Reflective array antenna

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This reflective array television antenna consists of eight "bowtie" dipole driven elements mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174-216 MHz) and UHF (470-700 MHz) bands. It has a gain of 5 dB VHF and 12 dB UHF and an 18 dB front-to-back ratio. The example shown is horizontally polarised. Eight bay bowtie TV antenna.jpg
This reflective array television antenna consists of eight "bowtie" dipole driven elements mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174216 MHz) and UHF (470700 MHz) bands. It has a gain of 5 dB VHF and 12 dB UHF and an 18 dB front-to-back ratio. The example shown is horizontally polarised.
Reflective array 'billboard' antenna of the SCR-270 radar, an early US Army radar system. It consists of 32 horizontal half wave dipoles mounted in front of a 17 m (55 ft) high screen reflector. With an operating frequency of 106 MHz and a wavelength of 3 m (10 ft) this large antenna was required to generate a sufficiently narrow beamwidth to locate enemy aircraft. SCR-270-set-up.jpg
Reflective array 'billboard' antenna of the SCR-270 radar, an early US Army radar system. It consists of 32 horizontal half wave dipoles mounted in front of a 17 m (55 ft) high screen reflector. With an operating frequency of 106 MHz and a wavelength of 3 m (10 ft) this large antenna was required to generate a sufficiently narrow beamwidth to locate enemy aircraft.

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. Other names are bedspring array [1] 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.

Contents

Reflective array antennas usually have a number of identical driven elements, fed in phase, in front of a flat, electrically large reflecting surface to produce a unidirectional beam of radio waves, increasing antenna gain and reducing radiation in unwanted directions. The larger the number of elements used, the higher the gain; the narrower the beam is and the smaller the sidelobes are. The individual elements are most commonly half wave dipoles, although they sometimes contain parasitic elements as well as driven elements. The reflector may be a metal sheet or more commonly a wire screen. A metal screen reflects radio waves as well as a solid metal sheet as long as the holes in the screen are smaller than about one-tenth of a wavelength, so screens are often used to reduce weight and wind loads on the antenna. They usually consist of a grill of parallel wires or rods, oriented parallel to the axis of the dipole elements.

The driven elements are fed by a network of transmission lines, which divide the power from the RF source equally between the elements. This often has the circuit geometry of a tree structure.

Basic concepts

Radio signals

When a radio signal passes a conductor, it induces an electrical current in it. Since the radio signal fills space, and the conductor has a finite size, the induced currents add up or cancel out as they move along the conductor. A basic goal of antenna design is to make the currents add up to a maximum at the point where the energy is tapped off. To do this, the antenna elements are sized in relation to the wavelength of the radio signal, with the aim of setting up standing waves of current that are maximized at the feed point.

This means that an antenna designed to receive a particular wavelength has a natural size. To improve reception, one cannot simply make the antenna larger; this will improve the amount of signal intercepted by the antenna, which is largely a function of area, but will lower the efficiency of the reception (at a given wavelength). Thus, in order to improve reception, antenna designers often use multiple elements, combining them together so their signals add up. These are known as antenna arrays .

Array phasing

In order for the signals to add together, they need to arrive in-phase. Consider two dipole antennas placed in a line end-to-end, or collinear. If the resulting array is pointed directly at the source signal, both dipoles will see the same instantaneous signal, and thus their reception will be in-phase. However, if one were to rotate the antenna so it was at an angle to the signal, the extra path from the signal to the more distant dipole means it receives the signal slightly out of phase. When the two signals are then added up, they no longer strictly reinforce each other, and the output drops. This makes the array more sensitive horizontally, while stacking the dipoles in parallel narrows the pattern vertically. This allows the designer to tailor the reception pattern, and thus the gain, by moving the elements about.

