8.829 km (5.486 mi) in 43 flights as of 19 February 2023
NASA and JPL insignias
Mars 2020 is a Mars rover mission that includes the rover Perseverance, the small robotic helicopter Ingenuity, and associated delivery systems, as part of NASA's Mars Exploration Program. Mars 2020 was launched from Earth on an Atlas Vlaunch vehicle at 11:50:01 UTC on 30 July 2020, and confirmation of touch down in the Martian crater Jezero was received at 20:55 UTC on 18 February 2021. On 5 March 2021, NASA named the landing site of the rover Octavia E. Butler Landing. As of 8 August 2023, Perseverance and Ingenuity have been on Mars for 877 sols (901 total days; 2 years, 171 days).
Perseverance is investigating an astrobiologically relevant ancient environment on Mars for its surface geological processes and history, and assessing its past habitability, the possibility of past life on Mars, and the potential for preservation of biosignatures within accessible geological materials. It will cache sample containers along its route for retrieval by a potential future Mars sample-return mission. The Mars 2020 mission was announced by NASA in December 2012 at the fall meeting of the American Geophysical Union in San Francisco. Perseverance's design is derived from the rover Curiosity, and it uses many components already fabricated and tested in addition to new scientific instruments and a core drill. The rover also employs nineteen cameras and two microphones, allowing for the audio recording of the Martian environment. On 30 April 2021, Perseverance became the first spacecraft to hear and record another spacecraft, the Ingenuity helicopter, on another planet.
The launch of Mars 2020 was the third of three space missions sent toward Mars during the July 2020 Mars launch window, with missions also launched by the national space agencies of the United Arab Emirates (the Emirates Mars Mission with the orbiter Hope on 19 July 2020) and China (the Tianwen-1 mission on 23 July 2020, with an orbiter, deployable and remote cameras, lander, and Zhurong rover).
Conception
The Mars 2020 mission was announced by NASA on 4 December 2012 at the fall meeting of the American Geophysical Union in San Francisco.
The selection of Mars as the target of NASA's flagship mission elicited
surprise from some members of the scientific community. Some criticized
NASA for continuing to focus on Mars exploration instead of other Solar
System destinations in constrained budget times. Support came from California U.S. Representative Adam Schiff, who said he was interested in the possibility of advancing the launch date, which would enable a larger payload. Science educator Bill Nye endorsed the Mars sample-return role, saying this would be "extraordinarily fantastic and world-changing and worthy."
Objectives
Sample tubes being loaded into the Perseverance rover
The mission will seek signs of habitable conditions on Mars in the ancient past, and will also search for evidence—or biosignatures—of past microbial life, and water. The mission was launched 30 July 2020 on an Atlas V-541, and the Jet Propulsion Laboratory managed the mission. The mission is part of NASA's Mars Exploration Program.
The Science Definition Team proposed that the rover collect and package
as many as 31 samples of rock cores and surface soil for a later
mission to bring back for definitive analysis on Earth.
In 2015, they expanded the concept, planning to collect even more
samples and distribute the tubes in small piles or caches across the
surface of Mars.
In September 2013, NASA launched an Announcement of Opportunity
for researchers to propose and develop the instruments needed, including
the Sample Caching System.
The science instruments for the mission were selected in July 2014
after an open competition based on the scientific objectives set one
year earlier. The science conducted by the rover's instruments will provide the context needed for detailed analyses of the returned samples.
The chairman of the Science Definition Team stated that NASA does not
presume that life ever existed on Mars, but given the recent Curiosity rover findings, past Martian life seems possible.
The Perseverance rover will explore a site likely to have
been habitable. It will seek signs of past life, set aside a returnable
cache with the most compelling rock core and soil samples, and
demonstrate the technology needed for the future human and robotic
exploration of Mars. A key mission requirement is that it must help
prepare NASA for its long-term Mars sample-return mission and crewed mission efforts.
The rover will make measurements and technology demonstrations to help
designers of a future human expedition understand any hazards posed by
Martian dust, and will test technology to produce a small amount of pure oxygen (O2) from Martian atmosphericcarbon dioxide (CO2).
Improved precision landing technology that enhances the
scientific value of robotic missions also will be critical for eventual
human exploration on the surface.
Based on input from the Science Definition Team, NASA defined the final
objectives for the 2020 rover. Those became the basis for soliciting
proposals to provide instruments for the rover's science payload in the
spring of 2014. The mission will also attempt to identify subsurface water, improve landing techniques, and characterize weather, dust, and other potential environmental conditions that could affect future astronauts living and working on Mars.
Campaign plans for 2021–2022The
heat shield (left) and back shell (right) together make up the
aeroshell for NASA's Mars 2020 mission. The diameter of each part is
about 15 feet (4.5 metres). The rover and its descent stage were
enclosed and shielded by the aeroshell during their deep space voyage to
Mars and upon entry into the Martian atmosphere, which generated
intense heat. The aeroshell was made by Lockheed Martin Space in Denver,
Colorado, where the photo was taken.
A key mission requirement for this rover is that it must help prepare NASA for its Mars sample-return mission (MSR) campaign, which is needed before any crewed mission takes place. Such effort would require three additional vehicles: an orbiter, a fetch rover, and a two-stage, solid-fueled Mars ascent vehicle (MAV). Between 20 and 30 drilled samples will be collected and cached inside small tubes by the Perseverance rover, and will be left on the surface of Mars for possible later retrieval by NASA in collaboration with ESA. A "fetch rover" would retrieve the sample caches and deliver them to a two-stage, solid-fueled Mars ascent vehicle (MAV). In July 2018, NASA contracted Airbus to produce a "fetch rover" concept study. The MAV would launch from Mars and enter a 500 km orbit and rendezvous with the Next Mars Orbiter or Earth Return Orbiter.
The sample container would be transferred to an Earth entry vehicle
(EEV) which would bring it to Earth, enter the atmosphere under a
parachute and hard-land for retrieval and analyses in specially designed
safe laboratories.
In the first science campaign Perseverance performs an
arching drive southward from its landing site to the Séítah unit to
perform a "toe dip" into the unit to collect remote-sensing measurements
of geologic targets. After that she will return to the Crater Floor Fractured Rough to collect the first core sample there. Passing by the Octavia B. Butler landing site concludes the first science campaign.
The second campaign shall start with several months of travel
towards the "Three Forks" where Perseverance can access geologic
locations at the base of the ancient delta of Neretva river, as well as
ascend the delta by driving up a valley wall to the northwest.
