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84-481: The Little Rock Marathon is famous for the world's largest finisher's medal (According to Runners World). Race organizers insist the medals get slightly bigger each year. The 2015 marathon medal measured 8 ½ x 8 ¼ inches and weighed just over 3 pounds. Another unique feature of the Little Rock Marathon is their themes; the event has a different theme each year. Each year the theme is reflected in everything from

168-415: A battery and relay . When the incoming radio wave reduced the resistance of the coherer, the current from the battery flowed through it, turning on the relay to ring a bell or make a mark on a paper tape in a siphon recorder . In order to restore the coherer to its previous nonconducting state to receive the next pulse of radio waves, it had to be tapped mechanically to disturb the metal particles. This

252-438: A battery -powered transceiver , connected to the athlete, that emits its unique code when it is interrogated. A passive transponder does not contain a power source inside the transponder. Instead, the transponder captures electromagnetic energy produced by a nearby exciter and utilizes that energy to emit a unique code. In both systems, an antenna is placed at the start, finish, and in some cases, intermediate time points and

336-468: A digital signal rather than an analog signal as AM and FM do. Its advantages are that DAB has the potential to provide higher quality sound than FM (although many stations do not choose to transmit at such high quality), has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth, and provides advanced user features such as electronic program guide , sports commentaries, and image slideshows. Its disadvantage

420-445: A feedback control system which monitors the average level of the radio signal at the detector, and adjusts the gain of the amplifiers to give the optimum signal level for demodulation. This is called automatic gain control (AGC). AGC can be compared to the dark adaptation mechanism in the human eye ; on entering a dark room the gain of the eye is increased by the iris opening. In its simplest form, an AGC system consists of

504-413: A radio frequency (RF) amplifier to increase its strength to a level sufficient to drive the demodulator; (3) the demodulator recovers the modulation signal (which in broadcast receivers is an audio signal , a voltage oscillating at an audio frequency rate representing the sound waves) from the modulated radio carrier wave ; (4) the modulation signal is amplified further in an audio amplifier , then

588-419: A radio receiver , also known as a receiver , a wireless , or simply a radio , is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna . The antenna intercepts radio waves ( electromagnetic waves of radio frequency ) and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts

672-570: A rectifier which converts the RF signal to a varying DC level, a lowpass filter to smooth the variations and produce an average level. This is applied as a control signal to an earlier amplifier stage, to control its gain. In a superheterodyne receiver, AGC is usually applied to the IF amplifier , and there may be a second AGC loop to control the gain of the RF amplifier to prevent it from overloading, too. In certain receiver designs such as modern digital receivers,

756-410: A wireless modem , is applied as input to a computer or microprocessor , which interacts with human users. In the simplest type of radio receiver, called a tuned radio frequency (TRF) receiver , the three functions above are performed consecutively: (1) the mix of radio signals from the antenna is filtered to extract the signal of the desired transmitter; (2) this oscillating voltage is sent through

840-444: A cable, as with rooftop television antennas and satellite dishes . Practical radio receivers perform three basic functions on the signal from the antenna: filtering , amplification , and demodulation : Radio waves from many transmitters pass through the air simultaneously without interfering with each other and are received by the antenna. These can be separated in the receiver because they have different frequencies ; that is,

924-420: A distance of 3500 km (2200 miles), which was received by a coherer. However the usual range of coherer receivers even with the powerful transmitters of this era was limited to a few hundred miles. The coherer remained the dominant detector used in early radio receivers for about 10 years, until replaced by the crystal detector and electrolytic detector around 1907. In spite of much development work, it

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1008-587: A few hills. Little Rock is a little salty. " The largest hill is found early in the race with a number of smaller hills throughout. The elevation change for the entire course is as follows: Landmarks along the course include the Arkansas State Capitol , River Market District , the Clinton Presidential Center , Governor's Mansion, MacArthur Birthplace/Military Museum, and Historic Little Rock Central High School . The Little Rock Marathon