If the antenna is properly aligned with the signal, at any given instant in time, all of the elements in an array will receive the same signal and be in-phase. However, the output from each element has to be gathered up at a single feed point, and as the signals travel across the antenna to that point, their phase is changing. In a two-element array this is not a problem because the feed point can be placed between them; any phase shift taking place in the transmission lines is equal for both elements. However, if one extends this to a four-element array, this approach no longer works, as the signal from the outer pair has to travel further and will thus be at a different phase than the inner pair when it reaches the center. To ensure that they all arrive with the same phase, it is common to see additional transmission wire inserted in the signal path, or for the transmission line to be crossed over to reverse the phase if the difference is greater than 12 a wavelength.

Reflectors

The gain can be further improved through the addition of a reflector. Generally any conductor in a flat sheet will act in a mirror-like fashion for radio signals, but this also holds true for non-continuous surfaces as long as the gaps between the conductors are less than about 110 of the target wavelength. [2] This means that wire mesh or even parallel wires or metal bars can be used, which is especially useful both for reducing the total amount of material as well as reducing wind loads.

Due to the change in signal propagation direction on reflection, the signal undergoes a reversal of phase. In order for the reflector to add to the output signal, it has to reach the elements in-phase. Generally this would require the reflector to be placed at 14 of a wavelength behind the elements, and this can be seen in many common reflector arrays like television antennas. However, there are a number of factors that can change this distance, and actual reflector positioning varies.

Reflectors also have the advantage of reducing the signal received from the back of the antenna. Signals received from the rear and re-broadcast from the reflector have not undergone a change of phase, and do not add to the signal from the front. This greatly improves the front-to-back ratio of the antenna, making it more directional. This can be useful when a more directional signal is desired, or unwanted signals are present. There are cases when this is not desirable, and although reflectors are commonly seen in array antennas, they are not universal. For instance, while UHF television antennas often use an array of bowtie antennas with a reflector, a bowtie array without a reflector is a relatively common design in the microwave region. [3]

Gain limits

As more elements are added to an array, the beamwidth of the antenna's main lobe decreases, leading to an increase in gain. In theory there is no limit to this process. However, as the number of elements increases, the complexity of the required feed network that keeps the signals in-phase increases. Ultimately, the rising inherent losses in the feed network become greater than the additional gain achieved with more elements, limiting the maximum gain that can be achieved.

Two element dipole array in front of a one wavelength square reflector used as gain standard Standard gain dipole array.png
Two element dipole array in front of a one wavelength square reflector used as gain standard

The gain of practical array antennas is limited to about 25–30 dB. Two half wave elements spaced a half wave apart and a quarter wave from a reflecting screen have been used as a standard gain antenna with about 9.8 dBi at its design frequency. [4] Common 4-bay television antennas have gains around 10 to 12 dB, [5] and 8-bay designs might increase this to 12 to 16 dB. [6] The 32-element SCR-270 had a gain around 19.8 dB. [7] Some very large reflective arrays have been constructed, notably the Soviet Duga radars which are hundreds of meters across and contain hundreds of elements. Active array antennas, in which groups of elements are driven by separate RF amplifiers, can have much higher gain, but are prohibitively expensive.

Since the 1980s, versions for use at microwave frequencies have been made with patch antenna elements mounted in front of a metal surface. [8]

Radiation pattern and beam steering

When driven in phase, the radiation pattern of the reflective array is a single main lobe perpendicular to the plane of the antenna, plus several sidelobes at equal angles to either side. The more elements used, the narrower the main lobe and the less power is radiated in the sidelobes.

The main lobe of the antenna can be steered electronically within a limited angle by phase shifting the drive signals applied to the individual elements. Each antenna element is fed through a phase shifter which can be controlled digitally, delaying each signal by a successive amount. This causes the wavefronts created by the superposition of the individual elements to be at an angle to the plane of the antenna. Antennas that use this technique are called phased arrays and often used in modern radar systems.

Another option for steering the beam is mounting the entire antenna structure on a pivoting platform and rotating it mechanically.

See also

Related Research Articles

<span class="mw-page-title-main">Collinear antenna array</span>

In telecommunications, a collinear antenna array is an array of dipole or quarter-wave antennas mounted in such a manner that the corresponding elements of each antenna are parallel and collinear; that is, they are located along a common axis.

