Horn antenna

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Pyramidal microwave horn antenna, with a bandwidth of 0.8 to 18 GHz. A coaxial cable feedline attaches to the connector visible at top. This type is called a ridged horn; the curving fins visible inside the mouth of the horn increase the antenna's bandwidth. Schwarzbeck BBHA 9120 D.jpg
Pyramidal microwave horn antenna, with a bandwidth of 0.8 to 18 GHz. A coaxial cable feedline attaches to the connector visible at top. This type is called a ridged horn; the curving fins visible inside the mouth of the horn increase the antenna's bandwidth.
The first modern horn antenna in 1938 with inventor Wilmer L. Barrow. Wilmer Barrow & horn antenna 1938.jpg
The first modern horn antenna in 1938 with inventor Wilmer L. Barrow.

A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. [1] They are used as feed antennas (called feed horns) for larger antenna structures such as parabolic antennas, as standard calibration antennas to measure the gain of other antennas, and as directive antennas for such devices as radar guns, automatic door openers, and microwave radiometers. [2] Their advantages are moderate directivity, low standing wave ratio (SWR), broad bandwidth, and simple construction and adjustment. [3]

Antenna (radio) electrical device which converts electric power into radio waves, and vice versa

In radio engineering, an antenna 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 the antenna radiates the energy from the current as electromagnetic 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.

Waveguide structure that guides waves, typically electromagnetic waves

A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting expansion to one dimension or two. There is a similar effect in water waves constrained within a canal, or guns that have barrels which restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of a waveguide, wave amplitudes decrease according to the inverse square law as they expand into three dimensional space.

An acoustic horn or waveguide is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air. Conversely, a horn can be used at the receiving end to optimize the transfer of sound from the air to a receiver.

Contents

One of the first horn antennas was constructed in 1897 by Bengali-Indian radio researcher Jagadish Chandra Bose in his pioneering experiments with microwaves. [4] [5] The modern horn antenna was invented independently in 1938 by Wilmer Barrow and G. C. Southworth [6] [7] [8] [9] The development of radar in World War 2 stimulated horn research to design feed horns for radar antennas. The corrugated horn invented by Kay in 1962 has become widely used as a feed horn for microwave antennas such as satellite dishes and radio telescopes. [9]

Jagadish Chandra Bose Bengali polymath, physicist, biologist, botanist and archaeologist

Sir Jagadish Chandra Bose (;, IPA: [dʒɔɡodiʃ tʃɔndro bosu]; 30 November 1858 – 23 November 1937), also spelled Jagdish and Jagadis, was a polymath, physicist, biologist, biophysicist, botanist and archaeologist, and an early writer of science fiction. He pioneered the investigation of radio and microwave optics, made significant contributions to plant science, and laid the foundations of experimental science in the Indian subcontinent. IEEE named him one of the fathers of radio science. Bose is considered the father of Bengali science fiction, and also invented the crescograph, a device for measuring the growth of plants. A crater on the moon has been named in his honour.

Radar object detection system based on radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

Satellite dish antenna for TV and radio reception

A satellite dish is a dish-shaped type of parabolic antenna designed to receive or transmit information by radio waves to or from a communication satellite. The term most commonly means a dish used by consumers to receive direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.

An advantage of horn antennas is that since they have no resonant elements, they can operate over a wide range of frequencies, a wide bandwidth. The usable bandwidth of horn antennas is typically of the order of 10:1, and can be up to 20:1 (for example allowing it to operate from 1 GHz to 20 GHz). [1] The input impedance is slowly varying over this wide frequency range, allowing low voltage standing wave ratio (VSWR) over the bandwidth. [1] The gain of horn antennas ranges up to 25 dBi, with 10 - 20 dBi being typical. [1]

Frequency is the number of occurrences of a repeating event per unit of time. It is also 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 (sound), radio waves, and light.

Bandwidth (signal processing) difference between the upper and lower frequencies in a continuous set of frequencies

Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz, and depending on context, may specifically refer to passband bandwidth or baseband bandwidth. Passband bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a band-pass filter, a communication channel, or a signal spectrum. Baseband bandwidth applies to a low-pass filter or baseband signal; the bandwidth is equal to its upper cutoff frequency.

In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line.

