The "Wadley-drift-canceling-loop" , also known as a "Wadley loop" , is a system of two oscillators , a frequency synthesizer , and two frequency mixers in the radio-frequency signal path. The system was designed by Dr. Trevor Wadley in the 1940s in South Africa . The circuit was first used for a stable wavemeter . (A wavemeter is used for measuring the wavelength and therefore also the frequency of a signal).
146-442: There is no regulation loop in a "Wadley-loop" , which is why the term is in quotation marks. However, the circuit configuration is not known by more accurate names. The "Wadley loop" was used in radio receivers from the 1950s to approximately 1980. The "Wadley loop" was mostly used in more expensive stationary radio receivers, but the "Wadley loop" was also used in a portable radio receiver (Barlow-Wadley XCR-30 Mark II). In
292-415: 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
438-468: 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
584-445: 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
730-413: 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
876-419: 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
1022-570: 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,
1168-410: 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
1314-405: A boom; the boom is only for support and not involved electrically. Only one of the elements is electrically connected to the transmitter or receiver, while the remaining elements are passive. The Yagi produces a fairly large gain (depending on the number of passive elements) and is widely used as a directional antenna with an antenna rotor to control the direction of its beam. It suffers from having
1460-444: 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,
1606-405: A current of 1 Ampere will require 63 Volts, and the antenna will radiate 63 Watts (ignoring losses) of radio frequency power. Now consider the case when the antenna is fed a signal with a wavelength of 1.25 m; in this case the current induced by the signal would arrive at the antenna's feedpoint out-of-phase with the signal, causing the net current to drop while the voltage remains
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#17330854567491752-463: A current will reflect when there are changes in the electrical properties of the material. In order to efficiently transfer the received signal into the transmission line, it is important that the transmission line has the same impedance as its connection point on the antenna, otherwise some of the signal will be reflected backwards into the body of the antenna; likewise part of the transmitter's signal power will be reflected back to transmitter, if there
1898-420: 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
2044-495: A fashion are known to be harmonically operated . Resonant antennas usually use a linear conductor (or element ), or pair of such elements, each of which is about a quarter of the wavelength in length (an odd multiple of quarter wavelengths will also be resonant). Antennas that are required to be small compared to the wavelength sacrifice efficiency and cannot be very directional. Since wavelengths are so small at higher frequencies ( UHF , microwaves ) trading off performance to obtain
2190-400: A feed-point impedance that matches that of a transmission line; a matching network between antenna terminals and the transmission line will improve power transfer to the antenna. A non-adjustable matching network will most likely place further limits the usable bandwidth of the antenna system. It may be desirable to use tubular elements, instead of thin wires, to make an antenna; these will allow
2336-602: 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
2482-436: A flux of 1 pW / m (10 Watts per square meter) and an antenna has an effective area of 12 m , then the antenna would deliver 12 pW of RF power to the receiver (30 microvolts RMS at 75 ohms). Since the receiving antenna is not equally sensitive to signals received from all directions, the effective area is a function of the direction to the source. Due to reciprocity (discussed above)
2628-403: A greater bandwidth. Or, several thin wires can be grouped in a cage to simulate a thicker element. This widens the bandwidth of the resonance. Amateur radio antennas that operate at several frequency bands which are widely separated from each other may connect elements resonant at those different frequencies in parallel. Most of the transmitter's power will flow into the resonant element while
2774-539: 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
2920-450: A long Beverage antenna can have significant directivity. For non directional portable use, a short vertical antenna or small loop antenna works well, with the main design challenge being that of impedance matching . With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance ; small loop antennas are tuned with parallel capacitors for this purpose. An antenna lead-in
3066-404: A new user, the feel of the first oscillator tuning control is counterintuitive. Although the knob moves in a continuous, analog fashion, its effect on the receiver operation is discrete , that is, the tuning advances in 1 MHz jumps. An example is Yaesu 's FRG-7 communications receiver, which uses the system to remove local oscillator drift. The Racal RA17 and Realistic DX-302 also used
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#17330854567493212-603: A number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array" (directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical. Log-periodic and frequency-independent antennas employ self-similarity in order to be operational over