1092-602: A filter increases with its center frequency, so as the TRF receiver is tuned to different frequencies its bandwidth varies. Most important, the increasing congestion of the radio spectrum requires that radio channels be spaced very close together in frequency. It is extremely difficult to build filters operating at radio frequencies that have a narrow enough bandwidth to separate closely spaced radio stations. TRF receivers typically must have many cascaded tuning stages to achieve adequate selectivity. The Advantages section below describes how

1176-633: A free, year-round training program for those interested in training for a race or improve/maintain their health and fitness. The Little Rock Marathon is a USATF sanctioned event on a certified 26.2-mile (42.2 km) single loop course. The course, which starts and finishes in Downtown Little Rock and also crosses over the Arkansas River, is scenic and includes many of Little Rock's famous and historic landmarks. The very spectator-friendly course features plenty of entertainment and crowd support along

1260-539: A limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver, atmospheric and internal noise , as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond

1344-481: A lot today. Other manufacturers developed their own proprietary RFID systems usually as an offshoot to more industrial applications. These latter systems attempted to get around the problem of reading large numbers of transponders in a read field by using the High Frequency 13.56 MHz RFID methodology that allowed transponders to use anti-collision algorithms to avoid tags interfering with each other's signal during

1428-448: A paper tape machine. The coherer's poor performance motivated a great deal of research to find better radio wave detectors, and many were invented. Some strange devices were tried; researchers experimented with using frog legs and even a human brain from a cadaver as detectors. By the first years of the 20th century, experiments in using amplitude modulation (AM) to transmit sound by radio ( radiotelephony ) were being made. So

1512-612: A related problem is DC offset of the signal. This is corrected by a similar feedback system. Radio waves were first identified in German physicist Heinrich Hertz 's 1887 series of experiments to prove James Clerk Maxwell's electromagnetic theory . Hertz used spark-excited dipole antennas to generate the waves and micrometer spark gaps attached to dipole and loop antennas to detect them. These primitive devices are more accurately described as radio wave sensors, not "receivers", as they could only detect radio waves within about 100 feet of

1596-423: A second goal of detector research was to find detectors that could demodulate an AM signal, extracting the audio (sound) signal from the radio carrier wave . It was found by trial and error that this could be done by a detector that exhibited "asymmetrical conduction"; a device that conducted current in one direction but not in the other. This rectified the alternating current radio signal, removing one side of

1680-491: A separate PC computer that is connected to the readers via serial or Ethernet communications. The software relates the raw transponder code and timestamp data to each entrant in a database and calculates gun and net times of runners, or the splits of a triathlete. In advanced systems these results are instantly calculated and published to the internet so that athletes and spectators have access to results via any web enabled device. Radio receiver In radio communications ,

1764-511: A single audio channel that is a combination (sum) of the left and right channels. While AM stereo transmitters and receivers exist, they have not achieved the popularity of FM stereo. Most modern radios are able to receive both AM and FM radio stations, and have a switch to select which band to receive; these are called AM/FM radios . Digital audio broadcasting (DAB) is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as

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1848-585: A strict protocol to ensure that multiple transponders and readers could be used between manufacturers. Much like the HF tags, the UHF tags were much cheaper to produce in volume and formed the basis in the next revolution in sports timing. Currently, many of the largest athletic events are timed using disposable transponders either placed on the back of a race number or on the runner's shoe. The low cost meant that transponders were now fully disposable and did not need to be returned to

1932-444: Is a transmitter and receiver combined in one unit. Below is a list of a few of the most common types, organized by function. A radio receiver is connected to an antenna which converts some of the energy from the incoming radio wave into a tiny radio frequency AC voltage which is applied to the receiver's input. An antenna typically consists of an arrangement of metal conductors. The oscillating electric and magnetic fields of

2016-485: Is a 26.2-mile running/walking tour through the historical sites of Little Rock. The course is certified by the UST&;F. The Little Rock Half Marathon (13.1 miles) is run on the same course as the big race, but it is half the distance. The course is certified by the UST&F. The Little Rock 10K (6.2 miles) will run with the 5K on the day before the marathon and half-marathon. The Little Rock 5K (3.1 miles) will run with

