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65-799: 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

130-478: A dipole, or a quarter-wave whip with an adequate size ground plane. Whips not mounted on the radio itself are usually fed with coaxial cable feedline of 50 ohm or 75 ohm impedance. In transmitting antennas the impedance of the antenna must be matched to the feedline for maximum power transfer. A half wave whip antenna (length of 1 2 λ {\displaystyle {\tfrac {1}{2}}\lambda } ) has somewhat higher gain than

195-495: A perfectly conducting ground plane under it maximum field strength is in horizontal directions, falling monotonically to zero on the axis. With a small or imperfectly conducting ground plane or no ground plane under it, the general result is to tilt the main lobe up so maximum power is no longer radiated horizontally but at an angle into the sky. Antennas longer than a half-wavelength have patterns consisting of several conical "lobes"; with radiation maxima at several elevation angles;

260-499: 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

325-619: 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,

390-441: A quarter wave ground plane antenna with horizontal ground wires is around 22 ohms, a poor match to coaxial cable feedline, and the main lobe of the radiation pattern is still tilted up toward the sky. Often (see pictures) the ground plane rods are sloped downward at a 45-degree angle, which has the effect of lowering the main lobe of the radiation pattern so more of the power is radiated in horizontal directions, and increases

455-416: A quarter wave whip, but it has a current node at its feedpoint at the base of the rod so it has very high input impedance. If it was infinitely thin the antenna would have an infinite input impedance, but the finite width gives typical, practical half wave whips an impedance of 800–1,500 ohms. These are usually fed through an impedance matching transformer or a quarter wave stub matching section (e.g.

520-462: A quarter-wavelength long connected to the opposite side of the feedline, extending horizontally from the base of the whip, is often used. This is called a ground plane antenna . These few short wire elements serve to receive the displacement current from the driven element and return it to the ground conductor of the transmission line, making the antenna behave somewhat as if it has a continuous conducting plane under it. The radiation resistance of

585-403: A quarter-wavelength, and still be resonant , by cancelling out the capacitive reactance of the short antenna. This is called an electrically short whip . The coil is added at the base of the whip (called a base-loaded whip) or occasionally in the middle (center-loaded whip). In the most widely used form, the rubber ducky antenna , the loading coil is integrated with the antenna itself by making

650-505: 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

715-414: A whip antenna is determined by the wavelength of the radio waves it is used with. Their length varies from compact electrically short antennas ⁠ 1 / 10  ⁠  wavelength long, up to ⁠ 5  / 8 ⁠   wavelength to improve directivity. The most common type is the quarter-wave whip , which is approximately ⁠ 1  / 4 ⁠ wavelength long. Whips are

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780-408: Is a monopole antenna , and like a vertical dipole has an omnidirectional radiation pattern , radiating equal radio power in all azimuthal directions (perpendicular to the antenna's axis), with the radiated power falling off with elevation angle to zero on the antenna's axis. Whip antennas less than one-half wavelength long, including the common quarter wave whip, have a single main lobe , and with

845-436: 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

910-501: 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

975-524: 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

1040-1061: 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 Line-of-sight propagation Line-of-sight propagation

1105-404: Is because they transmit (or receive) equally well in all horizontal directions, while radiating little radio energy up into the sky where it is wasted. Whip antennas are normally designed as resonant antennas; the rod acts as a resonator for radio waves, with standing waves of voltage and current reflected back and forth from its ends. Therefore, the length of the antenna rod is determined by

1170-457: 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

1235-558: 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

1300-404: Is never approached in actual antennas unless the ground plane is many wavelengths in diameter. 2 dBi is more typical for a whip with a ground plane of 1 2 λ   . {\displaystyle {\tfrac {1}{2}}\lambda ~.} Whips mounted on vehicles use the metal skin of the vehicle as a ground plane. In hand-held devices usually no explicit ground plane

1365-461: 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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1430-449: Is possible with coils at about one-half or one-third and two-thirds that do not affect the aerial much at the lowest band, but it creates the effect of stacked dipoles at a higher band (usually ×2 or ×3 frequency). At higher frequencies the feed coax can go up the centre of a tube. The insulated junction of the tube and whip is fed from the coax and the lower tube end where coax cable enters has an insulated mount. This kind of vertical whip

1495-426: Is provided, and the ground side of the antenna's feed line is just connected to the ground (common) on the device's circuit board. Therefore, the radio itself serves as a rudimentary ground plane. If the radio chassis is not a good deal larger than the antenna itself, the combination of whip and radio functions more as an asymmetrical dipole antenna than as a monopole antenna . The gain will be somewhat lower than