3358-448: 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
3504-427: A proper resonant antenna at the trap frequency. At substantially higher or lower frequencies the trap allows the full length of the broken element to be employed, but with a resonant frequency shifted by the net reactance added by the trap. The bandwidth characteristics of a resonant antenna element can be characterized according to its Q where the resistance involved is the radiation resistance , which represents
3650-516: A pure resistance. Sometimes the resulting (lower) electrical resonant frequency of such a system (antenna plus matching network) is described using the concept of electrical length , so an antenna used at a lower frequency than its resonant frequency is called an electrically short antenna For example, at 30 MHz (10 m wavelength) a true resonant 1 / 4 wave monopole would be almost 2.5 meters long, and using an antenna only 1.5 meters tall would require
3796-456: A rather limited bandwidth, restricting its use to certain applications. Rather than using one driven antenna element along with passive radiators, one can build an array antenna in which multiple elements are all driven by the transmitter through a system of power splitters and transmission lines in relative phases so as to concentrate the RF power in a single direction. What's more, a phased array can be made "steerable", that is, by changing
3942-612: 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
4088-423: 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
4234-443: A signal into the transmission line only when the source signal's frequency is close to that of the design frequency of the antenna, or one of the resonant multiples. This makes resonant antenna designs inherently narrow-band: Only useful for a small range of frequencies centered around the resonance(s). It is possible to use simple impedance matching techniques to allow the use of monopole or dipole antennas substantially shorter than
4380-511: 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
4526-440: A smaller physical size is usually not required. The quarter-wave elements imitate a series-resonant electrical element due to the standing wave present along the conductor. At the resonant frequency, the standing wave has a current peak and voltage node (minimum) at the feed. In electrical terms, this means that at that position, the element has minimum impedance magnitude , generating the maximum current for minimum voltage. This
Wadley loop - Misplaced Pages Continue
4672-498: A standard resistive impedance needed for its optimum operation. The feed point location(s) is selected, and antenna elements electrically similar to tuner components may be incorporated in the antenna structure itself, to improve the match . It is a fundamental property of antennas that most of the electrical characteristics of an antenna, such as those described in the next section (e.g. gain , radiation pattern , impedance , bandwidth , resonant frequency and polarization ), are
4818-477: A total 360 degree phase change, returning it to the original signal. The current in the element thus adds to the current being created from the source at that instant. This process creates a standing wave in the conductor, with the maximum current at the feed. The ordinary half-wave dipole is probably the most widely used antenna design. This consists of two 1 / 4 wavelength elements arranged end-to-end, and lying along essentially
4964-491: A traditional superheterodyne radio receiver , most oscillator drift and instability occur in the first frequency converter stage because it is tunable and operating at a high frequency. Unlike other drift-reducing techniques (such as crystal control or frequency synthesis ), the Wadley Loop does not attempt to stabilize the oscillator. Instead, it cancels the drift mathematically. The Wadley loop works by: Since
5110-473: A wide range of bandwidths . The most familiar example is the log-periodic dipole array which can be seen as a number (typically 10 to 20) of connected dipole elements with progressive lengths in an endfire array making it rather directional; it finds use especially as a rooftop antenna for television reception. On the other hand, a Yagi–Uda antenna (or simply "Yagi"), with a somewhat similar appearance, has only one dipole element with an electrical connection;
5256-455: Is 45 MHz, it is the third harmonic, because 45 - 3 = 42. At 46 MHz, it is the fourth harmonic, and so on. The oscillator does not have to be exactly 45, 46, and so on, only close enough to get through the 42 MHz band-pass filter . Let's say it is 45.1 . Then we get 42.1 from the filter, and 45.1 - 42.1 is still 3. When the high IF is mixed with the 42 MHz, the result is a band of signals from 3 MHz to 2 MHz, from which
5402-413: Is a monopole antenna, not balanced with respect to ground. The ground (or any large conductive surface) plays the role of the second conductor of a monopole. Since monopole antennas rely on a conductive surface, they may be mounted with a ground plane to approximate the effect of being mounted on the Earth's surface. More complex antennas increase the directivity of the antenna. Additional elements in
5548-444: 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
5694-434: Is a change in electrical impedance where the feedline joins the antenna. This leads to the concept of impedance matching , the design of the overall system of antenna and transmission line so the impedance is as close as possible, thereby reducing these losses. Impedance matching is accomplished by a circuit called an antenna tuner or impedance matching network between the transmitter and antenna. The impedance match between