2100-528: Is a technique for measuring performance in sport events. A transponder working on a radio-frequency identification (RFID) basis is attached to the athlete and emits a unique code that is detected by radio receivers located at the strategic points in an event. Prior to the use of this technology, races were either timed by hand (with operators pressing a stopwatch ) or using video camera systems. Generally, there are two types of transponder timing systems; active and passive. An active transponder consists of

2184-436: Is applied to a loudspeaker or earphone to convert it to sound waves. Although the TRF receiver is used in a few applications, it has practical disadvantages which make it inferior to the superheterodyne receiver below, which is used in most applications. The drawbacks stem from the fact that in the TRF the filtering, amplification, and demodulation are done at the high frequency of the incoming radio signal. The bandwidth of

2268-443: Is called the intermediate frequency (IF). The IF signal also has the modulation sidebands that carry the information that was present in the original RF signal. The IF signal passes through filter and amplifier stages, then is demodulated in a detector, recovering the original modulation. The receiver is easy to tune; to receive a different frequency it is only necessary to change the local oscillator frequency. The stages of

2352-475: Is commonly called a "radio". However radio receivers are very widely used in other areas of modern technology, in televisions , cell phones , wireless modems , radio clocks and other components of communications, remote control, and wireless networking systems. The most familiar form of radio receiver is a broadcast receiver, often just called a radio , which receives audio programs intended for public reception transmitted by local radio stations . The sound

2436-479: Is completed on the Little Rock Marathon course and finishes at the official Little Rock Marathon finish line. The Health and Fitness Expo is the official kickoff of the Little Rock Marathon weekend. The expo is held at the Statehouse Convention Center. Exhibitors sell running/fitness items, give out samples, and distribute their products to participants picking up race packets. Established in 2003,

2520-456: Is connected to a decoder. This decoder identifies the unique transponder code and calculates the exact time when the transponder passes a timing point. Some implementations of timing systems require the use of a mat on the ground at the timing points while other systems implement the timing points with vertically oriented portals. RFID was first used in the late 1980s primarily for motor racing and became more widely adopted in athletic events in

2604-428: Is first mixed with one local oscillator signal in the first mixer to convert it to a high IF frequency, to allow efficient filtering out of the image frequency, then this first IF is mixed with a second local oscillator signal in a second mixer to convert it to a low IF frequency for good bandpass filtering. Some receivers even use triple-conversion . At the cost of the extra stages, the superheterodyne receiver provides

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2688-499: Is much lower than that of conventional re-usable transponders and the race does not bother to collect them afterwards. As of 2015, many UHF timers use a combination of ground antennas with panel antenna(s) mounted on a tripod at the side of the race course. All RFID timing systems incorporate a box housing the reader(s) with peripherals like a microprocessor, serial or Ethernet communications and power source (battery). The readers are attached to one or more antennas that are designed for

2772-537: Is not the degree of amplification but random electronic noise present in the circuit, which can drown out a weak radio signal. After the radio signal is filtered and amplified, the receiver must extract the information-bearing modulation signal from the modulated radio frequency carrier wave . This is done by a circuit called a demodulator ( detector ). Each type of modulation requires a different type of demodulator Many other types of modulation are also used for specialized purposes. The modulation signal output by

2856-411: Is reproduced either by a loudspeaker in the radio or an earphone which plugs into a jack on the radio. The radio requires electric power , provided either by batteries inside the radio or a power cord which plugs into an electric outlet . All radios have a volume control to adjust the loudness of the audio, and some type of "tuning" control to select the radio station to be received. Modulation

2940-571: Is restricted. Also more equipment is needed for events that require multiple timing points. Wider timing points require more readers and antennas. For active systems a simple wire loop is all that is needed since the transponder has its own power source and the loop serves as a trigger to turn on the transponder, then receive the relatively strong signal from the transponder. Therefore, active systems need less readers (or decoders) per timing point width. All systems utilize specialized software to calculate results and splits. This software usually resides on

3024-445: Is that it is incompatible with previous radios so that a new DAB receiver must be purchased. As of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. The United States and Canada have chosen not to implement DAB. DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500 kHz bandwidth signal that carries from 9 to 12 channels from which

