Very high frequency

Very high frequency (VHF) is the ITU designation[1] for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).

Common uses for radio waves in the VHF band are 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 & ILS) work at distances of 100 kilometres (62 mi) or more to aircraft at cruising altitude.

In 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 format.

Very high frequency
Frequency range
30 MHz to 300 MHz
Wavelength range
10 to 1 m
Antenna
VHF television antennas used for broadcast television reception. These six antennas are a type known as a Yagi antenna, which is widely used at VHF

Propagation characteristics

Radio waves in the VHF band propagate mainly by line-of-sight and ground-bounce paths; unlike in the HF band there is only some reflection at lower frequencies from the ionosphere (skywave propagation).[2] 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 is less of a problem in the band 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 the atmosphere.

Line-of-sight calculation

Rabbit-ears dipole antenna with UHF loop 20090204
"Rabbit-ears" VHF television antenna (the small loop is a separate UHF antenna).

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 nautical miles = where is the height of the antenna in feet
  • distance in kilometers = where is the height of the antenna in meters.

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.[3]

Antennas

Superturnstile Tx Muehlacker
A VHF television broadcasting antenna. This is a common type called a super turnstile or batwing antenna.

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, 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 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.

Universal use

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

By country

VHF Usage
A plan showing VHF use in television, 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. 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 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 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, 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.

VHF radio is also used for marine Radio [4] as per its long-distance reachability comparing UHF frequencies.

Example allocation of VHF–UHF frequencies:[5]

  • Radionavigation 60: 84–86 MHz
  • Fixed Maritime Mobile: 130–135.7 MHz
  • Fixed Aeronautical radio navigation: 160–190 MHz
  • Broadcasting Aeronautical Radionavigation: 255–283.5 MHz
  • Aeronautical Radionavigation AUS 49 / Maritime Radionavigation (radiobeacons) 73: 315–325 MHz

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)

Until 2013, the four main Free-to-Air TV stations in New Zealand used 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 the end of 2013, all television channels stopped broadcasting on the VHF bands.[6]

Refer to Australasian television frequencies for more information.

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:

  • 30–49.6 MHz: Licensed 2-way land mobile communication, with various sub-bands.[a]
  • 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 it was moved to 88–108 MHz.
  • 50–54 MHz: Amateur radio 6 meter band
  • 54–72 and 76–88 MHz TV channels 2–6 (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 US and Canada, air navigation beacons 74.8–75.2 MHz.
  • 87.5–108 MHz: FM radio broadcasting (87.9–91.9 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 MHz Space research, space operations, meteorological satellite [8]
  • 138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services
  • 144–148 MHz: Amateur radio 2-meter band
  • 148–150 MHz: 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; 156.8 MHz (Channel 16) is the maritime emergency and contact frequency.
  • 159.81-161.565 MHz railways [b] 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, Weatheradio Canada Stations
  • 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,[8]
  • 222–225 MHz: 1.25 meters (US) (Canada 219–220, 222–225 MHz) amateur radio
  • 225 MHz and above(UHF): Military aircraft radio, 243 MHz is an emergency frequency (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104

Cable television, though not transmitted aerially, uses a spectrum of frequencies overlapping VHF.[9]

VHF television

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 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).[10]

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.

The FM broadcast channel at 87.9 MHz is normally off-limits for FM audio broadcasting; it 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.

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.[11]

See also

Notes

  1. ^ The 42 MHz Segment is still in current use by the California Highway Patrol, New Jersey State Police, Tennessee Highway Patrol, and other state law enforcement agencies.
  2. ^ The 160 and 161 areas are Association of American Railroads (AAR) 99 channel railroad radios, issued to the railroad. For example, AAR 21 is 160.425 MHz and that is issued to Tennessee Valley Railroad Museum, as well as other railroads that want AAR Channel 21.

