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A  short dipole  is a simple wire antenna. One end of it is open-circuited and the other end is fed with AC source. This dipole got its name because of its length. Frequency range The range of frequency in which short dipole operates is around 3KHz to 30MHz. This is mostly used in low frequency receivers. Construction & Working of Short Dipole The  Short dipole  is the dipole antenna having the length of its wire shorter than the wavelength. A voltage source is connected at one end while a dipole shape is made, i.e., the lines are terminated at the other end. The circuit diagram of a short dipole with length L is shown. The actual size of the antenna does not matter. The wire that leads to the antenna must be less than one-tenth of the wavelength. That is L < λ 10 L < λ 10 Where L  is the length of the wire of the short dipole. λ  is the wavelength. Another type of short dipole is infinitesimal dipole, whose length is far less than its wave length. Its constructiion is simila

If the length of the dipole, i.e. the total wire, equals the full wavelength  λ , then it is called as  full wave dipole . If a full wavelength dipole is used either for transmission or for reception, let us see how the radiation will be. Construction & Working of Full-wave Dipole The full-wave dipole with its voltage and current distribution is shown here. Both the positive and negative peaks of the wave induce positive and negative voltages respectively. However, as the induced voltages cancel out each other, there is no question of radiation. The above figure shows the voltage distribution of full-wave dipole whose length is  λ . It is seen that two half-wave dipoles are joined to make a full-wave dipole. The voltage pattern when induces its positive charges and negative charges at the same time, cancel out each other as shown in the figure. The induced charges make no further attempt of radiation since they are cancelled. The output radiation will be zero for a fullwave transmi

A folded dipole is an antenna, with two conductors connected on both sides, and folded to form a cylindrical closed shape, to which feed is given at the center. The length of the dipole is half of the wavelength. Hence, it is called as  half wave folded dipole antenna . Frequency range The range of frequency in which half wave folded dipole operates is around 3KHz to 300GHz. This is mostly used in television receivers. Construction & Working of Half-wave Folded Dipole This antenna is commonly used with the array type antennas to increase the feed resistance. The most commonly used one is with Yagi-Uda antenna. The following figure shows a half-wave folded dipole antenna. This antenna uses an extra conducting element (a wire or a rod) when compared with previous dipole antenna. This is continued by placing few conducting elements in parallel, with insulation in-between, in array type of antennas. The following figure explains the working of a half-wave folded dipole antenna, when it

The dipole antenna is cut and bent for effective radiation. The length of the total wire, which is being used as a dipole, equals half of the wavelength (i.e., l = λ/2). Such an antenna is called as  half-wave dipole antenna . This is the most widely used antenna because of its advantages. It is also known as  Hertz antenna . Frequency range The range of frequency in which half-wave dipole operates is around 3KHz to 300GHz. This is mostly used in radio receivers. Construction & Working of Half-wave Dipole It is a normal dipole antenna, where the frequency of its operation is  half of its wavelength . Hence, it is called as half-wave dipole antenna. The edge of the dipole has maximum voltage. This voltage is alternating (AC) in nature. At the positive peak of the voltage, the electrons tend to move in one direction and at the negative peak, the electrons move in the other direction. This can be explained by the figures given below. The figures given above show the working of a half-

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. 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. If one end of this wire is not connected, then the power in it tries to escape. This leads to wireless communication. If one end of the wire is bent, then the energy tries to escape from the transmission line, more effectively than before. This purposeful escape is known as  Radiation . For the radiation to take place effectively, the impedance of the open end of the transmission line should match with the impedance of the free-space. Consider a transmission line of a quarter-wave length size. The far end of it is kept open

Antennas have to be classified to understand their physical structure and functionality more clearly. There are many types of antennas depending upon the applications applications. Type of antenna Examples Applications Wire Antennas Dipole antenna, Monopole antenna, Helix antenna, Loop antenna Personal applications, buildings, ships, automobiles, space crafts Aperture Antennas Waveguide (opening), Horn antenna Flush-mounted applications, air-craft, space craft Reflector Antennas Parabolic reflectors, Corner reflectors Microwave communication, satellite tracking, radio astronomy Lens Antennas Convex-plane, Concave-plane, Convex-convex, Concaveconcave lenses Used for very highfrequency applications Micro strip Antennas Circular-shaped, Rectangularshaped metallic patch above the ground plane Air-craft, space-craft, satellites, missiles, cars, mobile phones etc. Array Antennas Yagi-Uda antenna, Micro strip patch array, Aperture array, Slotted wave guide array Used for very high gain applic