Description

Pyramidal horn antennas for a variety of frequencies. They have flanges at the top to attach to standard waveguides. ATM Horn Antennas.jpg
Pyramidal horn antennas for a variety of frequencies. They have flanges at the top to attach to standard waveguides.

A horn antenna is used to transmit radio waves from a waveguide (a metal pipe used to carry radio waves) out into space, or collect radio waves into a waveguide for reception. It typically consists of a short length of rectangular or cylindrical metal tube (the waveguide), closed at one end, flaring into an open-ended conical or pyramidal shaped horn on the other end. [10] The radio waves are usually introduced into the waveguide by a coaxial cable attached to the side, with the central conductor projecting into the waveguide to form a quarter-wave monopole antenna. The waves then radiate out the horn end in a narrow beam. In some equipment the radio waves are conducted between the transmitter or receiver and the antenna by a waveguide; in this case the horn is attached to the end of the waveguide. In outdoor horns, such as the feed horns of satellite dishes, the open mouth of the horn is often covered by a plastic sheet transparent to radio waves, to exclude moisture.

Coaxial cable A type of electrical cable with an inner conductor surrounded by concentric insulating layer and conducting shield

Coaxial cable, or coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English physicist, engineer, and mathematician Oliver Heaviside, who patented the design in 1880.

Transmitter Electronic device that emits radio waves

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.

Radio receiver radio device for receiving radio waves and converting them to a useful signal

In radio communications, a radio receiver, also known as a receiver, wireless or simply 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 and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts 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.

How it works

Corrugated conical horn antenna used as a feed horn on a Hughes Direcway home satellite dish. A transparent plastic sheet covers the horn mouth to keep out rain. Hughes Direcway LNB.jpg
Corrugated conical horn antenna used as a feed horn on a Hughes Direcway home satellite dish. A transparent plastic sheet covers the horn mouth to keep out rain.

A horn antenna serves the same function for electromagnetic waves that an acoustical horn does for sound waves in a musical instrument such as a trumpet. It provides a gradual transition structure to match the impedance of a tube to the impedance of free space, enabling the waves from the tube to radiate efficiently into space. [11]

Trumpet musical instrument with the highest register in the brass family

A trumpet is a brass instrument commonly used in classical and jazz ensembles. The trumpet group contains the instruments with the highest register in the brass family. Trumpet-like instruments have historically been used as signaling devices in battle or hunting, with examples dating back to at least 1500 BC; they began to be used as musical instruments only in the late 14th or early 15th century. Trumpets are used in art music styles, for instance in orchestras, concert bands, and jazz ensembles, as well as in popular music. They are played by blowing air through nearly-closed lips, producing a "buzzing" sound that starts a standing wave vibration in the air column inside the instrument. Since the late 15th century they have primarily been constructed of brass tubing, usually bent twice into a rounded rectangular shape.

The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields. For a transverse-electric-magnetic (TEM) plane wave traveling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium. In particular, for a plane wave travelling through empty space, the wave impedance is equal to the impedance of free space. The symbol Z is used to represent it and it is expressed in units of ohms. The symbol η (eta) may be used instead of Z for wave impedance to avoid confusion with electrical impedance.

If a simple open-ended waveguide is used as an antenna, without the horn, the sudden end of the conductive walls causes an abrupt impedance change at the aperture, from the wave impedance in the waveguide to the impedance of free space, (about 377 ohms). [2] [12] When radio waves travelling through the waveguide hit the opening, this impedance-step reflects a significant fraction of the wave energy back down the guide toward the source, so that not all of the power is radiated. This is similar to the reflection at an open-ended transmission line or a boundary between optical mediums with a low and high index of refraction, like at a glass surface. The reflected waves cause standing waves in the waveguide, increasing the SWR, wasting energy and possibly overheating the transmitter. In addition, the small aperture of the waveguide (less than one wavelength) causes significant diffraction of the waves issuing from it, resulting in a wide radiation pattern without much directivity.

The impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, Z0 = |E|/|H|, where |E| is the electric field strength and |H| is the magnetic field strength. Its presently accepted value is

Ohm SI derived unit of electrical resistance

The ohm is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. Although several empirically derived standard units for expressing electrical resistance were developed in connection with early telegraphy practice, the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time and of a convenient size for practical work as early as 1861. The definition of the ohm was revised several times. Today, the definition of the ohm is expressed from the quantum Hall effect.