Spacecraft
Cruise stage and EDLS
Animation of Mars 2020's trajectory from 30 July 2020 to 20 February 2021
Mars 2020
Sun
Earth
Mars
The three major components of the Mars 2020 spacecraft are the 539 kg (1,188 lb) cruise stage for travel between Earth and Mars; the Entry, Descent, and Landing System (EDLS) that includes the 575 kg (1,268 lb) aeroshell descent vehicle + 440 kg (970 lb) heat shield; and the 1,070 kg (2,360 lb) (fueled mass) descent stage needed to deliver Perseverance and Ingenuity
safely to the Martian surface. The Descent Stage carries 400 kg
(880 lb) landing propellant for the final soft landing burn after being
slowed down by a 21.5 m (71 ft)-wide, 81 kg (179 lb) parachute. The 1,025 kg (2,260 lb) rover is based on the design of Curiosity.
While there are differences in scientific instruments and the
engineering required to support them, the entire landing system
(including the descent stage
and heat shield) and rover chassis could essentially be recreated
without any additional engineering or research. This reduces overall
technical risk for the mission, while saving funds and time on
development.
One of the upgrades is a guidance and control technique called
"Terrain Relative Navigation" (TRN) to fine-tune steering in the final
moments of landing. This system allowed for a landing accuracy within 40 m (130 ft) and avoided obstacles. This is a marked improvement from the Mars Science Laboratory mission that had an elliptical area of 7 by 20 km (4.3 by 12.4 mi). In October 2016, NASA reported using the Xombie rocket
to test the Lander Vision System (LVS), as part of the Autonomous
Descent and Ascent Powered-flight Testbed (ADAPT) experimental
technologies, for the Mars 2020 mission landing, meant to increase the
landing accuracy and avoid obstacle hazards.
The cruise stage and EDLS carried both spacecraft to Mars.
Perseverance was designed with help from Curiosity's engineering team, as both are quite similar and share common hardware. Engineers redesigned Perseverance's wheels to be more robust than Curiosity's, which, after kilometres of driving on the Martian surface, have shown progressed deterioration. Perseverance will have thicker, more durable aluminium wheels, with reduced width and a greater diameter, 52.5 cm (20.7 in), than Curiosity's 50 cm (20 in) wheels. The aluminium wheels are covered with cleats for traction and curved titanium spokes for springy support. The combination of the larger instrument suite, new Sampling and Caching System, and modified wheels makes Perseverance 14 percent heavier than Curiosity, at 1,025 kg (2,260 lb) and 899 kg (1,982 lb), respectively.
The rover will include a five-jointed robotic arm measuring 2.1 m (6 ft
11 in) long. The arm will be used in combination with a turret to
analyze geologic samples from the Martian surface.
A Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), left over as a backup part for Curiosity during its construction, was integrated onto the rover to supply electrical power. The generator has a mass of 45 kg (99 lb) and contains 4.8 kg (11 lb) of plutonium dioxide as the source of steady supply of heat that is converted to electricity. The electrical power generated is approximately 110 watts at launch with little decrease over the mission time.
Two lithium-ion rechargeable batteries
are included to meet peak demands of rover activities when the demand
temporarily exceeds the MMRTG's steady electrical output levels. The
MMRTG offers a 14-year operational lifetime, and it was provided to NASA
by the United States Department of Energy.
Unlike solar panels, the MMRTG does not rely on the presence of the Sun
for power, providing engineers with significant flexibility in
operating the rover's instruments even at night and during dust storms,
and through the winter season.
Each Mars mission contributes to an ongoing innovation chain.
Each draws on prior operations or tested technologies and contributes
uniquely to upcoming missions. By using this strategy, NASA is able to
advance the frontiers of what is currently feasible while still
depending on earlier advancements.
The Curiosity rover, which touched down on Mars in 2012, is
directly responsible for a large portion of Perseverance's rover design,
including its entry, descent, and landing mechanism. With perseverance,
new technological innovations will be demonstrated, and entry, descent,
and landing capabilities will be improved. These advancements will help
open the door for future robotic and human missions to the Moon and
Mars.
Technology Displays on the Perseverance
Demonstrations of technology are risky experiments that put cutting-edge
equipment to the test. Promising technology can be tested on a smaller
scale.
Ingenuity is a robotic coaxial helicopter that demonstrated the technology for rotorcraft flight in the extremely thin atmosphere of Mars. The aircraft was deployed from the rover's deck, and has flown five times during its 30-day test campaign early in the mission.
Each flight took no more than 117 seconds, at altitudes ranging from 3
to 10 m (9.8 to 32.8 ft) off the ground, and a maximum distance of 266 m
(873 ft). It used autonomous control and communicated with Perseverance
directly after each landing. It is the first powered flight on another
planet, and NASA will be able to build on the design for future Mars
missions.
The Jezero craterdelta on Mars, where the Perseverance rover and Ingenuity helicopter landed. Clays are visible as green in this false colorCRISM / CTX image.1:02Jezero Crater Formation by asteroid impact
The mission will explore Jezero crater, which scientists speculate was a 250 m (820 ft) deep lake about 3.9 billion to 3.5 billion years ago.
Jezero today features a prominent river delta where water flowing
through it deposited much sediment over the eons, which is "extremely
good at preserving biosignatures".
The sediments in the delta likely include carbonates and hydrated
silica, known to preserve microscopic fossils on Earth for billions of
years. Prior to the selection of Jezero, eight proposed landing sites for the mission were under consideration by September 2015; Columbia Hills in Gusev crater, Eberswalde crater, Holden crater, Jezero crater, Mawrth Vallis, Northeastern Syrtis Major Planum, Nili Fossae, and Southwestern Melas Chasma.
A workshop was held on 8–10 February 2017 in Pasadena, California, to discuss these sites, with the goal of narrowing down the list to three sites for further consideration. The three sites chosen were Jezero crater, Northeastern Syrtis Major Planum, and Columbia Hills. Jezero crater was ultimately selected as the landing site in November 2018.
The "fetch rover" for returning the samples is expected to launch in
2026. The landing and surface operations of the "fetch rover" would take
place early in 2029. The earliest return to Earth is envisaged for 2031.
The launch window, when the positions of Earth and Mars were optimal
for traveling to Mars, opened on 17 July 2020 and lasted through 15
August 2020.