5840-417: Is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it. The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This
5986-490: Is a consequence of the reciprocity theorem of electromagnetics. Therefore, in discussions of antenna properties no distinction is usually made between receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving, whichever is more convenient. A necessary condition for the aforementioned reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal (or bilateral ) means that
Wadley loop - Misplaced Pages Continue
6132-410: Is adjusted according to the receiver tuning. On the other hand, log-periodic antennas are not resonant at any single frequency but can (in principle) be built to attain similar characteristics (including feedpoint impedance) over any frequency range. These are therefore commonly used (in the form of directional log-periodic dipole arrays ) as television antennas. Gain is a parameter which measures
6278-421: Is an electronic device that converts an alternating electric current into radio waves (transmitting), or radio waves into an electric current (receiving). It is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver . In transmission , a radio transmitter supplies an electric current to the antenna's terminals, and
6424-436: 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
6570-400: Is called an isotropic radiator ; however, these cannot exist in practice nor would they be particularly desired. For most terrestrial communications, rather, there is an advantage in reducing radiation toward the sky or ground in favor of horizontal direction(s). A dipole antenna oriented horizontally sends no energy in the direction of the conductor – this is called the antenna null – but
6716-443: 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
6862-475: 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
7008-407: Is connected to a transmission line . The conductor, or element , is aligned with the electrical field of the desired signal, normally meaning it is perpendicular to the line from the antenna to the source (or receiver in the case of a broadcast antenna). The radio signal's electrical component induces a voltage in the conductor. This causes an electrical current to begin flowing in the direction of
7154-413: Is divided into 30 1 MHz bands, which are then translated to a band at 44-45 MHz. To convert 0-1 MHz, the first oscillator must be 45 MHz; to convert 1-2 MHz it must be 46 MHz; and so on. Meanwhile, the first oscillator is also mixed with harmonics from a 1 MHz crystal and the results pass through a 42 MHz filter. Only one harmonic gets through. When the first oscillator
7300-438: Is equal to 1. Therefore, the effective area A eff in terms of the gain G in a given direction is given by: For an antenna with an efficiency of less than 100%, both the effective area and gain are reduced by that same amount. Therefore, the above relationship between gain and effective area still holds. These are thus two different ways of expressing the same quantity. A eff is especially convenient when computing
7446-428: 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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#17330854567497592-465: Is its radiation pattern . The frequency range or bandwidth over which an antenna functions well can be very wide (as in a log-periodic antenna) or narrow (as in a small loop antenna); outside this range the antenna impedance becomes a poor match to the transmission line and transmitter (or receiver). Use of the antenna well away from its design frequency affects its radiation pattern , reducing its directive gain. Generally an antenna will not have
7738-537: 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
7884-434: Is redirected toward the desired direction, increasing the antenna's gain by a factor of at least 2. Likewise, a corner reflector can insure that all of the antenna's power is concentrated in only one quadrant of space (or less) with a consequent increase in gain. Practically speaking, the reflector need not be a solid metal sheet, but can consist of a curtain of rods aligned with the antenna's polarization; this greatly reduces
8030-411: 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
8176-445: 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
8322-418: Is the transmission line , or feed line , which connects the antenna to a transmitter or receiver. The " antenna feed " may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called "aperture antenna", such as a horn or parabolic dish, the "feed" may also refer to a basic radiating antenna embedded in
8468-410: Is the ideal situation, because it produces the maximum output for the minimum input, producing the highest possible efficiency. Contrary to an ideal (lossless) series-resonant circuit, a finite resistance remains (corresponding to the relatively small voltage at the feed-point) due to the antenna's resistance to radiating , as well as any conventional electrical losses from producing heat. Recall that
8614-446: 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
8760-416: Is the radio equivalent of an optical lens . An antenna coupling network is a passive network (generally a combination of inductive and capacitive circuit elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be used to minimize losses on the feed line, by reducing transmission line's standing wave ratio , and to present the transmitter or receiver with
8906-440: Is unidirectional, designed for maximum response in the direction of the other station, whereas many other antennas are intended to accommodate stations in various directions but are not truly omnidirectional. Since antennas obey reciprocity the same radiation pattern applies to transmission as well as reception of radio waves. A hypothetical antenna that radiates equally in all directions (vertical as well as all horizontal angles)