3108-446: Is the process of adding information to a radio carrier wave . Two types of modulation are used in analog radio broadcasting systems; AM and FM. In amplitude modulation (AM) the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, and between 526 and 1706 kHz in

3192-413: The amplitude (voltage or current) of the signal. In most modern receivers, the electronic components which do the actual amplifying are transistors . Receivers usually have several stages of amplification: the radio signal from the bandpass filter is amplified to make it powerful enough to drive the demodulator, then the audio signal from the demodulator is amplified to make it powerful enough to operate

3276-460: The medium frequency (MF) range of the radio spectrum . AM broadcasting is also permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting. In frequency modulation (FM), the frequency of the radio signal is varied slightly by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in

3360-418: The very high frequency (VHF) range. The exact frequency ranges vary somewhat in different countries. FM stereo radio stations broadcast in stereophonic sound (stereo), transmitting two sound channels representing left and right microphones . A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A monaural receiver, in contrast, only receives

3444-445: The 10K on the day before the marathon and half-marathon. This race is designed for individuals who want an introduction into running or walking without weeks of training. The Little Rockers Kids Marathon is for children in 1st to 5th grades and gives them an opportunity to finish a marathon over an extended period of time. Prior to race day, participants run or walk 25.2 miles at their own pace and with adult supervision. The last mile

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3528-611: The Official Little Rock Marathon Training Program has helped thousands of athletes (runners, walkers, joggers) cross the finish line. The Little Rock Marathon Training Program is a free, year-round program for those at any skill-level interested in starting a running/walking program, maintaining their base mileage, or training for a race. The Little Rock Marathon Training Program is for those individuals wanting to : Transponder timing Transponder timing (also called chip timing or RFID timing )

3612-409: The advantage of greater selectivity than can be achieved with a TRF design. Where very high frequencies are in use, only the initial stage of the receiver needs to operate at the highest frequencies; the remaining stages can provide much of the receiver gain at lower frequencies which may be easier to manage. Tuning is simplified compared to a multi-stage TRF design, and only two stages need to track over

3696-440: The amplitude of the modulation does not vary with the radio signal strength, but in all types the demodulator requires a certain range of signal amplitude to operate properly. Insufficient signal amplitude will cause an increase of noise in the demodulator, while excessive signal amplitude will cause amplifier stages to overload (saturate), causing distortion (clipping) of the signal. Therefore, almost all modern receivers include

3780-431: The antenna is mixed with an unmodulated signal generated by a local oscillator (LO) in the receiver. The mixing is done in a nonlinear circuit called the " mixer ". The result at the output of the mixer is a heterodyne or beat frequency at the difference between these two frequencies. The process is similar to the way two musical notes at different frequencies played together produce a beat note . This lower frequency

3864-577: The course. The race is filled with loads of southern hospitality and several unique features, such as the couch potato mile and the Lipstick Stop (so runners can look good for their finish photos). It is the first race in the state to be transponder chip timed , and is also a qualifying event for the Boston Marathon . The Little Rock Marathon Course is described as relatively hilly. Running great Bill Rodgers once said " any marathon worth its salt has

3948-420: The demodulator is usually amplified to increase its strength, then the information is converted back to a human-usable form by some type of transducer . An audio signal , representing sound, as in a broadcast radio, is converted to sound waves by an earphone or loudspeaker . A video signal , representing moving images, as in a television receiver , is converted to light by a display . Digital data , as in

4032-424: The desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation . Radio receivers are essential components of all systems that use radio . The information produced by

4116-450: The desired signal. A single tunable RF filter stage rejects the image frequency; since these are relatively far from the desired frequency, a simple filter provides adequate rejection. Rejection of interfering signals much closer in frequency to the desired signal is handled by the multiple sharply-tuned stages of the intermediate frequency amplifiers, which do not need to change their tuning. This filter does not need great selectivity, but as

4200-407: The distance between the transponder and readers antenna is minimized offering the best capture rate. Transponders may be threaded onto the shoe laces for running. For triathlon a soft elastic ankle band holds the transponder to the leg and care is taken to ensure the transponder is in the correct orientation or polarity for maximum read performance. Transponders have also been placed on the race bib. In