The rocket was launched on 30 July 2020 at 11:50 UTC, and the rover
landed on Mars on 18 February 2021 at 20:55 UTC, with a planned surface
mission of at least one Mars year (668 sols or 687 Earth days). NASA was not the only Mars mission to use this window: the United Arab Emirates Space Agency launched its Emirates Mars Mission with the Hope orbiter on 20 July 2020, which arrived in Mars orbit on 8 February 2021, and China National Space Administration launched Tianwen-1 on 23 July 2020, arriving in orbit on 10 February 2021 and successfully soft landed with the Zhurong rover on 14 May 2021.
NASA announced that all of the trajectory correction maneuvers
(TCM) were a success. The spacecraft fired thrusters to adjust its
course toward Mars, shifting the probe's initial post-launch aim point
onto the Red Planet.
Entry, descent, and landing (EDL)
Diagram of the various stages of the EDL process for PerseveranceA new crater appears on Mars after the impact of the 77 kg (170 lb) piece of tungsten thrown down during the EDL stage
Prior to landing, the Science Team from an earlier NASA lander, InSight, announced that they would attempt to detect the entry, descent and landing
(EDL) sequence of the Mars 2020 mission using InSight's seismometers.
Despite being more than 3,400 km (2,100 mi) away from the Mars landing
site, the team indicated that there was a possibility that InSight's
instruments would be sensitive enough to detect the hypersonic impact of Mars 2020's cruise mass balance devices with the Martian surface.
The rover's landing was planned similar to the Mars Science Laboratory used to deploy Curiosity
on Mars in 2012. The craft from Earth was a carbon fiber capsule that
protected the rover and other equipment from heat during entry into the
Mars atmosphere and initial guidance towards the planned landing site.
Once through, the craft jettisoned the lower heat shield and deployed a
parachute from the backshell to slow the descent to a controlled speed.
With the craft moving under 320 km/h (200 mph) and about 1.9 km (1.2 mi)
from the surface, the rover and skycrane assembly detached from the
backshell, and rockets on the skycrane controlled the remaining descent
to the planet. As the skycrane moved closer to the surface, it lowered Perseverance via cables until it confirmed touchdown, detached the cables, and flew a distance away to avoid damaging the rover.
Perseverance successfully landed on the surface of Mars with
help of the skycrane on 18 February 2021 at 20:55 UTC, to begin its
science phase, and began sending images back to Earth. Ingenuity reported back to NASA via the communications systems on Perseverance the following day, confirming its status. The helicopter was not expected to be deployed for at least 60 days into the mission. NASA also confirmed that the on-board microphone on Perseverance had survived entry, descent and landing
(EDL), along with other high-end visual recording devices, and released
the first audio recorded on the surface of Mars shortly after landing, capturing the sound of a Martian breeze as well as a hum from the rover itself. On 7 May 2021, NASA confirmed that Perseverance managed to record both audio and video from Ingenuity's fourth flight which took place on 30 April 2021.
19 April 2021 – First major flight test of Ingenuity
20 April 2021 – Mars Oxygen ISRU Experiment (MOXIE) generated 5.37 g (0.189 oz) of oxygen gas from carbon dioxide on its first test on Mars
1 June 2021 – Perseverance begins its first science campaign.
8 June 2021 – Seventh flight of Ingenuity.
21 June 2021 – Eighth flight of Ingenuity. The "watchdog issue", a recurring issue which occasionally prevented Ingenuity from taking flight, is fixed.
5 July 2021 – Ninth flight of Ingenuity. This flight is the first to explore areas only an aerial vehicle can, by taking a shortcut over the Séítah unit. The sandy ripples of the Séítah unit would prove too difficult for Perseverance to travel through directly.
Mid-August 2021 – Perseverance will have acquired its first
sample from the ancient lakebed by drilling out "finger-size cores of
Martian rock for return to Earth".
3 May 2022 – After 27 flights of Ingenuity, the rover lost
contact with the helicopter. By holding all scientific operations on the
rover to search for and communicate to the helicopter, NASA was able to
regain contact with the helicopter to recharge its batteries and return
to the rover.
AM and FM modulated signals for radio. AM (amplitude modulation) and FM (frequency modulation) are types of modulation (coding). The sound of the program material, usually coming from a radio studio, is used to modulate (vary) a carrier wave of a specific frequency, then broadcast. In AM broadcasting, the amplitude of the carrier wave is modulated to encode the original sound. In FM broadcasting, the frequency of the carrier wave is modulated to encode the sound. A radio receiverextracts the original program sound from the modulated radio signal and reproduces the sound in a loudspeaker.Position of FM radio in the electromagnetic spectrumA commercial 35 kW FM radio transmitter built in the late 1980s. It belongs to FM radio station KWNR, in Henderson, Nevada, and broadcasts at 95.5 MHz.
FM broadcasting is a method of radio broadcasting that uses frequency modulation (FM) of the radio broadcast carrier wave. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to transmit high-fidelity sound over broadcast radio.
FM broadcasting offers higher fidelity—more accurate reproduction of
the original program sound—than other broadcasting techniques, such as AM broadcasting. It is also less susceptible to common forms of interference, having less static and popping sounds than are often heard on AM. Therefore, FM is used for most broadcasts of music and general audio (in the audio spectrum). FM radio stations use the very high frequency range of radio frequencies.
Throughout the world, the FM broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 to 108.0 MHz is used, or some portion of it, with few exceptions:
In the former Soviet republics, and some former Eastern Bloc
countries, the older 65.8–74 MHz band is also used. Assigned
frequencies are at intervals of 30 kHz. This band, sometimes referred to
as the OIRT band, is slowly phased out. Where the OIRT band is used, the 87.5–108.0 MHz band is referred to as the CCIR band.
In Brazil, until the late 2010s, FM broadcast stations only used the 88-108 MHz Band, but with the phasing out of analog television, the 76-88 MHz band (old band channels 5 and 6 in VHF television) are allocated for old local MW stations who have moved to FM in agreement with ANATEL.
The frequency of an FM broadcast station (more strictly its assigned
nominal center frequency) is usually a multiple of 100 kHz. In most of South Korea, the Americas, the Philippines, and the Caribbean,
only odd multiples are used. Some other countries follow this plan
because of the import of vehicles, principally from the United States,
with radios that can only tune to these frequencies. In some parts of Europe, Greenland, and Africa, only even multiples are used. In the United Kingdom, both odd and even are used. In Italy,
multiples of 50 kHz are used. In most countries the maximum permitted
frequency error of the unmodulated carrier is specified, which typically
should be within 2 kHz of the assigned frequency.