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#17330854567499052-449: Is usable in most other directions. A number of such dipole elements can be combined into an antenna array such as the Yagi–Uda in order to favor a single horizontal direction, thus termed a beam antenna. The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals. The vertical antenna
9198-448: The 1 / 4 or 1 / 2 wave , respectively, at which they are resonant. As these antennas are made shorter (for a given frequency) their impedance becomes dominated by a series capacitive (negative) reactance; by adding an appropriate size " loading coil " – a series inductance with equal and opposite (positive) reactance – the antenna's capacitive reactance may be cancelled leaving only
9344-413: 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
9490-405: The lens antenna . The antenna's power gain (or simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit. Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. A plot of the directional characteristics in the space surrounding the antenna
9636-460: 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
9782-403: The resonance principle. This relies on the behaviour of moving electrons, which reflect off surfaces where the dielectric constant changes, in a fashion similar to the way light reflects when optical properties change. In these designs, the reflective surface is created by the end of a conductor, normally a thin metal wire or rod, which in the simplest case has a feed point at one end where it
9928-418: 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
10074-416: The Wadley Loop in their design. An optical implementation of a Wadley Loop has recently been proposed. This allows a compact relatively unstable laser to be used as a local oscillator. The system's stability being obtained from a master 'comb source' (usually a pulsed laser, such as a mode-locked laser), possibly common to many receivers within an exchange. Radio receiver In radio communications ,
10220-467: The addition of a loading coil. Then it may be said that the coil has lengthened the antenna to achieve an electrical length of 2.5 meters. However, the resulting resistive impedance achieved will be quite a bit lower than that of a true 1 / 4 wave (resonant) monopole, often requiring further impedance matching (a transformer) to the desired transmission line. For ever shorter antennas (requiring greater "electrical lengthening")
10366-409: 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
10512-440: 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
10658-402: The antenna consisting of a thin conductor. Antennas for use over much broader frequency ranges are achieved using further techniques. Adjustment of a matching network can, in principle, allow for any antenna to be matched at any frequency. Thus the small loop antenna built into most AM broadcast (medium wave) receivers has a very narrow bandwidth, but is tuned using a parallel capacitance which
10804-431: 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
10950-412: The antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception , an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified . Antennas are essential components of all radio equipment. An antenna is an array of conductors ( elements ), electrically connected to
11096-400: The antenna structure, which need not be directly connected to the receiver or transmitter, increase its directionality. Antenna "gain" describes the concentration of radiated power into a particular solid angle of space. "Gain" is perhaps an unfortunately chosen term, by comparison with amplifier "gain" which implies a net increase in power. In contrast, for antenna "gain", the power increased in
11242-474: The antenna to the power radiated by a half-wave dipole antenna I dipole {\displaystyle I_{\text{dipole}}} ; these units are called decibels-dipole (dBd) Since the gain of a half-wave dipole is 2.15 dBi and the logarithm of a product is additive, the gain in dBi is just 2.15 decibels greater than the gain in dBd High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully at
11388-427: The broadside direction. If higher gain is needed one cannot simply make the antenna larger. Due to the constraint on the effective area of a receiving antenna detailed below , one sees that for an already-efficient antenna design, the only way to increase gain (effective area) is by reducing the antenna's gain in another direction. If a half-wave dipole is not connected to an external circuit but rather shorted out at
11534-466: The carrier cycles, leaving a pulsing DC current whose amplitude varied with the audio modulation signal. When applied to an earphone this would reproduce the transmitted sound. Below are the detectors that saw wide use before vacuum tubes took over around 1920. All except the magnetic detector could rectify and therefore receive AM signals: Antenna (radio) In radio engineering , an antenna ( American English ) or aerial ( British English )
11680-470: The degree of directivity of the antenna's radiation pattern . A high-gain antenna will radiate most of its power in a particular direction, while a low-gain antenna will radiate over a wide angle. The antenna gain , or power gain of an antenna is defined as the ratio of the intensity (power per unit surface area) I {\displaystyle I} radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by
11826-420: 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