4284-600: The down-link between transponder and reader. Active transponder systems continued to mature and despite their much higher cost they retained market share in the high speed sports like motor racing, cycling and ice skating. Active systems are also used at high-profile events such as the Olympics due to their very high read rates and time-stamping precision. By 2005 a newer RFID technology was becoming available, mostly for industrial applications. The first and second generation (UHF) transponders and readers that were being developed followed

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4368-508: The décor at the Health and Fitness Expo, race course areas, and post-race events to the huge finishers medal and all official merchandise. Many participants are also known to dress in costumes according to the year's race theme. The Little Rock Marathon bills itself as "Arkansas' Race for Every Pace". With an 8-hour time limit for the marathon and half-marathon, it is very friendly for walkers and beginner runners. The Little Rock Marathon also offers

4452-651: The earphone the signal sounded like a musical tone or buzz, and the Morse code "dots" and "dashes" sounded like beeps. The first person to use radio waves for communication was Guglielmo Marconi . Marconi invented little himself, but he was first to believe that radio could be a practical communication medium, and singlehandedly developed the first wireless telegraphy systems, transmitters and receivers, beginning in 1894–5, mainly by improving technology invented by others. Oliver Lodge and Alexander Popov were also experimenting with similar radio wave receiving apparatus at

4536-415: The exact net time to be calculated. Awards in a race are generally based on the "gun time" (which ignores any delay at the start) as per IAAF and USA Track and Field rules. However, some races use "net time" for presenting age group awards. In the past the transponder was almost always worn on the athletes running shoe, or on an ankle band. This enabled the transponder to be read best on antenna mats because

4620-485: The filtering at the lower intermediate frequency. One of the most important parameters of a receiver is its bandwidth , the band of frequencies it accepts. In order to reject nearby interfering stations or noise, a narrow bandwidth is required. In all known filtering techniques, the bandwidth of the filter increases in proportion with the frequency, so by performing the filtering at the lower f IF {\displaystyle f_{\text{IF}}} , rather than

4704-476: The frequency of the original radio signal f RF {\displaystyle f_{\text{RF}}} , a narrower bandwidth can be achieved. Modern FM and television broadcasting, cellphones and other communications services, with their narrow channel widths, would be impossible without the superheterodyne. The signal strength ( amplitude ) of the radio signal from a receiver's antenna varies drastically, by orders of magnitude, depending on how far away

4788-428: The incoming radio signal is at the resonant frequency, the resonant circuit has high impedance and the radio signal from the desired station is passed on to the following stages of the receiver. At all other frequencies the resonant circuit has low impedance, so signals at these frequencies are conducted to ground. The power of the radio waves picked up by a receiving antenna decreases with the square of its distance from

4872-417: The listener can choose. Broadcasters can transmit a channel at a range of different bit rates , so different channels can have different audio quality. In different countries DAB stations broadcast in either Band III (174–240 MHz) or L band (1.452–1.492 GHz). The signal strength of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within

4956-475: The mid-1990s upon the release of low cost 134 kHz transponders and readers from Texas Instruments . This technology formed the basis of electronic sports timing for the world's largest running events as well as for cycling, triathlon and skiing. Some manufacturers made improvements to the technology to handle larger numbers of transponders in the read field or improve the tolerance of their systems to low-frequency noise. These low-frequency systems are still used

5040-412: The organizers after the event. Very large running events (more than 10,000) and triathlons were the first events to be transponder (or chip) timed because it is near impossible to manually time them. Also for large runs there are delays in participants reaching the start line, which penalize their performance. Some races place antennas or timing mats at both the start line and the finish line, which allow

5124-432: The particular operating frequency. In the case of low or medium frequencies these consist of wire loops incorporated into mats that cover the entire width of the timing point. For UHF systems the antennas consist of patch antennas that are protected in a matting system. The patch antennas may also be placed on stands or a finish gantry pointing towards the oncoming athlete. In most cases the distance between reader and antennas