There are other unusual and obsolete FM broadcasting standards in
some countries, with non-standard spacings of 1, 10, 30, 74, 500, and
300 kHz. To minimise inter-channel interference, stations operating from
the same or nearby transmitter sites tend to keep to at least a 500 kHz
frequency separation even when closer frequency spacing is technically
permitted. The ITU publishes Protection Ratio graphs, which give the minimum spacing between frequencies based on their relative strengths. Only broadcast stations with large enough geographic separations between their coverage areas can operate on close frequencies.
Technology
FM has better rejection of static (RFI) than AM. This was shown in a dramatic demonstration by General Electric
at its New York lab in 1940. The radio had both AM and FM receivers.
With a million-volt arc as a source of interference behind it, the AM
receiver produced a roar of static, while the FM receiver clearly
reproduced a music program from Armstrong's experimental FM transmitter
in New Jersey.Crossed dipole antenna of station KENZ's 94.9 MHz, 48 kW transmitter on Lake Mountain, Utah. It radiates circularly polarized radio waves.
Modulation
Frequency modulation or FM is a form of modulation which conveys information by varying the frequency of a carrier wave; the older amplitude modulation or AM varies the amplitude of the carrier, with its frequency remaining constant. With FM, frequency deviation from the assigned carrier frequency
at any instant is directly proportional to the amplitude of the (audio)
input signal, determining the instantaneous frequency of the
transmitted signal. Because transmitted FM signals use significantly
more bandwidth than AM signals, this form of modulation is commonly used with the higher (VHF or UHF) frequencies used by TV, the FM broadcast band, and land mobile radio systems.
The maximum frequency deviation of the carrier is usually
specified and regulated by the licensing authorities in each country.
For a stereo broadcast, the maximum permitted carrier deviation is
invariably ±75 kHz, although a little higher is permitted in the United
States when SCA systems are used. For a monophonic broadcast, again the
most common permitted maximum deviation is ±75 kHz. However, some
countries specify a lower value for monophonic broadcasts, such as
±50 kHz.
Armstrong's first prototype FM broadcast transmitter, located in the Empire State Building,
New York City, which he used for secret tests of his system between
1934 and 1935. Licensed as experimental station W2XDG, it transmitted on
41 MHz at a power of 2 kW.Instantaneous spectrum and waterfall plot
in the FM broadcast band showing three strong local stations; speech
and music show different patterns of frequency vs. time. When the
transmitted audio is quiet, the 19 kHz stereo pilot tones can be
resolved in the spectrum.
Bandwidth
The bandwidth of a FM transmission is given by the Carson bandwidth rule which is the sum of twice the maximum deviation and twice the maximum modulating frequency. For a transmission that includes RDS this would be 2x75kHz + 2x60kHz = 270 kHz. This is also known as the Necessary Bandwidth.
Pre-emphasis and de-emphasis
Random noise has a triangularspectral distribution in an FM system, with the effect that noise occurs predominantly at the higher audio frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission
and reducing them by a corresponding amount in the receiver. Reducing
the high audio frequencies in the receiver also reduces the
high-frequency noise. These processes of boosting and then reducing
certain frequencies are known as pre-emphasis and de-emphasis, respectively.
The amount of pre-emphasis and de-emphasis used is defined by the time constant of a simple RC filter circuit. In most of the world a 50 µs time constant is used. In the Americas and South Korea, 75 µs is used. This applies to both mono and stereo transmissions. For stereo, pre-emphasis is applied to the left and right channels before multiplexing.
The use of pre-emphasis becomes a problem because many forms of
contemporary music contain more high-frequency energy than the musical
styles which prevailed at the birth of FM broadcasting. Pre-emphasizing
these high-frequency sounds would cause excessive deviation of the FM carrier.
Modulation control (limiter) devices are used to prevent this. Systems
more modern than FM broadcasting tend to use either programme-dependent
variable pre-emphasis; e.g., dbx in the BTSC TV sound system, or none at all.
Pre-emphasis and de-emphasis was used in the earliest days of FM broadcasting. According to a BBC report from 1946, 100 µs was originally considered in the US, but 75 µs subsequently adopted.
Stereo FM
Long
before FM stereo transmission was considered, FM multiplexing of other
types of audio level information was experimented with.
Edwin Armstrong, who invented FM, was the first to experiment with
multiplexing, at his experimental 41 MHz station W2XDG located on the
85th floor of the Empire State Building in New York City.
These FM multiplex transmissions started in November 1934 and consisted of the main channel audio program and three subcarriers:
a fax program, a synchronizing signal for the fax program and a
telegraph "order" channel. These original FM multiplex subcarriers were
amplitude modulated.
Two musical programs, consisting of both the Red and Blue Network
program feeds of the NBC Radio Network, were simultaneously transmitted
using the same system of subcarrier modulation as part of a
studio-to-transmitter link system. In April 1935, the AM subcarriers
were replaced by FM subcarriers, with much improved results.
The first FM subcarrier transmissions emanating from Major
Armstrong's experimental station KE2XCC at Alpine, New Jersey occurred
in 1948. These transmissions consisted of two-channel audio programs,
binaural audio programs and a fax program. The original subcarrier
frequency used at KE2XCC was 27.5 kHz. The IF bandwidth was ±5 kHz, as
the only goal at the time was to relay AM radio-quality audio. This
transmission system used 75 µs audio pre-emphasis like the main monaural
audio and subsequently the multiplexed stereo audio.
In the late 1950s, several systems to add stereo to FM radio were considered by the FCC. Included were systems from 14 proponents including Crosby, Halstead, Electrical and Musical Industries, Ltd (EMI), Zenith, and General Electric. The individual systems were evaluated for their strengths and weaknesses during field tests in Uniontown, Pennsylvania, using KDKA-FM in Pittsburgh as the originating station. The Crosby system was rejected by the FCC because it was incompatible with existing subsidiary communications authorization
(SCA) services which used various subcarrier frequencies including 41
and 67 kHz. Many revenue-starved FM stations used SCAs for
"storecasting" and other non-broadcast purposes. The Halstead system was
rejected due to lack of high frequency stereo separation and reduction
in the main channel signal-to-noise ratio. The GE and Zenith systems, so
similar that they were considered theoretically identical, were
formally approved by the FCC in April 1961 as the standard stereo FM
broadcasting method in the United States and later adopted by most other
countries.