Transmission line specialized cable or other structure designed to carry alternating current of radio frequency

In radio-frequency engineering, a transmission line is a specialized cable or other structure designed to conduct alternating current of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses.

To improve these poor characteristics, the ends of the waveguide are flared out to form a horn. The taper of the horn changes the impedance gradually along the horn's length. [12] This acts like an impedance matching transformer, allowing most of the wave energy to radiate out the end of the horn into space, with minimal reflection. The taper functions similarly to a tapered transmission line, or an optical medium with a smoothly varying refractive index. In addition, the wide aperture of the horn projects the waves in a narrow beam.

The horn shape that gives minimum reflected power is an exponential taper. [12] Exponential horns are used in special applications that require minimum signal loss, such as satellite antennas and radio telescopes. However conical and pyramidal horns are most widely used, because they have straight sides and are easier to design and fabricate.

Radiation pattern

The waves travel down a horn as spherical wavefronts, with their origin at the apex of the horn, a point called the phase center. The pattern of electric and magnetic fields at the aperture plane at the mouth of the horn, which determines the radiation pattern, is a scaled-up reproduction of the fields in the waveguide. Because the wavefronts are spherical, the phase increases smoothly from the edges of the aperture plane to the center, because of the difference in length of the center point and the edge points from the apex point. The difference in phase between the center point and the edges is called the phase error. This phase error, which increases with the flare angle, reduces the gain and increases the beamwidth, giving horns wider beamwidths than similar-sized plane-wave antennas such as parabolic dishes.

At the flare angle, the radiation of the beam lobe is down about 20 dB from its maximum value. [13]

As the size of a horn (expressed in wavelengths) is increased, the phase error increases, giving the horn a wider radiation pattern. Keeping the beamwidth narrow requires a longer horn (smaller flare angle) to keep the phase error constant. The increasing phase error limits the aperture size of practical horns to about 15 wavelengths; larger apertures would require impractically long horns. [14] This limits the gain of practical horns to about 1000 (30 dBi) and the corresponding minimum beamwidth to about 5 - 10°. [14]

Types

Horn antenna types Horn antenna types.svg
Horn antenna types
Stack of sectoral feed horns for air search radar antenna Stacked beam2.jpg
Stack of sectoral feed horns for air search radar antenna

Below are the main types of horn antennas. Horns can have different flare angles as well as different expansion curves (elliptic, hyperbolic, etc.) in the E-field and H-field directions, making possible a wide variety of different beam profiles.

Pyramidal horn (a, right) – a horn antenna with the horn in the shape of a four-sided pyramid, with a rectangular cross section. They are a common type, used with rectangular waveguides, and radiate linearly polarized radio waves. [12]
Sectoral horn – A pyramidal horn with only one pair of sides flared and the other pair parallel. It produces a fan-shaped beam, which is narrow in the plane of the flared sides, but wide in the plane of the narrow sides. These types are often used as feed horns for wide search radar antennas.
E-plane horn (b) – A sectoral horn flared in the direction of the electric or E-field in the waveguide.
H-plane horn (c) – A sectoral horn flared in the direction of the magnetic or H-field in the waveguide.
Conical horn (d) – A horn in the shape of a cone, with a circular cross section. They are used with cylindrical waveguides.
Exponential horn (e) – A horn with curved sides, in which the separation of the sides increases as an exponential function of length. Also called a scalar horn, they can have pyramidal or conical cross sections. Exponential horns have minimum internal reflections, and almost constant impedance and other characteristics over a wide frequency range. They are used in applications requiring high performance, such as feed horns for communication satellite antennas and radio telescopes.
Corrugated horn – A horn with parallel slots or grooves, small compared with a wavelength, covering the inside surface of the horn, transverse to the axis. Corrugated horns have wider bandwidth and smaller sidelobes and cross-polarization, and are widely used as feed horns for satellite dishes and radio telescopes.
Dual-mode conical horn – (The Potter horn [15] ) This horn can be used to replace the corrugated horn for use at sub-mm wavelengths where the corrugated horn is lossy and difficult to fabricate.
Diagonal horn – This simple dual-mode horn superficially looks like a pyramidal horn with a square output aperture. On closer inspection, however, the square output aperture is seen to be rotated 45° relative to the waveguide. These horns are typically machined into split blocks and used at sub-mm wavelengths. [16]
Ridged horn – A pyramidal horn with ridges or fins attached to the inside of the horn, extending down the center of the sides. The fins lower the cutoff frequency, increasing the antenna's bandwidth.
Septum horn – A horn which is divided into several subhorns by metal partitions (septums) inside, attached to opposite walls.
Aperture-limited horn – a long narrow horn, long enough so the phase error is a negligible fraction of a wavelength, [13] so it essentially radiates a plane wave. It has an aperture efficiency of 1.0 so it gives the maximum gain and minimum beamwidth for a given aperture size. The gain is not affected by the length but only limited by diffraction at the aperture. [13] Used as feed horns in radio telescopes and other high-resolution antennas.