11972-557: The design operating frequency, f o , and antennas are normally designed to be this size. However, feeding that element with 3 f o (whose wavelength is 1 / 3 that of f o ) will also lead to a standing wave pattern. Thus, an antenna element is also resonant when its length is 3 / 4 of a wavelength. This is true for all odd multiples of 1 / 4 wavelength. This allows some flexibility of design in terms of antenna lengths and feed points. Antennas used in such
12118-463: The desired direction is at the expense of power reduced in undesired directions. Unlike amplifiers, antennas are electrically " passive " devices which conserve total power, and there is no increase in total power above that delivered from the power source (the transmitter), only improved distribution of that fixed total. A phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves
12264-424: 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
12410-434: The desired signal is selected. This can potentially be accomplished with a conventional superheterodyne back-end converting 3-2 MHz to 455 kHz and finally demodulating the signal back to audio. The overall receiver drift consists of the crystal's drift plus the 3 MHz back-end. When we're listening to a 30 MHz signal, this receiver is about ten times as stable as one using a high-frequency tunable VFO . To
12556-450: 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
12702-651: 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
12848-405: The electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss . Antennas can be classified as omnidirectional , radiating energy approximately equally in all horizontal directions, or directional , where radio waves are concentrated in some direction(s). A so-called beam antenna
12994-427: The emission of energy from the resonant antenna to free space. The Q of a narrow band antenna can be as high as 15. On the other hand, the reactance at the same off-resonant frequency of one using thick elements is much less, consequently resulting in a Q as low as 5. These two antennas may perform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as
13140-418: The entire system of reflecting elements (normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in place of a (conductive) transmission line . An antenna counterpoise , or ground plane , is a structure of conductive material which improves or substitutes for
13286-526: The feedline and antenna is measured by a parameter called the standing wave ratio (SWR) on the feedline. Consider a half-wave dipole designed to work with signals with wavelength 1 m, meaning the antenna would be approximately 50 cm from tip to tip. If the element has a length-to-diameter ratio of 1000, it will have an inherent impedance of about 63 ohms resistive. Using the appropriate transmission wire or balun, we match that resistance to ensure minimum signal reflection. Feeding that antenna with
13432-430: The feedpoint, then it becomes a resonant half-wave element which efficiently produces a standing wave in response to an impinging radio wave. Because there is no load to absorb that power, it retransmits all of that power, possibly with a phase shift which is critically dependent on the element's exact length. Thus such a conductor can be arranged in order to transmit a second copy of a transmitter's signal in order to affect
13578-485: 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
13724-454: The first oscillator is cancelled out, it cannot be used to tune a particular signal. Instead, it selects an entire band of signals - which one depends on which harmonic was chosen in part 3 above. The size of the band is equal to the spacing of the crystal harmonics. A conventionally tuned "back end" selects the desired signal from the band of signals presented at the second IF. An example would be picking up signals from 0 to 30 MHz. This
13870-438: The focal point of parabolic reflectors for both transmitting and receiving. Starting in 1895, Guglielmo Marconi began development of antennas practical for long-distance, wireless telegraphy, for which he received the 1909 Nobel Prize in physics . The words antenna and aerial are used interchangeably. Occasionally the equivalent term "aerial" is used to specifically mean an elevated horizontal wire antenna. The origin of
14016-476: 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
14162-446: The gain of an antenna used for transmitting must be proportional to its effective area when used for receiving. Consider an antenna with no loss , that is, one whose electrical efficiency is 100%. It can be shown that its effective area averaged over all directions must be equal to λ /4π , the wavelength squared divided by 4π . Gain is defined such that the average gain over all directions for an antenna with 100% electrical efficiency
14308-439: The geometrical divergence of the transmitted wave. For a given incoming flux, the power acquired by a receiving antenna is proportional to its effective area . This parameter compares the amount of power captured by a receiving antenna in comparison to the flux of an incoming wave (measured in terms of the signal's power density in watts per square metre). A half-wave dipole has an effective area of about 0.13 λ seen from
14454-474: The ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this aids in the function of the natural ground, particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable . An electromagnetic wave refractor in some aperture antennas
