Helical antenna
A helical antenna is an antenna consisting of one or more conducting wires wound in the form of a helix. In most cases, directional helical antennas are mounted over a ground plane, while omnidirectional designs may not be; the feed line is connected between the bottom of the ground plane. Helical antennas can operate in one of two principal modes -- axial mode. In the normal mode or broadside helical antenna, the diameter and the pitch of the aerial are small compared with the wavelength; the antenna acts to an electrically short dipole or monopole, equivalent to a 1/4 wave vertical and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. For monofilar designs the radiation is linearly polarized parallel to the helix axis; these are used for compact antennas for portable hand held as well as mobile vehicle mount two-way radios, in larger scale for UHF television broadcasting antennas. In bifilar or quadrifilar implementations, broadside circularly polarized.
In the axial mode or end-fire helical antenna, the diameter and pitch of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis, it radiates circularly polarized radio waves. These are used for satellite communication. Axial mode operation was discovered by physicist John D. Kraus If the circumference of the helix is less than a wavelength and its pitch is less than a quarter wavelength, the antenna is called a normal-mode helix; the antenna acts similar to a monopole antenna, with an omnidirectional radiation pattern, radiating equal power in all directions perpendicular to the antenna's axis. However, because of the inductance added by the helical shape, the antenna acts like a inductively loaded monopole. Therefore, normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big.
As with other electrically short antennas, the gain, thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes "helicals" useful as antennas for mobile and portable communications equipment on the HF, VHF, UHF bands; the loading provided by the helix allows the antenna to be physically shorter than its electrical length of a quarter-wavelength. This means that for example a 1/4 wave antenna at 27MHz is 2.7 m long and is physicality quite unsuitable for mobile applications. The reduced size of a helical provides the same radiation pattern in a much more compact physical size with only a slight reduction in signal performance. An effect of using a helical conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohms to between 25 and 35 ohms base impedance; this does not seem to be adverse to operation or matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wavelength at the frequency of operation.
Another example of the type as used in mobile communications is "spaced constant turn" in which one or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance and inductance for the radiating element at a particular resonant frequency. Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s. To date many millions of these ‘helical antennas’ have been mass-produced for mobile vehicle use and reached peak production during the CB Radio boom-times during the 1970s to late 1980s and used worldwide. Multi-frequency versions with manual plug-in taps have become the mainstay for multi-band single-sideband modulation HF communications with frequency coverage over the whole HF spectrum from 1mHz to 30 MHz with from 2 to 6 dedicated frequency tap points tuned at dedicated and allocated frequencies in the land mobile and aircraft bands.
These antennas have been superseded by electronicly tuned antenna matching devices. Most examples were wound with copper wire using a fiberglass rod as a former; the flexible or ridged radiator is covered with a PVC or polyolefin heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna. The fibreglass rod was usually glued and/or crimped to a brass fitting and screw mounted onto an insulated base affixed to a vehicle roof, guard or bull-bar mount; this mounting provided a ground reflector for an effective vertical radiation pattern. These popular designs are still in common use as of 2018 and the ‘constant turn’ design originating in Australia have been universally adapted as standard FM receiving antennas for many factory produced motor vehicles as well as the existing basic style of aftermarket HF and VHF mobile helical. Another common use for broadside helixes is in the "rubber ducky antenna" found on most portable VHF and UHF radios using a steel or copper conductor as the radiating element and terminated to a BNC / TNC style or screw on connector for quick removal.
Specialized enlarged normal-mode helical antennas are used for Base Station transmitters for FM radio and television broadcasting stations on the VHF and UHF bands. When the helix circumference is near the wavelength of operation, the antenna operates in axial mode; this is a nonresonant t
Omnidirectional antenna
In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis, with power varying with angle to the axis, declining to zero on the axis. When graphed in three dimensions this radiation pattern is described as doughnut-shaped. Note that this is different from an isotropic antenna, which radiates equal power in all directions, having a spherical radiation pattern. Omnidirectional antennas oriented vertically are used for nondirectional antennas on the surface of the Earth because they radiate in all horizontal directions, while the power radiated drops off with elevation angle so little radio energy is aimed into the sky or down toward the earth and wasted. Omnidirectional antennas are used for radio broadcasting antennas, in mobile devices that use radio such as cell phones, FM radios, walkie-talkies, wireless computer networks, cordless phones, GPS, as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications.