Optimum horn

Corrugated horn antenna with a bandwidth of 3.7 to 6 GHz designed to attach to SMA waveguide feedline. This was used as a feedhorn for a parabolic antenna on a British military base. Rillenhorn.jpg
Corrugated horn antenna with a bandwidth of 3.7 to 6 GHz designed to attach to SMA waveguide feedline. This was used as a feedhorn for a parabolic antenna on a British military base.
Exponential feed horn for 85 ft Cassegrain spacecraft communication antenna at NASA's Goldstone Deep Space Communications Complex. Cassegrain 85ft antenna Goldstone Deep Space Center.jpg
Exponential feed horn for 85 ft Cassegrain spacecraft communication antenna at NASA's Goldstone Deep Space Communications Complex.

For a given frequency and horn length, there is some flare angle that gives minimum reflection and maximum gain. The internal reflections in straight-sided horns come from the two locations along the wave path where the impedance changes abruptly; the mouth or aperture of the horn, and the throat where the sides begin to flare out. The amount of reflection at these two sites varies with the flare angle of the horn (the angle the sides make with the axis). In narrow horns with small flare angles most of the reflection occurs at the mouth of the horn. The gain of the antenna is low because the small mouth approximates an open-ended waveguide. As the angle is increased, the reflection at the mouth decreases rapidly and the antenna's gain increases. In contrast, in wide horns with flare angles approaching 90° most of the reflection is at the throat. The horn's gain is again low because the throat approximates an open-ended waveguide. As the angle is decreased, the amount of reflection at this site drops, and the horn's gain again increases.

This discussion shows that there is some flare angle between 0° and 90° which gives maximum gain and minimum reflection. [17] This is called the optimum horn. Most practical horn antennas are designed as optimum horns. In a pyramidal horn, the dimensions that give an optimum horn are: [17] [18]

For a conical horn, the dimensions that give an optimum horn are: [17]

where

aE is the width of the aperture in the E-field direction
aH is the width of the aperture in the H-field direction
LE is the slant length of the side in the E-field direction
LH is the slant length of the side in the H-field direction.
d is the diameter of the cylindrical horn aperture
L is the slant length of the cone from the apex.
λ is the wavelength

An optimum horn does not yield maximum gain for a given aperture size. That is achieved with a very long horn (an aperture limited horn). The optimum horn yields maximum gain for a given horn length. Tables showing dimensions for optimum horns for various frequencies are given in microwave handbooks.

Large pyramidal horn used in 1951 to detect the 21 cm (1.43 GHz) radiation from hydrogen gas in the Milky Way galaxy. Currently on display at the Green Bank Observatory in Green Bank, West Virginia, USA. Green Banks - Ewen-Purcell Horn Antenna.jpg
Large pyramidal horn used in 1951 to detect the 21 cm (1.43 GHz) radiation from hydrogen gas in the Milky Way galaxy. Currently on display at the Green Bank Observatory in Green Bank, West Virginia, USA.

Gain

Horns have very little loss, so the directivity of a horn is roughly equal to its gain. [1] The gain G of a pyramidal horn antenna (the ratio of the radiated power intensity along its beam axis to the intensity of an isotropic antenna with the same input power) is: [18]

For conical horns, the gain is: [17]

where

A is the area of the aperture,
d is the aperture diameter of a conical horn
λ is the wavelength,
eA is a dimensionless parameter between 0 and 1 called the aperture efficiency ,

The aperture efficiency ranges from 0.4 to 0.8 in practical horn antennas. For optimum pyramidal horns, eA = 0.511., [17] while for optimum conical horns eA = 0.522. [17] So an approximate figure of 0.5 is often used. The aperture efficiency increases with the length of the horn, and for aperture-limited horns is approximately unity.