References

  1. ^ "Rec. ITU-R V.431-7, Nomenclature of the frequency and wavelength bands used in telecommunications" (PDF). ITU. Archived from the original (PDF) on 31 October 2013. Retrieved 20 February 2013.
  2. ^ Seybold, John S. (2005). Introduction to RF Propagation. John Wiley and Sons. pp. 9–10. ISBN 978-0471743682.
  3. ^ Grotticelli, Michael (2009-06-22). "DTV Transition Not So Smooth in Some Markets". Broadcast Engineering. Archived from the original on June 28, 2009. Retrieved 2009-06-24.
  4. ^ "Marine VHF radio". ACMA.
  5. ^ "Australian radiofrequency spectrum plan". Planning. ACMA.
  6. ^ "Going Digital - When is my area going digital?". goingdigital.co.nz. Ministry for Culture and Heritage. Archived from the original on 17 October 2011. Retrieved 20 October 2011.
  7. ^ [[1]
  8. ^ a b Canadian Table of Frequency Allocations 9 kHz – 275 GHz (2005 (revised February 2007) ed.). Industry Canada. February 2007. pp. 29–30.
  9. ^ "Cable TV Channel Frequencies". www.jneuhaus.com. Archived from the original on 23 August 2017. Retrieved 27 April 2018.
  10. ^ "What Ever Happened to Channel 1?". tech-notes.tv. Tech Notes. Table 1. Archived from the original on 17 March 2017. Retrieved 27 April 2018.
  11. ^ "Change to the law to allow the use of low power FM transmitters for MP3 players". Ofcom. 23 November 2006. Archived from the original on 7 August 2011. Retrieved 2 October 2012.
Band I

Band I is a range of radio frequencies within the very high frequency (VHF) part of the electromagnetic spectrum. The first time there was defined "for simplicity" in Annex 1 of "Final acts of the European Broadcasting Conference in the VHF and UHF bands - Stockholm, 1961". Band I ranges from 47 to 68 MHz for the European Broadcasting Area, and from 54 to 88 MHz for the Americas and it is primarily used for television broadcasting in line to ITU Radio Regulations (article 1.38). Channel spacings vary from country to country, with spacings of 6, 7 and 8 MHz being common.

Band II

Band II is the range of radio frequencies within the very high frequency (VHF) part of the electromagnetic spectrum from 87.5 to 108.0 megahertz (MHz).

Band III

Band III is the name of the range of radio frequencies within the very high frequency (VHF) part of the electromagnetic spectrum from 174 to 240 megahertz (MHz). It is primarily used for radio and television broadcasting. It is also called high-band VHF, in contrast to Bands I and II.

Dan Reinstein

Professor Dan Z. Reinstein, MD, MA(Cantab), FRCSC, DABO, FRCOphth, FEBO, Cert LRS, PGDip CRS is a specialist ophthalmic surgeon in the UK and is a board-certified registered specialist ophthalmologist in the USA, Canada and the UK, specialising in the field of refractive surgery (vision correction). He is medical director of the London Vision Clinic and a voluntary faculty member as Professor of Ophthalmology at Columbia University College of Physicians and Surgeons as well as a Visiting Professor at the University of Ulster, UK and Professeur Associé at the Faculty of Medicine, Sorbonne Université, Paris, France.

Debye–Falkenhagen effect

The increase in the conductivity of an electrolyte solution when the applied voltage has a very high frequency is known as Debye–Falkenhagen effect. Impedance measurements on water-p-dioxane and the methanol-toluene systems have confirmed Falkenhagen's predictions made in 1929.

FM broadcast band

The FM broadcast band, used for FM broadcast radio by radio stations, differs between different parts of the world. In Europe, Australia and Africa ((defined as International Telecommunication Union (ITU) region 1)), it spans from 87.5 to 108 megahertz (MHz) - also known as VHF Band II - while in the Americas (ITU region 2) it ranges from 88 to 108 MHz. The FM broadcast band in Japan uses 76 to 95 MHz. The International Radio and Television Organisation (OIRT) band in Eastern Europe is from 65.8 to 74.0 MHz, although these countries now primarily use the 87.5 to 108 MHz band, as in the case of Russia. Some other countries have already discontinued the OIRT band and have changed to the 87.5 to 108 MHz band.

Frequency modulation radio originated in the United States during the 1930s; the system was developed by the American electrical engineer Edwin Howard Armstrong. However, FM broadcasting did not become widespread, even in North America, until the 1960s.

Frequency-modulated radio waves can be generated at any frequency. All the bands mentioned in this article are in the very high frequency (VHF) range, which extends from 30 to 300 MHz.