<span class="mw-page-title-main">Log-periodic antenna</span> Multi-element, directional antenna useable over a wide band of frequencies

A log-periodic antenna (LP), also known as a log-periodic array or log-periodic aerial, is a multi-element, directional antenna designed to operate over a wide band of frequencies. It was invented by John Dunlavy in 1952.

<span class="mw-page-title-main">Antenna (radio)</span> Electrical device

In radio engineering, an antenna or aerial is an electronic device that converts an alternating electric current into radio waves, or radio waves into an electric current. 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 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.

<span class="mw-page-title-main">Parabolic antenna</span> 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 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.

<span class="mw-page-title-main">Yagi–Uda antenna</span> Type of radio antenna

A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods acting as half-wave dipoles. Yagi–Uda antennas consist of a single driven element connected to a radio transmitter or receiver through a transmission line, and additional passive radiators with no electrical connection, usually including one so-called reflector and any number of directors. It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan, with a lesser role played by his boss Hidetsugu Yagi.

<span class="mw-page-title-main">Directional antenna</span> Radio antenna which has greater performance in specific alignments

A directional antenna or beam antenna is an antenna which radiates or receives greater radio wave power in specific directions. Directional antennas can radiate radio waves in beams, when greater concentration of radiation in a certain direction is desired, or in receiving antennas receive radio waves from one specific direction only. This can increase the power transmitted to receivers in that direction, or reduce interference from unwanted sources. This contrasts with omnidirectional antennas such as dipole antennas which radiate radio waves over a wide angle, or receive from a wide angle.

<span class="mw-page-title-main">Dipole antenna</span> Antenna consisting of two rod-shaped conductors

In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely-used types of antenna; the other is the monopole. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each far end. A dipole antenna commonly consists of two identical conductive elements such as metal wires or rods. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feedline connected to it, and the other side connected to some type of ground. A common example of a dipole is the rabbit ears television antenna found on broadcast television sets. All dipoles are electrically equivalent to two monopoles mounted end-to-end and fed with opposite phases, with the ground plane between them made virtual by the opposing monopole.

<span class="mw-page-title-main">Whip antenna</span> Type of radio antenna

A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so that it does not break easily, and the name is derived from the whip-like motion that it exhibits when disturbed. Whip antennas for portable radios are often made of a series of interlocking telescoping metal tubes, so they can be retracted when not in use. Longer whips, made for mounting on vehicles and structures, are made of a flexible fiberglass rod around a wire core and can be up to 11 m long.

<span class="mw-page-title-main">Beverage antenna</span> Type of radio antenna

The Beverage antenna or "wave antenna" is a long-wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It is used by amateur radio operators, shortwave listeners, longwave radio DXers and for military applications.

<span class="mw-page-title-main">Reflector (antenna)</span>

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.

<span class="mw-page-title-main">Loop antenna</span> Type of radio antenna

A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that for transmitting is usually fed by a balanced power source or for receiving feeds a balanced load. Within this physical description there are two distinct types:

<span class="mw-page-title-main">Television antenna</span> Antenna used with a television to receive television programs

A television antenna, also called a television aerial, is an antenna specifically designed for use with a television receiver (TV) to receive terrestrial over-the-air (OTA) broadcast television signals from a television station. Terrestrial television is broadcast on frequencies from about 47 to 250 MHz in the very high frequency (VHF) band, and 470 to 960 MHz in the ultra high frequency (UHF) band in different countries.

<span class="mw-page-title-main">Monopole antenna</span> Type of radio antenna

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane, which is often the Earth. This contrasts with a dipole antenna which consists of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna.

<span class="mw-page-title-main">Corner reflector antenna</span> Type of directional 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 reflector antennas have moderate gain of 10–15 dB, high front-to-back ratio of 20–30 dB, and wide bandwidth.

