Fresnel antenna
Fresnel zone antennas belong to the category of reflector and lens antennas. Unlike traditional reflector and lens antennas, however, the focusing effect in a Fresnel zone antenna is achieved by controlling the phase shifting property of the surface and allows for flat [1] [6] or arbitrary antenna shapes. [4] For historical reasons, a flat Fresnel zone antenna is termed a Fresnel zone plate antenna. An offset Fresnel zone plate can be flush mounted to the wall or roof of a building, printed on a window, or made conformal to the body of a vehicle. [7]
The advantages of the Fresnel zone plate antenna are numerous. It is normally cheap to manufacture and install, easy to transport and package and can achieve high gain. Owing to its flat nature, the wind loading force of a Fresnel zone plate can be as little as 1/8 of that of conventional solid or wire-meshed reflectors of similar size. When used at millimetre wave frequencies, a Fresnel zone antenna can be an integrated with the millimetre-wave monolithic integrated circuit (MMIC) and thus becomes even more competitive than a printed antenna array.
The simplest Fresnel zone plate antenna is the circular half-wave zone plate invented in the nineteenth century. The basic idea is to divide a plane aperture into circular zones with respect to a chosen focal point on the basis that all radiation from each zone arrives at the focal point in phase within ±π/2 range. If the radiation from alternate zones is suppressed or shifted in phase by π, an approximate focus is obtained and a feed can be placed there to collect the received energy effectively. Despite its simplicity, the half-wave zone plate remained mainly as an optical device for a long time, primarily because its efficiency is too low (less than 20%) and the sidelobe level of its radiation pattern is too high to compete with conventional reflector antennas.
Compared with conventional reflector and lens antennas, reported research on microwave and millimetre-wave Fresnel zone antennas appears to be limited. In 1948, Maddaus published the design and experimental work on stepped half-wave lens antennas operating at 23 GHz and sidelobe levels of around −17 dB were achieved. In 1961, Buskirk and Hendrix reported an experiment on simple circular phase reversal zone plate reflector antennas for radio frequency operation. Unfortunately, the sidelobe they achieved was as high as −7 dB. In 1987, Black and Wiltse published their theoretical and experimental work on the stepped quarter-wave zone plate at 35 GHz. A sidelobe level of about −17 dB was achieved. A year later a phase reversal zone plate reflector operating at 94 GHz was reported by Huder and Menzel, and 25% efficiency and −19 dB sidelobe level were obtained. An experiment on a similar antenna at 11.8 GHz was reported by NASA researchers in 1989. 5% 3 dB bandwidth and −16 dB sidelobe level were measured. [1]
Until the 1980s, the Fresnel zone plate antenna was regarded as a poor candidate for microwave applications. Following the development of DBS services in the eighties, however, antenna engineers began to consider the use of Fresnel zone plates as candidate antennas for DBS reception, where antenna cost is an important factor. This, to some extent, provided a commercial push to the research on Fresnel zone antennas. [1] [3] [5]
Phase correcting antenna
To increase the efficiency of Fresnel zone plate antennas, one can divide each Fresnel zone into several sub-zones, such as quarter-wave sub-zones, and provide an appropriate phase shift in each of them, thus resulting in a sub-zone phase correcting zone plate. [9] The problem with dielectric based zone plate lens antenna is that whilst a dielectric is providing a phase shift to the transmitted wave, it inevitably reflects some of the energy back, so the efficiency of such a lens is limited. However, the low efficiency problem for a zone plate reflector is less severe, as total reflection can be achieved by using a conducting reflector behind the zone plate. [10] Based on the focal field analysis, it is demonstrated that high efficiency zone plate reflectors can be obtained by employing the multilayer phase correcting technique, which is to use a number of dielectric slabs of low permittivity and print different metallic zonal patterns on the different interfaces. The design and experiments of circular and offset multilayer phase correcting zone plate reflectors were presented in. [1]
A problem with the multilayer zone plate reflector is the complexity introduced, which might offset the advantage of using Fresnel zone plate antennas. One solution is to print an inhomogeneous array of conducting elements on a grounded dielectric plate, thus leading to the so-called single-layer printed flat reflector. [1] [11] This configuration bears much in common with the printed array antenna but it requires the use of a feed antenna instead of a corporate feed network. In contrast to the normal array antenna, the array elements are different and are arranged in a pseudo-periodic manner. The theory and design method of single layer printed flat reflectors incorporating conducting rings and experimental results on such an antenna operating in the X-band were given in. [5] Naturally, this leads to a more general antenna concept, the phase correcting reflective array.
Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequencies between 300 MHz and 300 GHz, broadly construed. A more common definition in radio-frequency engineering is the range between 1 and 100 GHz, or between 1 and 3000 GHz . The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range; rather, it indicates that microwaves are small, compared to the radio waves used in prior radio technology.