It is important that stereo broadcasts be compatible with mono
receivers. For this reason, the left (L) and right (R) channels are
algebraically encoded into sum (L+R) and difference (L−R) signals. A
mono receiver will use just the L+R signal so the listener will hear
both channels through the single loudspeaker. A stereo receiver will add
the difference signal to the sum signal to recover the left channel,
and subtract the difference signal from the sum to recover the right
channel.
The (L+R) signal is limited to 30 Hz to 15 kHz to protect a
19 kHz pilot signal. The (L−R) signal, which is also limited to 15 kHz,
is amplitude modulated onto a 38 kHz double-sideband suppressed-carrier (DSB-SC) signal, thus occupying 23 kHz to 53 kHz. A 19 kHz ± 2 Hz pilot tone, at exactly half the 38 kHz sub-carrier
frequency and with a precise phase relationship to it, as defined by
the formula below, is also generated. The pilot is transmitted at 8–10%
of overall modulation level and used by the receiver to identify a stereo transmission and to regenerate the 38 kHz sub-carrier
with the correct phase. The composite stereo multiplex signal contains
the Main Channel (L+R), the pilot tone, and the (L−R) difference signal.
This composite signal, along with any other sub-carriers, modulates the
FM transmitter. The terms composite, multiplex and even MPX are used interchangeably to describe this signal.
The instantaneous deviation of the transmitter carrier frequency due to the stereo audio and pilot tone (at 10% modulation) is
where A and B are the pre-emphasized left and right audio signals and =19 kHz
is the frequency of the pilot tone. Slight variations in the peak
deviation may occur in the presence of other subcarriers or because of
local regulations.
Another way to look at the resulting signal is that it alternates
between left and right at 38 kHz, with the phase determined by the
19 kHz pilot signal.
Most stereo encoders use this switching technique to generate the
38 kHz subcarrier, but practical encoder designs need to incorporate
circuitry to deal with the switching harmonics. Converting the multiplex
signal back into left and right audio signals is performed by a
decoder, built into stereo receivers. Again, the decoder can use a
switching technique to recover the left and right channels.
In addition, for a given RF level at the receiver, the signal-to-noise ratio and multipath distortion for the stereo signal will be worse than for the mono receiver.
For this reason many stereo FM receivers include a stereo/mono switch
to allow listening in mono when reception conditions are less than
ideal, and most car radios are arranged to reduce the separation as the
signal-to-noise ratio worsens, eventually going to mono while still
indicating a stereo signal is received. As with monaural transmission,
it is normal practice to apply pre-emphasis to the left and right
channels before encoding and to apply de-emphasis at the receiver after
decoding.
In the U.S. around 2010, using single-sideband modulation for the stereo subcarrier was proposed.
It was theorized to be more spectrum-efficient and to produce a 4 dB
s/n improvement at the receiver, and it was claimed that multipath
distortion would be reduced as well. A handful of radio stations around
the country broadcast stereo in this way, under FCC experimental
authority. It may not be compatible with very old receivers, but it is
claimed that no difference can be heard with most newer receivers. At
present, the FCC rules do not allow this mode of stereo operation.
Quadraphonic FM
In 1969, Louis Dorren
invented the Quadraplex system of single station, discrete, compatible
four-channel FM broadcasting. There are two additional subcarriers in
the Quadraplex system, supplementing the single one used in standard
stereo FM. The baseband layout is as follows:
50 Hz to 15 kHz main channel (sum of all 4 channels) (LF+LR+RF+RR) signal, for mono FM listening compatibility.
23 to 53 kHz (sine quadrature subcarrier) (LF+LR) − (RF+RR) left
minus right difference signal. This signal's modulation in algebraic sum
and difference with the main channel is used for 2 channel stereo
listener compatibility.
23 to 53 kHz (cosine quadrature 38 kHz subcarrier) (LF+RR) − (LR+RF)
Diagonal difference. This signal's modulation in algebraic sum and
difference with the main channel and all the other subcarriers is used
for the Quadraphonic listener.
61 to 91 kHz (sine quadrature 76 kHz subcarrier) (LF+RF) − (LR+RR)
Front-back difference. This signal's modulation in algebraic sum and
difference with the main channel and all the other subcarriers is also
used for the Quadraphonic listener.
105 kHz SCA subcarrier, phase-locked to 19 kHz pilot, for reading services for the blind, background music, etc.
The normal stereo signal can be considered as switching between left
and right channels at 38 kHz, appropriately band-limited. The
quadraphonic signal can be considered as cycling through LF, LR, RF, RR,
at 76 kHz.
Early efforts to transmit discrete four-channel quadraphonic
music required the use of two FM stations; one transmitting the front
audio channels, the other the rear channels. A breakthrough came in 1970
when KIOI (K-101)
in San Francisco successfully transmitted true quadraphonic sound from a
single FM station using the Quadraplex system under Special Temporary
Authority from the FCC.
Following this experiment, a long-term test period was proposed that
would permit one FM station in each of the top 25 U.S. radio markets to
transmit in Quadraplex. The test results hopefully would prove to the
FCC that the system was compatible with existing two-channel stereo transmission and reception and that it did not interfere with adjacent stations.
There were several variations on this system submitted by GE,
Zenith, RCA, and Denon for testing and consideration during the National
Quadraphonic Radio Committee field trials for the FCC. The original
Dorren Quadraplex System outperformed all the others and was chosen as
the national standard for Quadraphonic FM broadcasting in the United
States. The first commercial FM station to broadcast quadraphonic
program content was WIQB (now called WWWW-FM) in Ann Arbor/Saline, Michigan under the guidance of Chief Engineer Brian Jeffrey Brown.
Noise reduction
Various attempts to add analog noise reduction to FM broadcasting were carried out in the 1970s and 1980s:
A commercially unsuccessful noise reduction system used with FM radio in some countries during the late 1970s, Dolby FM was similar to Dolby B but used a modified 25 µs pre-emphasis time constant and a frequency selective companding
arrangement to reduce noise. The pre-emphasis change compensates for
the excess treble response that otherwise would make listening difficult
for those without Dolby decoders.
A similar system named High Com FM was tested in Germany between July 1979 and December 1981 by IRT. It was based on the TelefunkenHigh Com broadband compander system, but was never introduced commercially in FM broadcasting.
