaller towns. However, at

1118-455: 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 . Certain subparts of the VHF band have the same use around the world. Some national uses are detailed below. The VHF TV band in Australia was originally allocated channels 1 to 10-with channels 2, 7 and 9 assigned for

1161-487: The atmosphere with height ( vertical pressure variation ) is to bend ( refract ) radio waves down towards the surface of the Earth. This results in an effective Earth radius , increased by a factor around 4 ⁄ 3 . This k -factor can change from its average value depending on weather. The previous vacuum distance analysis does not consider the effect of atmosphere on the propagation path of RF signals. In fact, RF signals do not propagate in straight lines: Because of

1204-427: The atmosphere. 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

1247-402: The direct line-of-sight can cause diffraction effects that disrupt radio transmissions. For the best propagation, a volume known as the first Fresnel zone should be free of obstructions. Reflected radiation from the surface of the surrounding ground or salt water can also either cancel out or enhance the direct signal. This effect can be reduced by raising either or both antennas further from

1290-468: The end of 2013 , all television channels stopped broadcasting on the VHF bands, as New Zealand moved to digital television broadcasting, requiring all stations to either broadcast on UHF or satellite (where UHF was unavailable) utilising the Freeview service. Refer to Australasian television frequencies for more information. British television originally used VHF band I and band III . Television on VHF

1333-407: The ground: The reduction in loss achieved is known as height gain . See also Non-line-of-sight propagation for more on impairments in propagation. It is important to take into account the curvature of the Earth for calculation of line-of-sight paths from maps, when a direct visual fix cannot be made. Designs for microwave formerly used 4 ⁄ 3  Earth radius to compute clearances along

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1376-421: The height h is given in metres, and distance d in kilometres, If the height h is given in feet, and the distance d in statute miles, In the case, when there are two stations involve, e.g. a transmit station on ground with a station height h and a receive station in the air with a station height H , the line of sight distance can be calculated as follows: The usual effect of the declining pressure of

1419-552: 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. 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

1462-408: The line-of-sight horizon distance (on Earth) is: 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

1505-513: The owner had to buy a new TV. Several TV stations were allocated to VHF channels 3, 4 and 5, 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 Newcastle on channel 5. While some Channel 5 stations were moved to 5A in the 1970s and 80s, beginning in the 1990s, the Australian Broadcasting Authority began

1548-441: The path. Although the frequencies used by mobile phones (cell phones) are in the line-of-sight range, they still function in cities. This is made possible by a combination of the following effects: The combination of all these effects makes the mobile phone propagation environment highly complex, with multipath effects and extensive Rayleigh fading . For mobile phone services, these problems are tackled using: A Faraday cage

1591-414: The presence of obstructions, for example mountains or trees. Simple formulas that include the effect of the atmosphere give the range as: The simple formulas give a best-case approximation of the maximum propagation distance, but are not sufficient to estimate the quality of service at any location. In telecommunications , Earth bulge refers to the effect of earth's curvature on radio propagation. It

1634-486: The propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests. Low-powered microwave transmitters can be foiled by tree branches, or even heavy rain or snow. The presence of objects not in

1677-518: The radius of the Earth and h be the altitude of a telecommunication station. The line of sight distance d of this station is given by the Pythagorean theorem ; The altitude of the station h is much smaller than the radius of the Earth R. Therefore, h 2 {\displaystyle h^{2}} can be neglected compared with 2 ⋅ R ⋅ h {\displaystyle 2\cdot R\cdot h} . Thus: If

1720-448: The refractive effects of atmospheric layers, the propagation paths are somewhat curved. Thus, the maximum service range of the station is not equal to the line of sight vacuum distance. Usually, a factor k is used in the equation above, modified to be k  > 1 means geometrically reduced bulge and a longer service range. On the other hand, k  < 1 means a shorter service range. Under normal weather conditions, k

1763-440: The terrestrial airwaves. This is largely because the uptake of cable television may be higher than that of terrestrial television, thus giving broadcasters very little incentive to provide their high-definition services via the terrestrial airwaves. Very high frequency 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

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1806-406: Was deleted and several channels changed frequencies, then in 1946 with television going from 18 channels to 13 channels, again with different frequencies, and finally in 1948 with the removal of Channel 1 (analog channels 2–13 remain as they were, even on cable television ). Channels 14–19 later appeared on the UHF band, while channel 1 remains unused. 87.5–87.9 MHz

1849-399: 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 1960s). 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

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