FORTE

The Fast On-orbit Rapid Recording of Transient Events (FORTE, occasionally stylized as FORTÉ) is a lightweight satellite which was launched at about 8:30 AM on August 29, 1997 into a circular 800-kilometer (500 mi) low Earth orbit which is inclined 70 degrees relative to the Earth's equator, using a Pegasus XL rocket. It was developed and launched by the Sandia National Laboratory in cooperation with Los Alamos National Laboratory, as a testbed for technologies applicable to U.S. nuclear detonation detection systems used to monitor compliance with arms control treaties, and later to study lightning from space. The project was sponsored by the United States Department of Energy, and cost about US$35 million. It utilizes optical sensors, RF sensors, and an "event classifier" in order to make observations, including monitoring Very High Frequency (VHF) lightning emissions in the ionosphere occurring from between 50 to 600 miles (80 to 966 km) above the surface of the Earth, and it will be a component of the VHF Global Lightning and Severe Storm Monitor (V-GLASS) system. Its primary mission is to record and analyze bursts of RF energy rising from the surface of the Earth. FORTE is 7-foot (2.1 m) tall, weighs 470-pound (210 kg), and is the first all-composite spacecraft, its framework being made entirely of graphite-reinforced epoxy. It consists of three decks with aluminum honeycomb cores, and composite facing to support the onboard instruments.

Flight instruments

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with information about the flight situation of that aircraft, such as altitude, airspeed and direction. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank (artificial horizon), direction (directional gyro) and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into Instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.The term is sometimes used loosely as a synonym for cockpit instruments as a whole, in which context it can include engine instruments, navigational and communication equipment.

Many modern aircraft have electronic flight instrument systems.

Most regulated aircraft have these flight instruments as dictated by the US Code of Federal Regulations, Title 14, Part 91. They are grouped according to pitot-static system, compass systems, and gyroscopic instruments.

High frequency

High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters (ten to one hundred metres). Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (2.31–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.

Kent Island Research Facility

The Kent Island Research Facility is a U.S. National Security Agency facility located on Kent Island, Maryland, near the town of Chester.The facility was established in 1961 to conduct research on very high frequency and microwave antenna systems—specifically, on problems associated with anomalous propagation in communications interception. According to James Bamford in 1983, the location consists of the NSA Propagation Research Laboratory, a single-story, white, windowless building that houses automatically operating, unattended equipment, as well as a small, white cinder-block "control" building surrounded in a barbed-wire fence. Near the control building are a number of unusual antennas.

Maritime broadcast communications net

In telecommunication, a maritime broadcast communications net is a communications net that is used for international distress calling, including international lifeboat, lifecraft, and survival-craft high frequency (HF); aeronautical emergency very high frequency (VHF); survival ultra high frequency (UHF); international calling and safety very high frequency (VHF); combined scene-of-search-and-rescue; and other similar and related purposes.

Note: Basic international distress calling is performed at either medium frequency (MF) or at high frequency (HF).

Microwave chemistry sensor

Microwave chemistry sensor or Surface acoustic wave (SAW) sensors consist of an input transducer, a chemically adsorbent polymer film, and an output transducer on a piezoelectric substrate, which is typically quartz. The input transducer launches an acoustic wave that travels through the chemical film and is detected by the output transducer. The Sandia-made device runs at a very high frequency (approximately 525 MHz), and the velocity and attenuation of the signal are sensitive to the viscoelasticity and mass of the thin film . SAWS have been able to distinguish organophosphates, chlorinated hydrocarbons, ketones, alcohols, aromatic hydrocarbons, saturated hydrocarbons, and water . The SAW used in these tests have four channels—each channel consists of a transmitter and a receiver, separated by a small distance. Three of the four channels have a polymer deposited on the substrate between the transmitter and receiver. The purpose of the polymers is to adsorb chemicals of interest, with different polymers having different affinities to various chemicals. When a chemical is adsorbed, the mass of the polymer increases, causing a slight change in phase of the acoustic signal relative to the reference (fourth) channel, which does not contain a polymer. The SAW device also contains three Application Specific Integrated Circuit chips (ASICs), which contain the electronics to analyze the signals and provide a DC voltage signal proportional to the phase shift. The SAW device, containing the transducers and ASICs, is bonded to a piece of quartz glass, which is placed in a leadless chip carrier (LCC). Wire bonds connect the terminals of the leadless chip carrier to the SAW circuits.

Motor neuron

A motor neuron (or motoneuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

A single motor neuron may innervate many muscle fibres and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or a tetanic contraction. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau.