Common types of low-gain omnidirectional antennas are the whip antenna, "Rubber Ducky" antenna, ground plane antenna, vertically oriented dipole antenna, discone antenna, mast radiator, horizontal loop antenna and the halo antenna. Higher-gain omnidirectional antennas can be built. "Higher gain" in this case means that the antenna radiates less energy at higher and lower elevation angles and more in the horizontal directions. High-gain omnidirectional antennas are realized using collinear dipole arrays; these consist of multiple half-wave dipoles mounted collinearly. The coaxial collinear antenna uses transposed coaxial sections to produce in-phase half-wavelength radiators. A Franklin Array uses short U-shaped half-wavelength sections whose radiation cancels in the far-field to bring each half-wavelength dipole section into equal phase. Another type is the Omnidirectional Microstrip Antenna. Omnidirectional radiation patterns are produced by the simplest practical antennas and dipole antennas, consisting of one or two straight rod conductors on a common axis.
Antenna gain is defined as antenna efficiency multiplied by antenna directivity, expressed mathematically as: G = e D. A useful relationship between omnidirectional radiation pattern directivity in decibels and half-power beamwidth based on the assumption of a sin / b θ pattern shape is: D ≈ 10 log 10 dB. Choke ring antenna Directional antenna
Decibel
The decibel is a unit of measurement used to express the ratio of one value of a power or field quantity to another on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively. It can be used to express a change in an absolute value. In the latter case, it expresses the ratio of a value to a fixed reference value. For example, if the reference value is 1 volt the suffix is "V", if the reference value is one milliwatt the suffix is "m". Two different scales are used when expressing a ratio in decibels, depending on the nature of the quantities: power and field; when expressing a power ratio, the number of decibels is ten times its logarithm to base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level; when expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two so that the related power and field levels change by the same number of decibels in, for example, resistive loads.
The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell. Today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics and control theory. In electronics, the gains of amplifiers, attenuation of signals, signal-to-noise ratios are expressed in decibels. In the International System of Quantities, the decibel is defined as a unit of measurement for quantities of type level or level difference, which are defined as the logarithm of the ratio of power- or field-type quantities; the decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was Miles of Standard Cable. 1 MSC corresponded to the loss of power over a 1 mile length of standard telephone cable at a frequency of 5000 radians per second, matched the smallest attenuation detectable to the average listener.
The standard telephone cable implied was "a cable having uniformly distributed resistance of 88 Ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile". In 1924, Bell Telephone Laboratories received favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the Transmission Unit. 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. The definition was conveniently chosen such that 1 TU approximated 1 MSC. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio, it was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell. The bel is used, as the decibel was the proposed working unit; the naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: Since the earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized.
The introduction of cable in 1896 afforded a stable basis for a convenient unit and the "mile of standard" cable came into general use shortly thereafter. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work; the new transmission unit is used among the foreign telephone organizations and it was termed the "decibel" at the suggestion of the International Advisory Committee on Long Distance Telephony. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 100.1 and any two amounts of power differ by N decibels when they are in the ratio of 10N. The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio; this method of designating the gain or loss of power in telephone circuits permits direct addition or subtraction of the units expressing the efficiency of different parts of the circuit... In 1954, J. W. Horton argued that the use of the decibel as a unit for quantities other than transmission loss led to confusion, suggested the name'logit' for "standard magnitudes which combine by addition".
In April 2003, the International Committee for Weights and Measures considered a recommendation for the inclusion of the decibel in the International System of Units, but decided against the proposal. However, the decibel is recognized by other international bodies such as the International Electrotechnical Commission and International Organization for Standardization; the IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios. The term field quantity is deprecated by ISO 80000-1. In spite of their widespread use, suffixes are not recognized by the IEC or ISO. ISO 80000-3 describes definitions for units of space and time; the decibel for use in acoustics is defined in ISO 80000-8. The major difference from the article below is that for acoustics the decibel has no
Frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency; the period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals, radio waves, light. For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics and radio, frequency is denoted by a Latin letter f or by the Greek letter ν or ν; the relation between the frequency and the period T of a repeating event or oscillation is given by f = 1 T.