Horn-reflector antenna

A type of antenna that combines a horn with a parabolic reflector is known as a Hogg-horn, or horn-reflector antenna, invented by Alfred C. Beck and Harald T. Friis in 1941 [19] and further developed by David C. Hogg at Bell labs in 1961. [20] It is also referred to as the "sugar scoop" due to its characteristic shape. It consists of a horn antenna with a reflector mounted in the mouth of the horn at a 45 degree angle so the radiated beam is at right angles to the horn axis. The reflector is a segment of a parabolic reflector, and the focus of the reflector is at the apex of the horn, so the device is equivalent to a parabolic antenna fed off-axis. [21] The advantage of this design over a standard parabolic antenna is that the horn shields the antenna from radiation coming from angles outside the main beam axis, so its radiation pattern has very small sidelobes. [22] Also, the aperture isn't partially obstructed by the feed and its supports, as with ordinary front-fed parabolic dishes, allowing it to achieve aperture efficiencies of 70% as opposed to 55-60% for front-fed dishes. [21] The disadvantage is that it is far larger and heavier for a given aperture area than a parabolic dish, and must be mounted on a cumbersome turntable to be fully steerable. This design was used for a few radio telescopes and communication satellite ground antennas during the 1960s. Its largest use, however, was as fixed antennas for microwave relay links in the AT&T Long Lines microwave network. [20] [22] [23] Since the 1970s this design has been superseded by shrouded parabolic dish antennas, which can achieve equally good sidelobe performance with a lighter more compact construction. Probably the most photographed and well-known example is the 15 meter (50 foot) long Holmdel Horn Antenna [20] at Bell Labs in Holmdel, New Jersey, with which Arno Penzias and Robert Wilson discovered cosmic microwave background radiation in 1965, for which they won the 1978 Nobel Prize in Physics. Another more recent horn-reflector design is the cass-horn, which is a combination of a horn with a cassegrain parabolic antenna using two reflectors. [24]

Horn Antenna-in Holmdel, New Jersey.jpeg
50 ft. Holmdel horn antenna at Bell labs in Holmdel, New Jersey, USA, with which Arno Penzias and Robert Wilson discovered cosmic microwave background radiation in 1964.
Relay 1 antenna USA.jpg
Large 177 ft. horn reflector antenna at AT&T satellite communications facility in Andover, Maine, USA, used in 1960s to communicate with the first direct relay communications satellite, Telstar.
Hogg horn antennas.jpg
AT&T Long-Lines KS-15676 C-band (4-6 GHz) microwave relay horn-reflector antennas [23] on roof of AT&T telephone switching center, Seattle, Washington, USA
Horn-reflector antennas

See also

Related Research Articles

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Cassegrain antenna type of parabolic antenna with a convex secondary reflector

In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites.

Reflective array antenna

In telecommunications and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are a type of array antenna. They are often used in the VHF and UHF frequency bands. VHF examples are generally large and resemble a highway billboard, so they are sometimes called billboard antennas, or in Britain hoarding antennas. Other names are bedspring array and bowtie array depending on the type of elements making up the antenna. The curtain array is a larger version used by shortwave radio broadcasting stations.

Parabolic antenna type of antenna

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it has high directivity. It functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used.

Directional antenna

A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance over dipole antennas—or omnidirectional antennas in general—when greater concentration of radiation in a certain direction is desired.

Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz (GHz). This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar. This frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, and numerous short range terrestrial data links. They are also used for heating in industrial microwave heating, medical diathermy, microwave hyperthermy to treat cancer, and to cook food in microwave ovens.

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 helix and the ground plane. Helical antennas can operate in one of two principal modes — normal mode or axial mode.