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5208-469: The past 5 years the newer UHF systems use transponders placed on the shoe lace, or stuck to the race number bib. In both cases, care must be taken to ensure the UHF tag does not directly touch a large part of the skin as this affects read performance. Despite this, UHF Systems have read performances as good (if not better) than the conventional low and high frequency systems. Because these UHF tags are made in huge volumes for industrial applications, their price

5292-416: The radio transmitter is, how powerful it is, and propagation conditions along the path of the radio waves. The strength of the signal received from a given transmitter varies with time due to changing propagation conditions of the path through which the radio wave passes, such as multipath interference ; this is called fading . In an AM receiver, the amplitude of the audio signal from the detector, and

5376-410: The radio wave from each transmitter oscillates at a different rate. To separate out the desired radio signal, the bandpass filter allows the frequency of the desired radio transmission to pass through, and blocks signals at all other frequencies. The bandpass filter consists of one or more resonant circuits (tuned circuits). The resonant circuit is connected between the antenna input and ground. When

5460-401: The radio wave push the electrons in the antenna back and forth, creating an oscillating voltage. The antenna may be enclosed inside the receiver's case, as with the ferrite loop antennas of AM radios and the flat inverted F antenna of cell phones; attached to the outside of the receiver, as with whip antennas used on FM radios , or mounted separately and connected to the receiver by

5544-565: The radio wave to demodulate the later amplitude modulated (AM) radio transmissions that carried sound. In a long series of experiments Marconi found that by using an elevated wire monopole antenna instead of Hertz's dipole antennas he could transmit longer distances, beyond the curve of the Earth, demonstrating that radio was not just a laboratory curiosity but a commercially viable communication method. This culminated in his historic transatlantic wireless transmission on December 12, 1901, from Poldhu, Cornwall to St. John's, Newfoundland ,

5628-445: The receiver after the mixer operates at the fixed intermediate frequency (IF) so the IF bandpass filter does not have to be adjusted to different frequencies. The fixed frequency allows modern receivers to use sophisticated quartz crystal , ceramic resonator , or surface acoustic wave (SAW) IF filters that have very high Q factors , to improve selectivity. The RF filter on the front end of

5712-420: The receiver is needed to prevent interference from any radio signals at the image frequency . Without an input filter the receiver can receive incoming RF signals at two different frequencies,. The receiver can be designed to receive on either of these two frequencies; if the receiver is designed to receive on one, any other radio station or radio noise on the other frequency may pass through and interfere with

5796-466: The receiver is tuned to different frequencies it must "track" in tandem with the local oscillator. The RF filter also serves to limit the bandwidth applied to the RF amplifier, preventing it from being overloaded by strong out-of-band signals. To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a dual-conversion or double-conversion superheterodyne. The incoming RF signal

5880-431: The receiver may be in the form of sound, video ( television ), or digital data . A radio receiver may be a separate piece of electronic equipment, or an electronic circuit within another device. The most familiar type of radio receiver for most people is a broadcast radio receiver, which reproduces sound transmitted by radio broadcasting stations, historically the first mass-market radio application. A broadcast receiver

5964-467: The same time in 1894–5, but they are not known to have transmitted Morse code during this period, just strings of random pulses. Therefore, Marconi is usually given credit for building the first radio receivers. The first radio receivers invented by Marconi, Oliver Lodge and Alexander Popov in 1894-5 used a primitive radio wave detector called a coherer , invented in 1890 by Edouard Branly and improved by Lodge and Marconi. The coherer

6048-420: The sound volume, is proportional to the amplitude of the radio signal, so fading causes variations in the volume. In addition as the receiver is tuned between strong and weak stations, the volume of the sound from the speaker would vary drastically. Without an automatic system to handle it, in an AM receiver, constant adjustment of the volume control would be required. With other types of modulation like FM or FSK

6132-402: The speaker. The degree of amplification of a radio receiver is measured by a parameter called its sensitivity , which is the minimum signal strength of a station at the antenna, measured in microvolts , necessary to receive the signal clearly, with a certain signal-to-noise ratio . Since it is easy to amplify a signal to any desired degree, the limit to the sensitivity of many modern receivers