The SI derived unit of frequency is the hertz, named after the German physicist Heinrich Hertz. One hertz means. If a TV has a refresh rate of 1 hertz the TV's screen will change its picture once a second. A previous name for this unit was cycles per second; the SI unit for period is the second. A traditional unit of measure used with rotating mechanical devices is revolutions per minute, abbreviated r/min or rpm. 60 rpm equals one hertz. As a matter of convenience and slower waves, such as ocean surface waves, tend to be described by wave period rather than frequency. Short and fast waves, like audio and radio, are described by their frequency instead of period; these used conversions are listed below: Angular frequency denoted by the Greek letter ω, is defined as the rate of change of angular displacement, θ, or the rate of change of the phase of a sinusoidal waveform, or as the rate of change of the argument to the sine function: y = sin = sin = sin d θ d t = ω = 2 π f Angular frequency is measured in radians per second but, for discrete-time signals, can be expressed as radians per sampling interval, a dimensionless quantity.
Angular frequency is larger than regular frequency by a factor of 2π. Spatial frequency is analogous to temporal frequency, but the time axis is replaced by one or more spatial displacement axes. E.g.: y = sin = sin d θ d x = k Wavenumber, k, is the spatial frequency analogue of angular temporal frequency and is measured in radians per meter. In the case of more than one spatial dimension, wavenumber is a vector quantity. For periodic waves in nondispersive media, frequency has an inverse relationship to the wavelength, λ. In dispersive media, the frequency f of a sinusoidal wave is equal to the phase velocity v of the wave divided by the wavelength λ of the wave: f = v λ. In the special case of electromagnetic waves moving through a vacuum v = c, where c is the speed of light in a vacuum, this expression becomes: f = c λ; when waves from a monochrome source travel from one medium to another, their frequency remains the same—only their wavelength and speed change. Measurement of frequency can done in the following ways, Calculating the frequency of a repeating event is accomplished by counting the number of times that event occurs within a specific time period dividing the count by the length of the time period.
For example, if 71 events occur within 15 seconds the frequency is: f = 71 15 s ≈ 4.73 Hz If the number of counts is not large, it is more accurate to measure the time interval for a predetermined number of occurrences, rather than the number of occurrences within a specified time. The latter method introduces a random error into the count of between zero and one count, so on average half a count; this is called gating error and causes an average error in the calculated frequency of Δ f = 1 2 T
Ultra high frequency
Ultra high frequency is the ITU designation for radio frequencies in the range between 300 megahertz and 3 gigahertz known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency or microwave frequency range. Lower frequency signals fall into lower bands. UHF radio waves propagate by line of sight, they are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, numerous other applications. The IEEE defines the UHF radar band as frequencies between 1 GHz. Two other IEEE radar bands overlap the ITU UHF band: the L band between 1 and 2 GHz and the S band between 2 and 4 GHz. Radio waves in the UHF band travel entirely by line-of-sight propagation and ground reflection. UHF radio waves are blocked by hills and cannot travel far beyond the horizon, but can penetrate foliage and buildings for indoor reception.
Since the wavelengths of UHF waves are comparable to the size of buildings, trees and other common objects and diffraction from these objects can cause fading due to multipath propagation in built-up urban areas. Atmospheric moisture reduces, or attenuates, the strength of UHF signals over long distances, the attenuation increases with frequency. UHF TV signals are more degraded by moisture than lower bands, such as VHF TV signals. Since UHF transmission is limited by the visual horizon to 30–40 miles and to shorter distances by local terrain, it allows the same frequency channels to be reused by other users in neighboring geographic areas. Public safety, business communications and personal radio services such as GMRS, PMR446, UHF CB are found on UHF frequencies as well as IEEE 802.11 wireless LANs. The adopted GSM and UMTS cellular networks use UHF cellular frequencies. Radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required; when conditions are right, UHF radio waves can travel long distances by tropospheric ducting as the atmosphere warms and cools throughout the day.
The length of an antenna is related to the length of the radio waves used. Due to the short wavelengths, UHF antennas are conveniently short. UHF wavelengths are short enough that efficient transmitting antennas are small enough to mount on handheld and mobile devices, so these frequencies are used for two way land mobile radio systems, such as walkie-talkies, two way radios in vehicles, for portable wireless devices. Omnidirectional UHF antennas used on mobile devices are short whips, sleeve dipoles, rubber ducky antennas or the planar inverted F antenna used in cellphones. Higher gain omnidirectional UHF antennas can be made of collinear arrays of dipoles and are used for mobile base stations and cellular base station antennas; the short wavelengths allow high gain antennas to be conveniently small. High gain antennas for point-to-point communication links and UHF television reception are Yagi, log periodic, corner reflectors, or reflective array antennas. At the top end of the band slot antennas and parabolic dishes become practical.