<span class="mw-page-title-main">Turnstile antenna</span>

A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase. The name reflects the notion the antenna looks like a turnstile when mounted horizontally. The antenna can be used in two possible modes. In normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis.

<span class="mw-page-title-main">Antenna array</span> Set of multiple antennas which work together

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.

Fresnel zone antennas are antennas that focus the signal by using the phase shifting property of the antenna surface or its shape. There are several types of Fresnel zone antennas, namely, Fresnel zone plate, offset Fresnel zone plate antennas, phase correcting reflective array or "Reflectarray" antennas and 3 Dimensional Fresnel antennas. They are a class of diffractive antennas and have been used from radio frequencies to X rays.

<span class="mw-page-title-main">Curtain array</span> Class of large multielement directional wire radio transmitting antennas

Curtain arrays are a class of large multielement directional radio transmitting wire antennas, used in the short-wave radio bands. They constitute a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often positioned 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, reaching heights of up to 90 m high. Primarily employed for long-distance skywave transmission, they emit a beam of radio waves at a shallow angle into the sky just above the horizon, which is then reflected by the ionosphere back to Earth beyond the horizon. Curtain arrays are extensively used by international short-wave radio stations for broadcasting to large areas at transcontinental distances.

In radio systems, many different antenna types are used whose properties are especially crafted for particular applications.

<span class="mw-page-title-main">Driven and parasitic elements</span>


In an antenna array made of multiple conductive elements, a driven element or active element is electrically connected to the receiver or transmitter while a parasitic element is not.

References

  1. US Navy (September 1998). NAVEDTRA 14183 - Navy Electricity and Electronics Training Series. Vol. Module 11 - Microwave principles. Lulu Press. p. 236. ISBN   1-329-66770-0.
  2. "The Effects of Earth's Upper Atmosphere on Radio Signals". NASA.
  3. Raut, S. (July 2014). "Wideband printed bowtie array for spectrum monitoring". 2014 IEEE Antennas and Propagation Society International Symposium (APSURSI). pp. 235–236. doi:10.1109/APS.2014.6904449. ISBN   978-1-4799-3540-6. S2CID   42085218.
  4. "Standard Gain Antennas".
  5. "ULTRAtenna 60". Channel Master.
  6. "EXTREMEtenna 80". Channel Master.
  7. Burrows, Chas. R. (2013-10-22). Radio Wave Propagation. Academic Press. p. 460. ISBN   978-1-4832-5854-6.
  8. Huang, john. Reflectarray antennas.

PD-icon.svg This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

Reflective array, bow tie, grid, or panel UHF television antenna,.jpg
A modern form of reflective array is the "bow tie" UHF television antenna. This example has two dipole driven elements in front of a grill reflector. The "bow-tie" dipoles, consisting of two V-shaped elements, have a larger bandwidth than ordinary dipoles, allowing the antenna to cover the wide UHF television band. This example is installed for reception of vertically polarized TV transmissions.
Woodpecker array.jpg
Enormous reflective array antenna of the Duga or "Steel Yard" over-the-horizon (OTH) radar system, Chernobyl, Ukraine, part of the Soviet early-warning network. It transmits at frequencies between 7 and 19 MHz. The pairs of cylindrical cages at right are the half wave dipole driven elements. Behind them is a reflector screen of horizontal wires, just visible in center.
06issoudun alliss Gange.JPG
An ALLISS antenna, a modular type of curtain array used by international shortwave stations for broadcasting to distant areas by skywave.
Reflective array VHF television antenna 1954.jpg
VHF reflective array TV antenna from 1954. The driven elements are folded dipoles, the two long ones cover 5488 MHz the 4 short ones 174216 MHz.
Dipole array TV broadcast antenna.jpg
"Panel array" VHF TV broadcasting antenna. This type is widely used for UHF today
Bowtie UHF TV antenna 1954.png
"Bowtie" UHF TV antenna from 1954
SCR-270 Radar Antenna2.jpg
Closeup of SCR-20 reflective array from the 1950s