Radio noise

In radio reception, noise is unwanted random electrical signals always present in a radio receiver in addition to the desired radio signal. Radio noise is a combination of natural electromagnetic atmospheric noise ("spherics", static) created by electrical processes in the atmosphere like lightning, manmade radio frequency interference (RFI) from other electrical devices picked up by the receiver's antenna, and thermal noise present in the receiver input circuits. The level of noise determines the maximum sensitivity and reception range of a radio receiver; if no noise were picked up with radio signals, even weak transmissions could be received at virtually any distance by making a radio receiver that was sensitive enough. With noise present, if a radio source is so weak and far away that the radio signal in the receiver has a lower amplitude than the average noise, the noise will drown out the signal.

The limiting noise source in a receiver depends on the frequency range in use. At frequencies below about 40 MHz, particularly in the mediumwave and longwave bands and below, atmospheric noise and nearby radio frequency interference from electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources tends to be above the thermal noise floor in the receiver's circuits. These noises are often referred to as static. Conversely, at very high frequency and ultra high frequency and above, these sources are often lower, and thermal noise is usually the limiting factor. In the most sensitive receivers at these frequencies, radio telescopes and satellite communication antennas, thermal noise is reduced by cooling the RF front end of the receiver to cryogenic temperatures. Cosmic background noise is experienced at frequencies above about 15 MHz when highly directional antennas are pointed toward the sun or to certain other regions of the sky such as the center of the Milky Way Galaxy.

Electromagnetic noise can interfere with electronic equipment in general, causing malfunction, and in recent years standards have been laid down for the levels of electromagnetic radiation that electronic equipment is permitted to radiate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility (EMC).

Rapid update cycle

The Rapid Update Cycle (RUC) was an atmospheric prediction system that consisted primarily of a numerical forecast model and an analysis system to initialize the model.

The RUC was designed to provide accurate short-range (0- to 12-hr, later expanded to 18-hr in 2010) numerical forecast guidance for weather-sensitive users, such as those in the aviation community. Significant weather forecasting problems that occur in the 0- to 12-hr range include severe weather in all seasons (for example, tornadoes, thunderstorms, snow, and ice storms) and hazards to aviation (for example, clear air turbulence, icing, and downbursts).

The RUC ran at the highest frequency of any forecast model at the National Centers for Environmental Prediction (NCEP), assimilating recent observations to provide very high frequency updates of current conditions and short-range forecasts. This update frequency was only once an hour (the standard interval for ASOS observation reporting), and with computational limitations and the time required to assimilate all of the data, there is approximately a one-hour delay in producing the forecasts. Because of this, it was common practice to use a one-hour forecast from the RUC as a current analysis, as the one-hour forecast would come out only a few minutes before the time it is forecasting for. There is also little possibility for error in a one-hour forecast, meaning that the RUC's one-hour forecast would not usually vary greatly from the actual state of the atmosphere at that particular point in time.

The RUC was decommissioned on May 1, 2012; it was replaced by the Rapid Refresh (RR or RAP) model, based on the WRF. Like the RUC, the Rapid Refresh model also runs hourly out to 18 hours on a 13 km (8.1 mi) grid spacing, but also covers a wider area. An experimental High Resolution Rapid Refresh (HRRR) ran by the Earth System Research Laboratory (ESRL) offers 3 km (1.9 mi) resolution at 15-minute intervals A backup version of the RUC continued to run until that too was stopped on May 15, 2013, thus formally bringing an end to the model.

Tropospheric propagation

Tropospheric propagation describes electromagnetic propagation in relation to the troposphere.

The service area from a VHF or UHF radio transmitter extends to just beyond the optical horizon, at which point signals start to rapidly reduce in strength. Viewers living in such a "deep fringe" reception area will notice that during certain conditions, weak signals normally masked by noise increase in signal strength to allow quality reception. Such conditions are related to the current state of the troposphere.

Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface and extending to some 25,000 feet (7,620 m). Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm anticyclonic weather (i.e., high pressure), usually weak signals from distant transmitters improve in strength. Another symptom during such conditions may be interference to the local transmitter resulting in co-channel interference, usually horizontal lines or an extra floating picture with analog broadcasts and break-up with digital broadcasts. A settled high-pressure system gives the characteristic conditions for enhanced tropospheric propagation, in particular favouring signals which travel along the prevailing isobar pattern (rather than across it). Such weather conditions can occur at any time, but generally the summer and autumn months are the best periods. In certain favourable locations, enhanced tropospheric propagation may enable reception of ultra high frequency (UHF) TV signals up to 1,000 miles (1,600 km) or more.