For satellite communication and turnstile antennas are used since satellites employ circular polarization, not sensitive to the relative orientation of the transmitting and receiving antennas. For television broadcasting specialized vertical radiators that are modifications of the slot antenna or reflective array antenna are used: the slotted cylinder, zig-zag, panel antennas. UHF television broadcasting fulfilled the demand for additional over-the-air television channels in urban areas. Today, much of the bandwidth has been reallocated to land mobile, trunked radio and mobile telephone use. UHF channels are still used for digital television. UHF spectrum is used worldwide for land mobile radio systems for commercial, public safety, military purposes. Many personal radio services use frequencies allocated in the UHF band, although exact frequencies in use differ between countries. Major telecommunications providers have deployed voice and data cellular networks in UHF/VHF range; this allows mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet.
UHF radars are said to be effective at tracking stealth fighters, if not stealth bombers. UHF citizens band: 476–477 MHz Television broadcasting uses UHF channels between 503 and 694 MHz Fixed point-to-point Link 450.4875 - 451.5125 MHz Land mobile service 457.50625 - 459.9875 MHz Mobile satellite service: 406.0000 - 406.1000 MHz Segment and Service examples: Land mobile for private, Australian and Territory Government, Rail industry and Mobile-Satellite 430–450 MHz: Amateur radio 470–806 MHz: Terrestrial television 1452–1492 MHz: Digital Audio Broadcasting Many other frequency assignments for Canada and Mexico are similar to their US counterparts 380–399.9 MHz: Terrestrial Trunked Radio service for emergency use 430–440 MHz: Amateur ra
Whip antenna
A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to transmitter; the antenna is designed to be flexible so that it does not break and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer ones, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 35 ft long; the length of the whip antenna is determined by the wavelength of the radio waves it is used with. The most common type is the quarter-wave whip, one-quarter of a wavelength long. Whips are the most common type of monopole antenna, are used in the higher frequency HF, VHF and UHF radio bands, they are used as the antennas for hand-held radios, cordless phones, walkie-talkies, FM radios, boom boxes, Wi-Fi enabled devices, are attached to vehicles as the antennas for car radios and two-way radios for wheeled vehicles and for aircraft.
Larger versions mounted on roofs and radio masts are used as base station antennas for police, ambulance and other vehicle dispatchers. The whip antenna is a monopole antenna, like a vertical dipole has an omnidirectional radiation pattern, radiating equal radio power in all azimuthal directions, with the radiated power falling off with elevation angle to zero on the antenna's axis. Whip antennas 1/4 wavelength long or less have a single main lobe, with field strength maximum in horizontal directions, falling monotonically to zero on the axis. Antennas longer than a quarter wavelength have patterns consisting of several conical "lobes". Vertical whip antennas are used for nondirectional radio communication on the surface of the Earth, where the direction to the transmitter is unknown or changing, for example in portable FM radio receivers, walkie-talkies, two-way radios in vehicles; this is because they transmit well in all horizontal directions, while radiating little radio energy up into the sky where it is wasted.
Whip antennas are designed as resonant antennas. Therefore, the length of the antenna rod is determined by the wavelength of the radio waves used; the most common length is one-quarter of the wavelength, called a "quarter-wave whip". For example, the common quarter-wave whip antennas used on FM radios in the USA are 75 cm long, one-quarter the length of radio waves in the FM radio band, which are 2.78 to 3.41 meters long. Half-wave antennas are common. A quarter wave vertical antenna working against a perfect infinite ground will have a gain of 5.19 dBi and about 36.8 ohms of radiation resistance. Whips mounted on vehicles use the metal skin of the vehicle as a ground plane. In hand-held devices no explicit ground plane is provided, the ground side of the antenna's feed line is just connected to the ground on the device's circuit board. Therefore, the radio itself, the user's hand, serves as a rudimentary ground plane. Since these are no larger than the size of the antenna itself, the combination of whip and radio functions more as an asymmetrical dipole antenna than as a monopole antenna.
The gain will suffer somewhat compared to a half wave metallic diople or a whip with a well defined ground plane. With stationary whips mounted on structures, an artificial "ground plane" consisting of three or four rods a quarter-wavelength long extending horizontally from the base of the whip is used; this provides a stable input impedance and pattern by helping prevent RF currents in the supporting mast and along the outside of the feed line. This type of antenna is called a ground plane antenna; the ground plane rods are sloped downward toward the ground, which lowers the main lobe of the radiation pattern and increases the normal 36.8 ohm radiation resistance closer to 50 ohms to provide a better impedance match with standard 50 ohm coaxial cable feedline. To reduce the length of a whip antenna to make it less cumbersome, an inductor is added in series with it; this allows the antenna to be made much shorter than the normal length of a quarter-wavelength, still be resonant, by cancelling out the capacitive reactance of the short antenna.
