
Types of Antennas. Radiation Mechanism
The antenna is a device that is used for transmitting and receiving signals, which represents some information. They are designed for wireless communication and have the capability to propagate both radio and microwave signals. Basically, it is just like a conductor and the resonant device, which works on a very narrow frequency band.They are categorized into different types according to their functionality and shape:
YagiUda: YagiUda is the directional antenna and is also known as the Yagi It is the most common type of antenna and is used in old household appliances like TVs and Radios. In this type, many numbers of parallel elements (halfwave dipoles) are arranged in a single straight line. It is used as a bridge in TV reception.
Aperture: An antenna with the opening that is capable of radiating the energy is known as the Aperture antenna. The opening of this is known to be the aperture. These types of antennas have the capability to send and receive signals in all directions, and the radiation is also more significant than the twowire transmission line.These kinds of antennas are used in surface search radars and microwave applications. Horn antenna is a wellknown aperture antenna.
Reflector: It is an antenna that consists of one or more reflecting surfaces with a system that transmits or absorbs the electromagnetic waves. These antennas are primarily used in satellite communication and remote sensing. Parabolic reflectors and corner reflectors are examples of reflector antennas.

Fundamental parameters of antennas.Radiation Pattern
Typical parameters of antennas are gain, bandwidth, radiation pattern, beamwidth, polarization, and impedance.
Antenna radiation patterns are graphical representations of elements of the radiation characteristics of an antenna. The antenna pattern is usually a graphical representation of the antenna's directional characteristic. It represents the relative intensity of the energy radiation or the amount of the electric or magnetic field strength as a function of the direction to the antenna. Antenna diagrams are measured or generated by simulation programs on the computer, for example, to graphically display the directivity of a radar antenna and thus estimate its performance

Beam efficiency. Bandwidth. Polarization. Input impedance
The beam efficiency of an antenna can be defined as the ratio of the power radiated within the main beam to the total power radiated. The beam efficiency is derived for ideal rectangular and circular apertures, as a function of the edgetocenter amplitude ratio.
The bandwidth of an antenna refers to the range of frequencies over which the antenna can operate correctly. The bandwidth can be described in terms of percentage of the center frequency of the band.

Linear wire Antennas. Infinitesimal Dipole. Small Dipole.
Linear wire antennas are the basic types of antennas. These are well known and widely used antennas. To have a better idea of these wire antennas, first let us have a look at the transmission lines.The wire or the transmission line has some power, which travels from one end to the other end. If both the ends of transmission line are connected to circuits, then the information will be transmitted or received using this wire between these two circuits.
A dipole whose length is far less than wavelength is infitesimal dipole.
The small dipole is the dipole antenna having the length of its wire shorter than the wavelength.

Loop antennas. Small circular loop.
A loop antenna is a radio antenna consisting of at least one loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line. It may be in any shape such as circular, rectangular, triangular, square etc.Loop antennas are of two types: Large loop antennas and small loop antennas.
Small loop antennas are also called as magnetic loop antennas. These are less resonant. These are mostly used as receivers.These antennas are of the size of onetenth of the wavelength.
L=(wavelength)/10

Linear, planar and circular arrays. Twoelement array.
Linear array is the simplest array geometry. All elements are aligned along a straight line. The minimum length linear array is the 2element array.
Planar array is a flat twodimensional array of antennas in which all of the elements, both active and parasitic, are in one plane. A planar array provides a large aperture and may be used for directional beam control by varying the relative phase of each element. A planar array may be used with a reflecting screen behind the active plane.
Circular arrays have become popular with the recent advancements in wireless communications. Though the design of circular arrays involves complex calculations, it has beam steering as inherent property which makes it a convenient candidate for beam forming. Every element in the circular array is characterised by three parameters, namely, excitation amplitude , phase , and interelement spacing . These three are known as steering parameters in array design. They are capable enough of modifying the array radiation pattern.

Triangular, cosine, and cosinesquared amplitude distributions.Linesource phase distributions.
Two dimensional antenna patterns have been determined for several aperture distributions having symmetrical amplitude and nonlinear phase by a method of calculation employing automatic punchcard machines to perform numerical integrations. These patterns have been arranged in sets which permit one to predict the pattern characteristics expected from a given aperture phase distribution. The method of calculation used here permits direct evaluation of the pattern integral with negligible error for any phase and amplitude distributions. The results presented extend previous work to the cases of quadratic and cubic phase distributions combined in various proportions together with uniform, tapered, and cosine amplitude distributions. Phase patterns, which permit determination of the effective aperture phase center, are presented as well as the magnitude patterns.
A phasetype distribution is the distribution of the time to absorption in a finite state absorbing Markov chain.The underlying Markov chain is called a representation of the distribution.

Integral equation method.Finite diameter wires. Moment method solution.
Integral equation method and numerical solution procedure is presented for problems of three‐dimensional elastic wave radiation and scattering from arbitrarily shaped obstacles. The formulation is explicitly in terms of surface traction and displacement, rather than wave potentials, and the BIE on which numerical work is based is written in a form entirely free of Cauchy principal value integrals. Indeed, the subsequent computational process, based on quadratic isoparametric boundary elements, renders all integrals free of singularities, so that ordinary Gaussian quadrature may be used. Numerical examples include scattering from spherical surfaces and radiation from a cube.
Moment method solution is a rigorous, fullwave numerical technique for solving open boundary electromagnetic problems. Using this technique, you can analyze electromagnetic radiation, scattering and wave propagation problems with relatively short computation times and modest computing resources. The method of moments is an integral equation technique; it solves the integral form of Maxwell’s equations as opposed to their differential forms that are used in the finite element or finite difference time domain methods.

Broadband dipoles ands matching techniques. Biconical antenna.
The operation of an antenna system over a frequency range is not completely dependent upon the frequency response of the antenna element itself but rather on the frequency characteristic of the transmission lineantenna element combination. In practice, the characteristic impedance of the transmission line is usually real whereas that of the antenna element is complex. Also the variation of each as a function of frequency is not the same. Thus efficient couplingmatching network must be designed which attempt to couplematch the characteristics of the two devices over the designed frequency range.Ideal matching at a given frequency can be accomplished by placing a short or opencircuited shunt a distance s from the transmissionlineantenna element connection. Assuming a real characteristic impedance, the length s is controlled so as to make the real part of the antenna element impedance equal to the characteristic impedance.
In radio systems, a biconical antenna is a broadbandwidth antenna made of two roughly conical conductive objects, nearly touching at their points. Biconical antennas are broadband dipole antennas, typically exhibiting a bandwidth of three octaves or more.

Aperture antennas. Field equivalence principle: Huygens’ principle
An Antenna with an aperture at the end can be termed as an Aperture antenna. Waveguide is an example of aperture antenna. The edge of a transmission line when terminated with an opening, radiates energy. This opening which is an aperture, makes it an Aperture antenna.The main types of aperture antennas are −wave guide antenna, horn antenna,slot antenna.
Huygens principle states that every point on a wavefront is in itself the source of spherical wavelets. The sum of these spherical wavelets forms the wavefront.
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