Yet another system was the CX-based noise reduction system FMX implemented in some radio broadcasting stations in the United States in the 1980s.
Other subcarrier services
Typical spectrum of composite baseband signal, including DirectBand and a subcarrier on 92 kHz
FM broadcasting has included subsidiary communications authorization
(SCA) services capability since its inception, as it was seen as
another service which licensees could use to create additional income. Use of SCAs was particularly popular in the US, but much less so elsewhere. Uses for such subcarriers include radio reading services for the blind,
which became common and remain so, private data transmission services
(for example sending stock market information to stockbrokers or stolen
credit card number denial lists to stores)
subscription commercial-free background music services for shops,
paging ("beeper") services, non-native-language programming, and
providing a program feed for AM transmitters of AM/FM stations. SCA
subcarriers are typically 67 kHz and 92 kHz. Initially the users of SCA
services were private analog audio channels which could be used
internally or leased, for example Muzak-type services. There were experiments with quadraphonic sound. If a station does not broadcast in stereo, everything from 23 kHz on up can be used for other services. The guard band
around 19 kHz (±4 kHz) must still be maintained, so as not to trigger
stereo decoders on receivers. If there is stereo, there will typically
be a guard band between the upper limit of the DSBSC stereo signal
(53 kHz) and the lower limit of any other subcarrier.
Digital data services are also available. A 57 kHz subcarrier (phase locked to the third harmonic of the stereo pilot tone) is used to carry a low-bandwidth digital Radio Data System signal, providing extra features such as station name, Alternative Frequency (AF), traffic data for commercial GPS receivers and Radio text (RT). This narrowband signal runs at only 1,187.5 bits per second, thus is only suitable for text. A few proprietary systems are used for private communications. A variant of RDS is the North American RBDS
or "smart radio" system. In Germany the analog ARI system was used
prior to RDS to alert motorists that traffic announcements were
broadcast (without disturbing other listeners). Plans to use ARI for
other European countries led to the development of RDS as a more
powerful system. RDS is designed to be capable of use alongside ARI
despite using identical subcarrier frequencies.
The
output power of an FM broadcasting transmitter is one of the parameters
that governs how far a transmission will cover. The other important
parameters are the height of the transmitting antenna and the antenna gain.
Transmitter powers should be carefully chosen so that the required
area is covered without causing interference to other stations further
away. Practical transmitter powers range from a few milliwatts to
80 kW. As transmitter powers increase above a few kilowatts, the
operating costs become high and only viable for large stations. The
efficiency of larger transmitters is now better than 70% (AC power in to
RF power out) for FM-only transmission. This compares to 50% before
high efficiency switch-mode power supplies and LDMOS amplifiers were
used. Efficiency drops dramatically if any digital HD Radio service is
added.
Reception distance
VHF radio waves usually do not travel far beyond the visual horizon,
so reception distances for FM stations are typically limited to 30–40
miles (50–60 km). They can also be blocked by hills and to a lesser
extent by buildings. Individuals with more-sensitive receivers or
specialized antenna systems, or who are located in areas with more
favorable topography, may be able to receive useful FM broadcast signals
at considerably greater distances.
The knife edge effect
can permit reception where there is no direct line of sight between
broadcaster and receiver. The reception can vary considerably depending
on the position. One example is the Učka mountain range, which makes constant reception of Italian signals from Veneto and Marche possible in a good portion of Rijeka, Croatia, despite the distance being over 200 km (125 miles).Other radio propagation effects such as tropospheric ducting and Sporadic E
can occasionally allow distant stations to be intermittently received
over very large distances (hundreds of miles), but cannot be relied on
for commercial broadcast purposes. Good reception across the country is
one of the main advantages over DAB/+ radio.
This is still less than the range of AM radio waves, which because of their lower frequencies can travel as ground waves or reflect off the ionosphere,
so AM radio stations can be received at hundreds (sometimes thousands)
of miles. This is a property of the carrier wave's typical frequency
(and power), not its mode of modulation.
The range of FM transmission is related to the transmitter's RF power, the antenna gain, and antenna height.
Interference from other stations is also a factor in some places. In
the U.S, the FCC publishes curves that aid in calculation of this
maximum distance as a function of signal strength at the receiving
location. Computer modelling is more commonly used for this around the
world.
Many FM stations, especially those located in severe multipath areas, use extra audio compression/processing
to keep essential sound above the background noise for listeners, often
at the expense of overall perceived sound quality. In such instances,
however, this technique is often surprisingly effective in increasing
the station's useful range.
The first radio station to broadcast in FM in Brazil was Rádio Imprensa, which began broadcasting in Rio de Janeiro
in 1955, on the 102.1 MHz frequency, founded by businesswoman Anna
Khoury. Due to the high import costs of FM radio receivers,
transmissions were carried out in circuit closed to businesses and
stores, which played ambient music offered by radio. Until 1976, Rádio
Imprensa was the only station operating in FM in Brazil. From the second
half of the 1970s onwards, FM radio stations began to become popular in
Brazil, causing AM radio to gradually lose popularity.
In 2021, the Brazilian Ministry of Communications expanded the FM
radio band from 87.5-108.0 MHz to 76.1-108.0 MHz to enable the
migration of AM radio stations in Brazilian capitals and large cities.
United States
One of the first FM radio stations, Edwin Armstrong's experimental station W2XMN in Alpine, New Jersey, USA. The insets show a part of the transmitter, and a map of FM stations in 1940. The tower still stands today.
FM broadcasting began in the late 1930s, when it was initiated by a handful of early pioneer experimental stations, including W1XOJ/W43B/WGTR (shut down in 1953) and W1XTG/WSRS, both transmitting from Paxton, Massachusetts (now listed as Worcester, Massachusetts); W1XSL/W1XPW/W65H/WDRC-FM/WFMQ/WHCN, Meriden, Connecticut; and W2XMN, KE2XCC, and WFMN, Alpine, New Jersey (owned by Edwin Armstrong himself, closed down upon Armstrong's death in 1954). Also of note were General Electric
stations W2XDA Schenectady and W2XOY New Scotland, New York—two
experimental FM transmitters on 48.5 MHz—which signed on in 1939. The
two began regular programming, as W2XOY, on November 20, 1940.
Over the next few years this station operated under the call signs
W57A, W87A and WGFM, and moved to 99.5 MHz when the FM band was relocated to the 88–108 MHz portion of the radio spectrum. General Electric sold the station in the 1980s. Today this station is WRVE.