The observable characteristics of such high-pressure systems are usually clear, cloudless days with little or no wind. At sunset the upper air cools, as does the surface temperature, but at different rates. This produces a boundary or temperature gradient, which allows an inversion level to form – a similar effect occurs at sunrise. The inversion is capable of allowing very high frequency (VHF) and UHF signal propagation well beyond the normal radio horizon distance.

The inversion effectively reduces sky wave radiation from a transmitter – normally VHF and UHF signals travel on into space when they reach the horizon, the refractive index of the ionosphere preventing signal return. With temperature inversion, however, the signal is to a large extent refracted over the horizon rather than continuing along a direct path into outer space.

Fog also produces good tropospheric results, again due to inversion effects. Fog occurs during high-pressure weather, and if such conditions result in a large belt of fog with clear sky above, there will be heating of the upper fog level and thus an inversion. This situation often arises towards night fall, continues overnight and clears with the sunrise over a period of around 4 – 5 hours.

Ultra high frequency

Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimeter). Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.

The IEEE defines the UHF radar band as frequencies between 300 MHz and 1 GHz. Two other IEEE radar bands overlap the ITU UHF band: the L band between 1 and 2 GHz and the S band between 2 and 4 GHz.

VHF omnidirectional range

Very High Frequency (VHF) Omni-Directional Range (VOR) is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons. It uses frequencies in the very high frequency (VHF) band from 108.00 to 117.95 MHz. Developed in the United States beginning in 1937 and deployed by 1946, VOR is the standard air navigational system in the world, used by both commercial and general aviation. By 2000 there were about 3,000 VOR stations around the world including 1,033 in the US, reduced to 967 by 2013 with more stations being decommissioned with the widespread adoption of GPS.

A VOR ground station sends out an omnidirectional master signal, and a highly directional second signal is propagated by a phased antenna array and rotates clockwise in space 30 times a second. This signal is timed so that its phase (compared to the master) varies as the secondary signal rotates, and this phase difference is the same as the angular direction of the 'spinning' signal, (so that when the signal is being sent 90 degrees clockwise from north, the signal is 90 degrees out of phase with the master). By comparing the phase of the secondary signal with the master, the angle (bearing) to the aircraft from the station can be determined. This line of position is called the "radial" from the VOR. The intersection of radials from two different VOR stations can be used to fix the position of the aircraft, as in earlier radio direction finding (RDF) systems.

VOR stations are fairly short range: the signals are line of sight between transmitter and receiver and are useful for up to 200 miles. Each station broadcasts a VHF radio composite signal including the navigation signal, station's identifier and voice, if so equipped. The navigation signal allows the airborne receiving equipment to determine a bearing from the station to the aircraft (direction from the VOR station in relation to Magnetic North). The station's identifier is typically a three-letter string in Morse code. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts. At some locations, this voice signal is a continuous recorded broadcast of Hazardous Inflight Weather Advisory Service or HIWAS.

Weather radio

A weather radio is a specialized radio receiver that is designed to receive a public broadcast service, typically from government-owned radio stations, dedicated to airing weather reports on a continual basis, with the routine weather reports being interrupted by emergency weather reports whenever needed. Weather radios are typically equipped with a standby alerting function—if the radio is muted or tuned to another band and a severe weather bulletin is transmitted, it can automatically sound an alarm and/or switch to a pre-tuned weather channel for emergency weather information.

Weather radio services may also broadcast non-weather-related emergency information, such as in the event of a natural disaster, a child abduction alert, or a terrorist attack. They generally broadcast in a pre-allocated very high frequency (VHF) range using FM. Usually a radio scanner or a dedicated weather radio receiver is needed for listening, although in some locations a weather radio broadcast may be re-transmitted on an AM or FM broadcast station, on terrestrial television stations, or local public, educational, and government access (PEG) cable TV channels or during weather or other emergencies.

Broadcast bands with Roman numeral designations
VHF
UHF
SHF
Visible (optical)
Microwaves
Radio
Wavelength types
History
Pioneers
Transmission
media
Network topology
and switching
Multiplexing
Networks
Systems
Color systems
Video
Sound
Modulation
Transmission
Frequencies & Bands
Propagation
Testing
Artifacts
Terrestrial
Satellite
Codecs
Subcarrier signals

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