The coil is added at the base of the whip or in the middle. In the most used form, the rubber ducky antenna, the loading coil is integrated with the antenna itself by making the whip out of a narrow helix of springy wire; the helix distributes the inductance along the antenna's length, improving the radiation pattern, makes it more flexible. Another alternative used to shorten the antenna is to add a "capacity hat", a metal screen or radiating wires, at the end; however all these electrically short whips have lower gain than a full length quarter-wave whip. Multi-band operation is possible with coils at about one-half or one-third and two-thirds that do not affect the aerial much at the lowest band, but it creates the effect of stacked dipoles at a higher band. At higher frequencies (2.4 GHz, but military whips for 50 MHz to 80 MHz band exist
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either mechanical. Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators. Another example is quartz crystals used in electronic devices such as radio transmitters and quartz watches to produce oscillations of precise frequency. A cavity resonator is one. In electronics and radio, microwave cavities consisting of hollow metal boxes are used in microwave transmitters and test equipment to control frequency, in place of the tuned circuits which are used at lower frequencies. Acoustic cavity resonators, in which sound is produced by air vibrating in a cavity with one opening, are known as Helmholtz resonators. A physical system can have as many resonant frequencies.
Systems with one degree of freedom, such as a mass on a spring, balance wheels, LC tuned circuits have one resonant frequency. Systems with two degrees of freedom, such as coupled pendulums and resonant transformers can have two resonant frequencies. A crystal lattice composed of N atoms bound together can have N resonant frequencies; as the number of coupled harmonic oscillators grows, the time it takes to transfer energy from one to the next becomes significant. The vibrations in them begin to travel through the coupled harmonic oscillators in waves, from one oscillator to the next; the term resonator is most used for a homogeneous object in which vibrations travel as waves, at an constant velocity, bouncing back and forth between the sides of the resonator. The material of the resonator, through which the waves flow, can be viewed as being made of millions of coupled moving parts. Therefore, they can have millions of resonant frequencies, although only a few may be used in practical resonators.
The oppositely moving waves interfere with each other, at its resonant frequencies reinforce each other to create a pattern of standing waves in the resonator. If the distance between the sides is d, the length of a round trip is 2 d. To cause resonance, the phase of a sinusoidal wave after a round trip must be equal to the initial phase so the waves self-reinforce; the condition for resonance in a resonator is that the round trip distance, 2 d, is equal to an integer number of wavelengths λ of the wave: 2 d = N λ, N ∈ If the velocity of a wave is c, the frequency is f = c / λ so the resonant frequencies are: f = N c 2 d N ∈ So the resonant frequencies of resonators, called normal modes, are spaced multiples of a lowest frequency called the fundamental frequency. The above analysis assumes the medium inside the resonator is homogeneous, so the waves travel at a constant speed, that the shape of the resonator is rectilinear. If the resonator is inhomogeneous or has a nonrectilinear shape, like a circular drumhead or a cylindrical microwave cavity, the resonant frequencies may not occur at spaced multiples of the fundamental frequency.
They are called overtones instead of harmonics. There may be several such series of resonant frequencies in a single resonator, corresponding to different modes of vibration. An electrical circuit composed of discrete components can act as a resonator when both an inductor and capacitor are included. Oscillations are limited by the inclusion of resistance, either via a specific resistor component, or due to resistance of the inductor windings; such resonant circuits are called RLC circuits after the circuit symbols for the components. A distributed-parameter resonator has capacitance and resistance that cannot be isolated into separate lumped capacitors, inductors, or resistors. An example of this, much used in filtering, is the helical resonator. A single layer coil, used as a secondary or tertiary winding in a Tesla coil or magnifying transmitter is a distributed resonator. A cavity resonator is a hollow closed conductor such as a metal box or a cavity within a metal block, containing electromagnetic waves reflecting back and forth between the cavity's walls.
When a source of radio waves at one of the cavity's resonant frequencies is applied, the oppositely-moving waves form standing waves, the cavity stores electromagnetic energy. Since the cavity's lowest resonant frequency, the fundamental frequency, is that at which the width of the cavity is equal to a half-wavelength, cavity resonators are only used at microwave frequencies and above, where wavelengths are short enough that the cavity is conveniently small in size. Due to the low resistance of their conductive walls, cavity resonators have very