Holmdel Horn Antenna United States national historic site

The Holmdel Horn Antenna is a large microwave horn antenna that was used as a satellite communication antenna and radio telescope during the 1960s at Bell Telephone Laboratories in Holmdel Township, New Jersey, United States. It was designated a National Historic Landmark in 1988 because of its association with the research work of two radio astronomers, Arno Penzias and Robert Wilson. In 1965 while using this antenna, Penzias and Wilson discovered the cosmic microwave background radiation (CMBR) that permeates the universe. This was one of the most important discoveries in physical cosmology since Edwin Hubble demonstrated in the 1920s that the universe was expanding. It provided the evidence that confirmed George Gamow's and Georges Lemaître's "Big Bang" theory of the creation of the universe. This helped change the science of cosmology, the study of the history of the universe, from a field for unlimited theoretical speculation into a discipline of direct observation. In 1978 Penzias and Wilson received the Nobel Prize for Physics for their discovery.

Slotted waveguide

A slotted waveguide is a waveguide that is used as an antenna in microwave radar applications. Prior to its use in surface search radar, such systems used a parabolic segment reflector. The slotted waveguide antenna was the result of collaborative radar research carried on by McGill University and the National Research Council of Canada during World War II. The co-inventors, W.H. Watson and E.W. Guptill of McGill, were granted a United States patent for the device, described as a "directive antenna for microwaves", in 1951.

Microwave antenna

A microwave antenna is a physical transmission device used to broadcast microwave transmissions between two or more locations. In addition to broadcasting, antennas are also used in radar, radio astronomy and electronic warfare.

Reflector (antenna) part of radio antenna

An antenna reflector is a device that reflects electromagnetic waves. Antenna reflectors can exist as a standalone device for redirecting radio frequency (RF) energy, or can be integrated as part of an antenna assembly.

Short backfire antenna

A short backfire antenna is a type of a directional antenna, characterized by high gain, relatively small size, and narrow band. It has a shape of a disc with a straight edge, with a vertical pillar with a dipole acting as the driven element in roughly the middle and a conductive disc at the top acting as a sub-reflector. The bottom disc has the diameter of two wavelengths, and its collar (edge) is quarter the wavelength tall. The center pillar consists of two coaxial tubes, with a quarter-wavelength slot cut into the outer tube

Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it. Typical parameters of antennas are gain, radiation pattern, beamwidth, polarization, and impedance.

Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse.

Corner reflector antenna

A corner reflector antenna is a type of directional antenna used at VHF and UHF frequencies. It was invented by John D. Kraus in 1938. It consists of a dipole driven element mounted in front of two flat rectangular reflecting screens joined at an angle, usually 90°. Corner reflectors have moderate gain of 10-15 dB, high front-to-back ratio of 20-30 dB, and wide bandwidth.

Leaky-Wave Antenna (LWA) belong to the more general class of Traveling wave antenna, that use a traveling wave on a guiding structure as the main radiating mechanism. Traveling-wave antenna fall into two general categories, slow-wave antennas and fast-wave antennas, which are usually referred to as leaky-wave antennas.

Antenna array set of multiple antennas which work together as a single antenna

An antenna array is a set of multiple connected antennas which work together as a single antenna, to transmit or receive radio waves. The individual antennas are usually connected to a single receiver or transmitter by feedlines that feed the power to the elements in a specific phase relationship. The radio waves radiated by each individual antenna combine and superpose, adding together to enhance the power radiated in desired directions, and cancelling to reduce the power radiated in other directions. Similarly, when used for receiving, the separate radio frequency currents from the individual antennas combine in the receiver with the correct phase relationship to enhance signals received from the desired directions and cancel signals from undesired directions. More sophisticated array antennas may have multiple transmitter or receiver modules, each connected to a separate antenna element or group of elements.

Curtain array class of large multielement directional wire radio transmitting antennas

Curtain arrays are a class of large multielement directional wire radio transmitting antennas, used in the shortwave radio bands. They are a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often in front of a "curtain" reflector made of a flat vertical screen of many long parallel wires. These are suspended by support wires strung between pairs of tall steel towers, up to 300 ft (90 m) high. They are used for long-distance skywave transmission; they transmit a beam of radio waves at a shallow angle into the sky just above the horizon, which is reflected by the ionosphere back to Earth beyond the horizon. Curtain antennas are mostly used by international short wave radio stations to broadcast to large areas at transcontinental distances.

In radio systems, many different antenna types are used with specialized properties for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.

References

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