6216-471: The superheterodyne receiver overcomes these problems. The superheterodyne receiver, invented in 1918 by Edwin Armstrong is the design used in almost all modern receivers except a few specialized applications. In the superheterodyne, the radio frequency signal from the antenna is shifted down to a lower " intermediate frequency " (IF), before it is processed. The incoming radio frequency signal from

6300-683: The transmitter, and were not used for communication but instead as laboratory instruments in scientific experiments. The first radio transmitters , used during the initial three decades of radio from 1887 to 1917, a period called the spark era , were spark gap transmitters which generated radio waves by discharging a capacitance through an electric spark . Each spark produced a transient pulse of radio waves which decreased rapidly to zero. These damped waves could not be modulated to carry sound, as in modern AM and FM transmission. So spark transmitters could not transmit sound, and instead transmitted information by radiotelegraphy . The transmitter

6384-404: The transmitting antenna. Even with the powerful transmitters used in radio broadcasting stations, if the receiver is more than a few miles from the transmitter the power intercepted by the receiver's antenna is very small, perhaps as low as picowatts or femtowatts . To increase the power of the recovered signal, an amplifier circuit uses electric power from batteries or the wall plug to increase

6468-427: The tuning range. The total amplification of the receiver is divided between three amplifiers at different frequencies; the RF, IF, and audio amplifier. This reduces problems with feedback and parasitic oscillations that are encountered in receivers where most of the amplifier stages operate at the same frequency, as in the TRF receiver. The most important advantage is that better selectivity can be achieved by doing

6552-464: The visual horizon to about 30–40 miles (48–64 km). Radios are manufactured in a range of styles and functions: Radio receivers are essential components of all systems that use radio . Besides the broadcast receivers described above, radio receivers are used in a huge variety of electronic systems in modern technology. They can be a separate piece of equipment (a radio ), or a subsystem incorporated into other electronic devices. A transceiver

6636-605: The visual horizon; limiting reception distance to about 40 miles (64 km), and can be blocked by hills between the transmitter and receiver. However FM radio is less susceptible to interference from radio noise ( RFI , sferics , static) and has higher fidelity ; better frequency response and less audio distortion , than AM. So in countries that still broadcast AM radio, serious music is typically only broadcast by FM stations, and AM stations specialize in radio news , talk radio , and sports radio . Like FM, DAB signals travel by line of sight so reception distances are limited by

6720-415: Was a glass tube with metal electrodes at each end, with loose metal powder between the electrodes. It initially had a high resistance . When a radio frequency voltage was applied to the electrodes, its resistance dropped and it conducted electricity. In the receiver the coherer was connected directly between the antenna and ground. In addition to the antenna, the coherer was connected in a DC circuit with

6804-418: Was a very crude unsatisfactory device. It was not very sensitive, and also responded to impulsive radio noise ( RFI ), such as nearby lights being switched on or off, as well as to the intended signal. Due to the cumbersome mechanical "tapping back" mechanism it was limited to a data rate of about 12-15 words per minute of Morse code , while a spark-gap transmitter could transmit Morse at up to 100 WPM with

6888-406: Was called a " detector ". Since there were no amplifying devices at this time, the sensitivity of the receiver mostly depended on the detector. Many different detector devices were tried. Radio receivers during the spark era consisted of these parts: The signal from the spark gap transmitter consisted of damped waves repeated at an audio frequency rate, from 120 to perhaps 4000 per second, so in

6972-415: Was done by a "decoherer", a clapper which struck the tube, operated by an electromagnet powered by the relay. The coherer is an obscure antique device, and even today there is some uncertainty about the exact physical mechanism by which the various types worked. However it can be seen that it was essentially a bistable device, a radio-wave-operated switch, and so it did not have the ability to rectify

7056-496: Was switched on and off rapidly by the operator using a telegraph key , creating different length pulses of damped radio waves ("dots" and "dashes") to spell out text messages in Morse code . Therefore, the first radio receivers did not have to extract an audio signal from the radio wave like modern receivers, but just detected the presence of the radio signal, and produced a sound during the "dots" and "dashes". The device which did this

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