14600-425: The high-IF of part 1 drifts in the same direction and the same amount as the "synthetic oscillator" of part 3, when they are mixed in part 4, the drift terms cancel out and the result is a crystal-stable signal at a second intermediate frequency. However, the drift makes it impossible to use high-IF selectivity to reject undesired signals. Instead, the high IF is designed with a band-pass characteristic. Also, since
14746-428: 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
14892-413: The increase in signal power due to an amplifying device placed at the front-end of the system, such as a low-noise amplifier . The effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which the antenna delivers to its terminals, expressed in terms of an equivalent area. For instance, if a radio wave passing a given location has
15038-405: The intensity I iso {\displaystyle I_{\text{iso}}} radiated at the same distance by a hypothetical isotropic antenna which radiates equal power in all directions. This dimensionless ratio is usually expressed logarithmically in decibels , these units are called decibels-isotropic (dBi) A second unit used to measure gain is the ratio of the power radiated by
15184-417: 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
15330-417: The loading coil, relative to the decreased radiation resistance, entail a reduced electrical efficiency , which can be of great concern for a transmitting antenna, but bandwidth is the major factor that sets the size of antennas at 1 MHz and lower frequencies. The radiant flux as a function of the distance from the transmitting antenna varies according to the inverse-square law , since that describes
15476-435: The log-periodic principle it obtains the unique property of maintaining its performance characteristics (gain and impedance) over a very large bandwidth. When a radio wave hits a large conducting sheet it is reflected (with the phase of the electric field reversed) just as a mirror reflects light. Placing such a reflector behind an otherwise non-directional antenna will insure that the power that would have gone in its direction
15622-553: The material has the same response to an electric current or magnetic field in one direction, as it has to the field or current in the opposite direction. Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators , made of nonreciprocal materials such as ferrite . These can be used to give the antenna a different behavior on receiving than it has on transmitting, which can be useful in applications like radar . The majority of antenna designs are based on
15768-653: The other parasitic elements interact with the electromagnetic field in order to realize a highly directional antenna but with a narrow bandwidth. Even greater directionality can be obtained using aperture antennas such as the parabolic reflector or horn antenna . Since high directivity in an antenna depends on it being large compared to the wavelength, highly directional antennas (thus with high antenna gain ) become more practical at higher frequencies ( UHF and above). At low frequencies (such as AM broadcast ), arrays of vertical towers are used to achieve directionality and they will occupy large areas of land. For reception,
15914-409: The other antenna. An example of a high-gain antenna is a parabolic dish such as a satellite television antenna. Low-gain antennas have shorter range, but the orientation of the antenna is relatively unimportant. An example of a low-gain antenna is the whip antenna found on portable radios and cordless phones . Antenna gain should not be confused with amplifier gain , a separate parameter measuring
16060-464: The other side connected to ground or an equivalent ground plane (or counterpoise ). Monopoles, which are one-half the size of a dipole, are common for long-wavelength radio signals where a dipole would be impractically large. Another common design is the folded dipole which consists of two (or more) half-wave dipoles placed side by side and connected at their ends but only one of which is driven. The standing wave forms with this desired pattern at
16206-401: The others present a high impedance. Another solution uses traps , parallel resonant circuits which are strategically placed in breaks created in long antenna elements. When used at the trap's particular resonant frequency the trap presents a very high impedance (parallel resonance) effectively truncating the element at the location of the trap; if positioned correctly, the truncated element makes
16352-461: The phases applied to each element the radiation pattern can be shifted without physically moving the antenna elements. Another common array antenna is the log-periodic dipole array which has an appearance similar to the Yagi (with a number of parallel elements along a boom) but is totally dissimilar in operation as all elements are connected electrically to the adjacent element with a phase reversal; using
16498-488: The power that would be received by an antenna of a specified gain, as illustrated by the above example. The radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna at different angles in the far field. It is typically represented by a three-dimensional graph, or polar plots of the horizontal and vertical cross sections. The pattern of an ideal isotropic antenna , which radiates equally in all directions, would look like
16644-444: The radiation pattern (and feedpoint impedance) of the element electrically connected to the transmitter. Antenna elements used in this way are known as passive radiators . A Yagi–Uda array uses passive elements to greatly increase gain in one direction (at the expense of other directions). A number of parallel approximately half-wave elements (of very specific lengths) are situated parallel to each other, at specific positions, along