1560-592: 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

1625-470: Is split into a horizontal lobe and a small second lobe at a 60° angle, so high angle radiation is poor. The input impedance is around 40 ohms. In a whip antenna not mounted on a conductive surface, such as one mounted on a mast, the lack of reflected radio waves from the ground plane causes the lobe of the radiation pattern to be tilted up toward the sky so less power is radiated in horizontal directions, undesirable for terrestrial communication. Also

1690-433: 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

1755-454: 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,

1820-686: 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

1885-447: The J-pole antenna ). An advantage is that because it acts as a dipole it does not need a ground plane. The maximum horizontal gain of a monopole antenna is achieved at a length of five eighths of a wavelength 5 8 λ {\displaystyle {\tfrac {5}{8}}\lambda } so this is also a popular length for whips. However at this length the radiation pattern

1950-611: 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

2015-428: 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

Very high frequency - Misplaced Pages Continue

2080-418: 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

2145-404: 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,

2210-415: The wavelength ( λ {\displaystyle \lambda } ) of the radio waves used. The most common length is approximately one-quarter of the wavelength ( 1 4 λ {\displaystyle {\tfrac {1}{4}}\lambda } ), called a "quarter-wave whip" (although often shortened by the use of a loading coil ; see Electrically short whips below). For example,

2275-435: 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

2340-538: 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

2405-453: 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

2470-548: 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

2535-468: 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

2600-455: 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

2665-487: 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

Very high frequency - Misplaced Pages Continue

2730-427: 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

2795-625: The common quarter-wave whip antennas used on FM radios in the USA are approximately 75 cm (2.5 feet) long, which is roughly one-quarter the length of radio waves in the FM radio band, which are 2.78 to 3.41 m (9 to 11 feet) long. Half-wave whips ( 1 2 λ {\displaystyle {\tfrac {1}{2}}\lambda } long) which have greater gain , and five-eighth wave whips ( 5 8 λ {\displaystyle {\tfrac {5}{8}}\lambda } long) which have

2860-402: 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

2925-467: 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

2990-407: 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

3055-421: 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

3120-552: 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

3185-413: The input impedance for a good match to standard 50-ohm coaxial cable . To match 75-ohm coaxial cable, the ends of the ground plane can be turned downward or a folded monopole driven element can be used. To reduce the length of a whip antenna to make it less cumbersome, an inductor ( loading coil ) is often added in series with it. This allows the antenna to be made much shorter than the normal length of

3250-408: 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

3315-523: The longer the electrical length of the antenna, the more lobes the pattern has. A vertical whip radiates vertically polarized radio waves, with the electric field vertical and the magnetic field horizontal. Vertical whip antennas are widely used for nondirectional radio communication on the surface of the Earth, where the direction to the transmitter (or the receiver) is unknown or constantly changing, for example in portable FM radio receivers, walkie-talkies , and two-way radios in vehicles. This

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3380-462: The maximum horizontal gain achievable by a monopole, are also common lengths. The gain and input impedance of the antenna is dependent on the length of the whip element, compared to a wavelength, but also on the size and shape of the ground plane used (if any). A quarter wave vertical antenna working against a perfectly conducting, infinite ground will have a gain of 5.19  dBi and a radiation resistance of about 36.8  ohms . However this gain

3445-590: The most common type of monopole antenna , and are used in the higher frequency HF , VHF and UHF radio bands. They are widely used as the antennas for hand-held radios, cordless phones , walkie-talkies , FM radios , boom boxes , and Wi-Fi enabled devices, and are attached to vehicles as the antennas for car radios and two-way radios for wheeled vehicles and for aircraft. Larger versions mounted on roofs, balconies and radio masts are used as base station antennas for amateur radio and police, fire, ambulance, taxi, and other vehicle dispatchers. The whip antenna

3510-403: The name is derived from the whip -like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer whips, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 11 m (35 feet) long. The length of

3575-513: 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

3640-441: 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

3705-414: 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

3770-486: 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

3835-518: 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

3900-448: 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

3965-425: The service range of a station at an altitude of 1500 m with respect to receivers at sea level can be found as, Whip antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter . A whip antenna is a form of monopole antenna . The antenna is designed to be flexible so that it does not break easily, and

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4030-405: The unbalanced impedance of the monopole element causes RF currents in the supporting mast and on the outside of the ground shield conductor of the coaxial feedline, causing these structures to radiate radio waves, which usually has a deleterious effect on the radiation pattern. To prevent this, with stationary whips mounted on structures, an artificial "ground plane" consisting of three or four rods

4095-441: The whip out of a narrow helix of springy wire. The helix distributes the inductance along the antenna's length, improving the radiation pattern, and also makes it more flexible. Another alternative occasionally used to shorten the antenna is to add a "capacity hat", a metal screen or radiating wires, at the end. However all these electrically short whips have lower gain than a full-length quarter-wave whip. Multi-band operation

4160-405: 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

4225-399: 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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