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.
A Fresnel zone, named after physicist Augustin-Jean Fresnel, is one of a series of confocal prolate ellipsoidal regions of space between and around a transmitter and a receiver. The primary wave will travel in a relative straight line from the transmitter to the receiver. Aberrant transmitted radio, sound, or light waves which are transmitted at the same time can follow slightly different paths before reaching a receiver, especially if there are obstructions or deflecting objects between the two. The two waves can arrive at the receiver at slightly different times and the aberrant wave may arrive out of phase with the primary wave due to the different path lengths. Depending on the magnitude of the phase difference between the two waves, the waves can interfere constructively or destructively. The size of the calculated Fresnel zone at any particular distance from the transmitter and receiver can help to predict whether obstructions or discontinuities along the path will cause significant interference.
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 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.
In radio engineering, an antenna or aerial 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.
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.
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
A zone plate is a device used to focus light or other things exhibiting wave character. Unlike lenses or curved mirrors, zone plates use diffraction instead of refraction or reflection. Based on analysis by French physicist Augustin-Jean Fresnel, they are sometimes called Fresnel zone plates in his honor. The zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from an opaque disc.
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. They are used as feed antennas 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. Their advantages are moderate directivity, broad bandwidth, low losses, and simple construction and adjustment.
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.
Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often microscopic, structures to produce unusual physical properties. Antenna designs incorporating metamaterials can step-up the antenna's radiated power.
A flat lens is a lens whose flat shape allows it to provide distortion-free imaging, potentially with arbitrarily-large apertures. The term is also used to refer to other lenses that provide a negative index of refraction. Flat lenses require a refractive index close to −1 over a broad angular range. In recent years, flat lenses based on metasurfaces were also demonstrated.
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.
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.
A photon sieve is a device for focusing light using diffraction and interference. It consists of a flat sheet of material full of pinholes that are arranged in a pattern which is similar to the rings in a Fresnel zone plate, but with the ability to bring light to much sharper focus. The sieve concept, first developed in 2001, is versatile because the characteristics of the focusing behaviour can be altered to suit the application by manufacturing a sieve containing holes of several different sizes and different arrangement of the pattern of holes.
A lens antenna is a directional antenna that uses a shaped piece of microwave-transparent material to bend and focus microwaves by refraction, as an optical lens does for light. Typically it consists of a small feed antenna such as a patch antenna or horn antenna which radiates radio waves, with a piece of dielectric or composite material in front which functions as a converging lens to collimate the radio waves into a beam. Conversely, in a receiving antenna the lens focuses the incoming radio waves onto the feed antenna, which converts them to electric currents which are delivered to a radio receiver. They can also be fed by an array of feed antennas, called a focal plane array (FPA), to create more complicated radiation patterns.
A transmitarray antenna is a phase-shifting surface (PSS), a structure capable of focusing electromagnetic radiation from a source antenna to produce a high-gain beam. Transmitarrays consist of an array of unit cells placed above a source (feeding) antenna. Phase shifts are applied to the unit cells, between elements on the receive and transmit surfaces, to focus the incident wavefronts from the feeding antenna. These thin surfaces can be used instead of a dielectric lens. Unlike phased arrays, transmitarrays do not require a feed network, so losses can be greatly reduced. Similarly, they have an advantage over reflectarrays in that feed blockage is avoided.
A reflectarray antenna consists of an array of unit cells, illuminated by a feeding antenna. The feeding antenna is usually a horn. The unit cells are usually backed by a ground plane, and the incident wave reflects off them towards the direction of the beam, but each cell adds a different phase delay to the reflected signal. A phase distribution of concentric rings is applied to focus the wavefronts from the feeding antenna into a plane wave . A progressive phase shift can be applied to the unit cells to steer the beam direction. It is common to offset the feeding antenna to prevent blockage of the beam. In this case, the phase distribution on the reflectarray surface needs to be altered. A reflectarray focuses a beam in a similar way to a parabolic reflector (dish), but with a much thinner form factor.
Oleg V. Minin was born on March 22, 1960, in Novosibirsk, Academytown, Russia. He is a Russian physicist, a corresponding member of the Russian Academy of Metrology, and a full professor of Physics at the Tomsk Polytechnic University. He is renowned for his contributions to creating new scientific directions, including THz 3D Zone plate, Mechatronics, hyper cumulative shaped charge, subwavelength structured light, encompassing acoustics and surface plasmon.
Igor V. Minin, is a Russian physicist, corresponding member of the Russian Academy of Metrology, and a full professor of Physics at the Tomsk Polytechnic University. He became known for his contribution to the creation of new directions in science: THz 3D Zone plate, Mesotronics, and subwavelength structured light, including acoustics and surface plasmon.