A commercial FM broadcasting band was formally established in the United States as of January 1, 1941, with the first fifteen construction permits announced on October 31, 1940.
These stations primarily simulcast their AM sister stations, in
addition to broadcasting lush orchestral music for stores and offices,
classical music to an upmarket listenership in urban areas, and
educational programming.
On June 27, 1945 the FCC announced the reassignment of the FM
band to 80 channels from 88–106 MHz (which was soon expanded to 100
channels from 88–108 MHz).
This shift, which the AM-broadcaster RCA had pushed for, made all the
Armstrong-era FM receivers useless and delayed the expansion of FM. In 1961 WEFM (in the Chicago area) and WGFM (in Schenectady, New York) were reported as the first stereo stations. By the late 1960s, FM had been adopted for broadcast of stereo "A.O.R.—'Album Oriented Rock'
Format", but it was not until 1978 that listenership to FM stations
exceeded that of AM stations in North America. In most of the 70s FM was
seen as highbrow radio associated with educational programming and
classical music, which changed during the 1980s and 1990s when Top 40
music stations and later even country music stations largely abandoned
AM for FM.
Today AM is mainly the preserve of talk radio, news, sports, religious
programming, ethnic (minority language) broadcasting and some types of
minority interest music. This shift has transformed AM into the
"alternative band" that FM once was. (Some AM stations have begun to
simulcast on, or switch to, FM signals to attract younger listeners and
aid reception problems in buildings, during thunderstorms, and near
high-voltage wires. Some of these stations now emphasize their presence
on the FM band.)
Europe
The medium wave band (known as the AM band because most stations using it employ amplitude modulation) was overcrowded
in western Europe, leading to interference problems and, as a result,
many MW frequencies are suitable only for speech broadcasting.
Belgium, the Netherlands, Denmark and particularly Germany were among the first countries to adopt FM on a widespread scale. Among the reasons for this were:
The medium wave
band in Western Europe became overcrowded after World War II, mainly
due to the best available medium wave frequencies used at high power
levels by the Allied Occupation Forces, both for broadcasting entertainment to their troops and for broadcasting Cold War propaganda across the Iron Curtain.
After World War II, broadcasting frequencies were reorganized and reallocated by delegates of the victorious countries in the Copenhagen Frequency Plan. German broadcasters were left with only two remaining AM frequencies and were forced to look to FM for expansion.
Public service broadcasters in Ireland and Australia were far slower at adopting FM radio than those in either North America or continental Europe.
Netherlands
Hans Idzerda operated a broadcasting station, PCGG, at The Hague from 1919 to 1924, which employed narrow-band FM transmissions.
United Kingdom
In the United Kingdom the BBC conducted tests during the 1940s, then began FM broadcasting in 1955, with three national networks: the Light Programme, Third Programme and Home Service.
These three networks used the sub-band 88.0–94.6 MHz. The sub-band
94.6–97.6 MHz was later used for BBC and local commercial services.
However, only when commercial broadcasting was introduced to the
UK in 1973 did the use of FM pick up in Britain. With the gradual
clearance of other users (notably Public Services such as police, fire
and ambulance) and the extension of the FM band to 108.0 MHz between
1980 and 1995, FM expanded rapidly throughout the British Isles and
effectively took over from LW and MW as the delivery platform of choice
for fixed and portable domestic and vehicle-based receivers. In
addition, Ofcom
(previously the Radio Authority) in the UK issues on demand Restricted
Service Licences on FM and also on AM (MW) for short-term local-coverage
broadcasting which is open to anyone who does not carry a prohibition
and can put up the appropriate licensing and royalty fees. In 2010
around 450 such licences were issued.
When the BBC's radio networks were renamed Radio 2, Radio 3 and Radio 4 respectively in 1967 to coincide with the launch of Radio 1,
the new station was the only one of the main four to not have an FM
frequency allocated, which was the case for 21 years. Instead, Radio 1
shared airtime with Radio 2 FM, on Saturday afternoons, Sunday evenings,
weekday evenings (10pm to midnight) and Bank Holidays, eventually having its own FM frequency starting in London in October 1987 on 104.8 MHz at Crystal Palace. Eventually in 1987
a frequency range of 97.6-99.8 MHz was allocated as police relay
transmitters were moved from the 100 MHz frequency, starting in London
before being completed by 1989, where Radio 1 in London moved from the
latter frequency to 98.8 MHz to the BBC's Wrotham transmitter. This followed the BBC Radio 1 FM frequencies rolled out to the rest of the UK.
Italy
Italy adopted FM broadcast widely in the early 1970s, but first experiments made by RAI dated back to 1950,
when the "movement for free radio", developed by so-called "pirates",
forced the recognition of free speech rights also through the use of
"free radio media such as Broadcast transmitters", and took the case to
the Constitutional Court of Italy. The court finally decided in favor of
Free Radio. Just weeks after the court's final decision there was an
"FM radio boom" involving small private radio stations across the
country. By the mid-1970s, every city in Italy had a crowded FM radio
spectrum.
Greece
Greece
was another European country where the FM radio spectrum was used at
first by the so-called "pirates" (both in Athens and Thessaloniki, the
two major Greek cities) in the mid-1970s, before any national stations
had started broadcasting on it; there were many AM (MW) stations in use
for the purpose. No later than the end of 1977, the national public
service broadcasting company EIRT (later also known as ERT) placed in
service its first FM transmitter in the capital, Athens. By the end of
the 1970s, most of Greek territory was covered by three National FM
programs, and every city had many FM "pirates" as well.
The adaptation of the FM band for privately owned commercial radio
stations came far later, in 1987.
Australia
FM broadcasting started in Australian capital cities in 1947 on an "experimental" basis, using an ABC national network
feed, consisting largely of classical music and Parliament, as a
programme source. It had a very small audience and was shut down in 1961
ostensibly to clear the television
band: TV channel 5 (102.250 video carrier) if allocated would fall
within the VHF FM band (98–108 MHz). The official policy on FM at the
time was to eventually introduce it on another band, which would have
required FM tuners custom-built for Australia. This policy was finally
reversed and FM broadcasting was reopened in 1975 using the VHF band,
after the few encroaching TV stations had been moved. Subsequently, it
developed steadily until in the 1980s many AM stations transferred to FM
due to its superior sound quality and lower operating costs. Today, as
elsewhere in the developed world, most urban Australian broadcasting is
on FM, although AM talk stations are still very popular. Regional
broadcasters still commonly operate AM stations due to the additional
range the broadcasting method offers. Some stations in major regional
centres simulcast on AM and FM bands. Digital radio using the DAB+
standard has been rolled out to capital cities.