Antennas radiate Electromagnetic energy to transmit or to receive information. Therefore, the terms  Energy  and  Power  are associated with these electromagnetic waves and we have to discuss them. An electromagnetic wave has both electric and magnetic fields. Consider the wave at any instant, which can be viewed in both the vectors. The following figure shows the representation of electric and magnetic field components in an Electromagnetic wave. The electric wave is present vertical to the propagation of EM wave, while the magnetic wave is horizontally located. Both the fields are at right angles to each other. Poynting Vector Poynting vector describes the energy of the EM Wave per unit time per unit area at any given instant of time.  John Henry Poynting  first derived this vector in 1884 and hence it was named after him. Definition  − “Poynting vector gives the rate of energy transfer per unit area” or “The energy that a wave carries per unit time per unit area is given by the Poyn

An antenna can be used as both transmitting antenna and receiving antenna. While using so, we may come across a question whether the properties of the antenna might change as its operating mode is changed. Fortunately, we need not worry about that. The properties of antenna being unchangeable is called as the property of  reciprocity . Properties under Reciprocity The properties of transmitting and receiving antenna that exhibit the reciprocity are − Equality of Directional patterns. Equality of Directivities. Equality of Effective lengths. Equality of Antenna impedances. Let us see how these are implemented. Equality of Directional patterns The  radiation pattern  of transmitting antenna1, which transmits to the receiving antenna2 is equal to the radiation pattern of antenna2, if it transmits and antenna1 receives the signal. Equality of Directivities Directivity  is same for both transmitting and receiving antennas, if the value of directivity is same for both the cases i.e. the dire

In this chapter, we shall discuss about another important factor in the radiation pattern of an antenna, known as  beam width . In the radiation pattern of an antenna, the main lobe is the main beam of the antenna where maximum and constant energy radiated by the antenna flows. Beam width  is the aperture angle from where most of the power is radiated. The two main considerations of this beam width are Half Power Beam Width  (HPBW)  and First Null Beam Width  (FNBW) . Half-Power Beam Width According to the standard definition, “The angular separation, in which the magnitude of the radiation pattern decreases by 50% (or -3dB) from the peak of the main beam, is the  Half Power Beam Width .” In other words, Beam width is the area where most of the power is radiated, which is the peak power.  Half power beam width  is the angle in which relative power is more than 50% of the peak power, in the effective radiated field of the antenna. Indication of HPBW When a line is drawn between radiatio

This chapter deals with the parameters of radiated beam of the antenna. These parameters help us to know about the beam specifications. Beam Area According to the standard definition, “Beam area is the solid angle through which all the power radiated by the antenna would stream if P (θ, Ø) maintained its maximum value over Ω A  and was zero elsewhere.” The radiated beam of the antenna comes out from an angle at the antenna, known as solid angle, where the power radiation intensity is maximum. This  solid beam angle  is termed as the  beam area . It is represented by  Ω A . The radiation intensity P (θ, Ø) should be maintained constant and maximum throughout the solid beam angle Ω A , its value being zero elsewhere. P o w e r   r a d i a t e d = P ( θ , Φ ) Ω A w a t t s P o w e r   r a d i a t e d = P ( θ , Φ ) Ω A w a t t s Beam angle is a set of angles between the half power points of the main lobe. Mathematical Expression The mathematical expression for beam area is Ω A = ∫ 2 π 0 ∫

In the previous chapter, we have gone through the radiation pattern. To have a better analysis regarding the radiation of an antenna, a referential point is necessary. The radiation of an isotropic antenna, fills this space. Definition Isotropic radiation  is the radiation from a point source, radiating uniformly in all directions, with same intensity regardless of the direction of measurement. The improvement of radiation pattern of an antenna is always assessed using the isotropic radiation of that antenna. If the radiation is equal in all directions, then it is known as  isotropic radiation . The point source is an example of isotropic radiator. However, this isotropic radiation is practically impossible, because every antenna radiates its energy with some directivity. The isotropic radiation is nothing but  Omni-directional radiation . It has a doughnut-shaped pattern when viewed in 3D and a figure-of-eight pattern when viewed in 2D. The figures given above show the radiation patte

Radiation is the term used to represent the emission or reception of wave front at the antenna, specifying its strength. In any illustration, the sketch drawn to represent the radiation of an antenna is its  radiation pattern . One can simply understand the function and directivity of an antenna by having a look at its radiation pattern. The power when radiated from the antenna has its effect in the near and far field regions. Graphically, radiation can be plotted as a function of  angular position  and  radial distance  from the antenna. This is a mathematical function of radiation properties of the antenna represented as a function of spherical co-ordinates, E (θ, Ø) and H (θ, Ø). Radiation Pattern The energy radiated by an antenna is represented by the  Radiation pattern  of the antenna. Radiation Patterns are diagrammatical representations of the distribution of radiated energy into space, as a function of direction. Let us look at the pattern of energy radiation. The figure given