16790-421: The radiation resistance plummets (approximately according to the square of the antenna length), so that the mismatch due to a net reactance away from the electrical resonance worsens. Or one could as well say that the equivalent resonant circuit of the antenna system has a higher Q factor and thus a reduced bandwidth, which can even become inadequate for the transmitted signal's spectrum. Resistive losses due to
16936-416: 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
17082-410: 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
17228-401: 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
17374-565: 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 ,
17520-480: The radio waves into a beam or other desired radiation pattern . Strong directivity and good efficiency when transmitting are hard to achieve with antennas with dimensions that are much smaller than a half wavelength . The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the 1867 electromagnetic theory of James Clerk Maxwell . Hertz placed dipole antennas at
17666-445: 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
17812-420: 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
17958-466: 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
18104-431: 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
18250-399: The receiver or transmitter. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally ( omnidirectional antennas ), or preferentially in a particular direction ( directional , or high-gain, or "beam" antennas). An antenna may include components not connected to the transmitter, parabolic reflectors , horns , or parasitic elements , which serve to direct
18396-441: The reflector's weight and wind load . Specular reflection of radio waves is also employed in a parabolic reflector antenna, in which a curved reflecting surface effects focussing of an incoming wave toward a so-called feed antenna ; this results in an antenna system with an effective area comparable to the size of the reflector itself. Other concepts from geometrical optics are also employed in antenna technology, such as with
18542-458: The same axis (or collinear ), each feeding one side of a two-conductor transmission wire. The physical arrangement of the two elements places them 180 degrees out of phase, which means that at any given instant one of the elements is driving current into the transmission line while the other is pulling it out. The monopole antenna is essentially one half of the half-wave dipole, a single 1 / 4 wavelength element with
18688-467: 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
18834-425: The same whether the antenna is transmitting or receiving . For example, the "receiving pattern" (sensitivity to incoming signals as a function of direction) of an antenna when used for reception is identical to the radiation pattern of the antenna when it is driven and functions as a radiator, even though the current and voltage distributions on the antenna itself are different for receiving and sending. This
18980-432: The same. Electrically this appears to be a very high impedance. The antenna and transmission line no longer have the same impedance, and the signal will be reflected back into the antenna, reducing output. This could be addressed by changing the matching system between the antenna and transmission line, but that solution only works well at the new design frequency. The result is that the resonant antenna will efficiently feed
19126-469: The signal's instantaneous field. When the resulting current reaches the end of the conductor, it reflects, which is equivalent to a 180 degree change in phase. If the conductor is 1 / 4 of a wavelength long, current from the feed point will undergo 90 degree phase change by the time it reaches the end of the conductor, reflect through 180 degrees, and then another 90 degrees as it travels back. That means it has undergone
19272-420: 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
19418-402: 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
19564-471: 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
19710-683: 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
19856-404: 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
20002-427: 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
20148-464: 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
20294-605: 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
20440-560: The word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi . In the summer of 1895, Marconi began testing his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials" suspended from a pole. In Italian a tent pole is known as l'antenna centrale , and the pole with the wire was simply called l'antenna . Until then wireless radiating transmitting and receiving elements were known simply as "terminals". Because of his prominence, Marconi's use of
20586-548: The word antenna spread among wireless researchers and enthusiasts, and later to the general public. Antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etc., in addition to the actual RF current-carrying components. A receiving antenna may include not only the passive metal receiving elements, but also an integrated preamplifier or mixer , especially at and above microwave frequencies. Antennas are required by any radio receiver or transmitter to couple its electrical connection to
20732-415: 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
20878-418: 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
21024-406: 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
21170-415: 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
21316-496: 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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