New Zealand
Like
Australia, New Zealand adopted the FM format relatively late. As was
the case with privately owned AM radio in the late 1960s, it took a
spate of 'pirate' broadcasters to persuade a control-oriented,
technology-averse government to allow FM to be introduced after at least
five years of consumer campaigning starting in the mid-1970s,
particularly in Auckland. An experimental FM station, FM 90.7, was broadcast in Whakatane in early 1982. Later that year, Victoria University of Wellington's Radio Active began full-time FM transmissions. Commercial FM licences were finally approved in 1983, with Auckland-based 91FM and 89FM being the first to take up the offer. Broadcasting was deregulated in 1989.
Like many other countries in Africa and Asia that drive on the
left, New Zealand imports vehicles from Japan. The standard radios in
these vehicles operate on 76-to-90 MHz, which is not compatible with the
88-to-108 MHz range. Imported cars with Japanese radios can have FM
expanders installed which down-convert the higher frequencies above
90 MHz. New Zealand has no indigenous car manufacturers.
Trinidad and Tobago
Trinidad and Tobago's first FM Radio station was 95.1FM, now rebranded as 951 Remix, was launched in March 1976 by the TBC Radio Network.
Turkey
In Turkey,
FM broadcasting began in the late 1960s, carrying several shows from
the One television network which was transferred from the AM frequency
(also known as MW in Turkey). In subsequent years, more MW stations were
slowly transferred to FM, and by the end of the 1970s, most radio
stations that were previously on MW had been moved to FM, though many
talk, news and sport, but mostly religious stations, still remain on MW.
Other countries
Most
other countries implemented FM broadcasting through the 1960s and
expanded their use of FM through the 1990s. Because it takes a large
number of FM transmitting stations to cover a geographically large
country, particularly where there are terrain difficulties, FM is more
suited to local broadcasting than for national networks. In such
countries, particularly where there are economic or infrastructural
problems, "rolling out" a national FM broadcast network to reach the
majority of the population can be a slow and expensive process. Despite
this, mostly in east European countries, national FM broadcast networks
were established in the late 1960s and 1970s. In all Soviet-dependent
countries except GDR,
the OIRT band was used. First restricted to 68–73 MHz with 100 kHz
channel spacing, then in the 1970s eventually expanded to
65.84–74.00 MHz with 30 kHz channel spacing.
The use of FM for domestic radio encouraged listeners to acquire
cheap FM-only receivers and so reduced the number able to listen to
longer-range AM foreign broadcasters. Similar considerations led to
domestic radio in South Africa switching to FM in the 1960s.
ITU Conferences about FM
The frequencies available for FM were decided by some important conferences of ITU. The milestone of those conferences is the Stockholm agreement of 1961 among 38 countries.
A 1984 conference in Geneva made some modifications to the original
Stockholm agreement particularly in the frequency range above 100 MHz.
In 2017, Norway became the first country so far to completely switch to Digital audio broadcasting,
the exception being some local stations remaining on FM until 2022, and
might be extended to 2031. The switchover to DAB+ meant that especially
rural areas obtained a far more diverse radio content compared to the
FM-only period; several new radio stations had started transmissions on
DAB+ in the years before the FM switch-off.
In some countries, small-scale (Part 15 in United States terms) transmitters are available that can transmit a signal from an audio device (usually an MP3 player
or similar) to a standard FM radio receiver; such devices range from
small units built to carry audio to a car radio with no audio-in
capability (often formerly provided by special adapters for audio cassette
decks, which are no longer common on car radio designs) up to
full-sized, near-professional-grade broadcasting systems that can be
used to transmit audio throughout a property, including systems that
synchronize holiday decorative lighting with music. Most such units
transmit in full stereo, though some models designed for beginner
hobbyists might not. Similar transmitters are often included in satellite radio receivers and some toys.
Legality of these devices varies by country. The U.S. Federal Communications Commission and Industry Canada allow them. Starting on 1 October 2006, these devices became legal in most countries in the European Union. Devices made to the harmonized European specification became legal in the UK on 8 December 2006.
The FM broadcast band is also used by some inexpensive wireless microphones sold as toys for karaoke
or similar purposes, allowing the user to use an FM radio as an output
rather than a dedicated amplifier and speaker. Professional-grade
wireless microphones generally use bands in the UHF region so they can run on dedicated equipment without broadcast interference.
Some wireless headphones
transmit in the FM broadcast band, with the headphones tunable to only a
subset of the broadcast band. Higher-quality wireless headphones use infrared transmission or UHF ISM bands such as 315 MHz, 863 MHz, 915 MHz, or 2.4 GHz instead of the FM broadcast band.
Assistive listening
Some assistive listening devices
are based on FM radio, mostly using the 72.1 to 75.8 MHz band. Aside
from the assisted listening receivers, only certain kinds of FM
receivers can tune to this band.
Microbroadcasting
Low-power
transmitters such as those mentioned above are also sometimes used for
neighborhood or campus radio stations, though campus radio stations are
often run over carrier current. This is generally considered a form of microbroadcasting. As a general rule, enforcement towards low-power FM stations is stricter than with AM stations, due to problems such as the capture effect, and as a result, FM microbroadcasters generally do not reach as far as their AM competitors.
Clandestine use of FM transmitters
FM transmitters have been used to construct miniature wireless microphones for espionage and surveillance purposes (covert listening devices
or so-called "bugs"); the advantage to using the FM broadcast band for
such operations is that the receiving equipment would not be considered
particularly suspect. Common practice is to tune the bug's transmitter
off the ends of the broadcast band, into what in the United States would
be TV channel 6 (<87.9 MHz) or aviation navigation frequencies
(>107.9 MHz); most FM radios with analog tuners have sufficient
overcoverage to pick up these slightly-beyond-outermost frequencies,
although many digitally tuned radios have not.
Constructing a "bug" is a common early project for electronics
hobbyists, and project kits to do so are available from a wide variety
of sources. The devices constructed, however, are often too large and
poorly shielded for use in clandestine activity.
In addition, much pirate radio
activity is broadcast in the FM range, because of the band's greater
clarity and listenership, the smaller size and lower cost of equipment.
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