Category: antenna Theory

Antenna Theory – Slot Slot Antenna is an example of Aperture antenna. A rectangular slot is made on the conducting sheet. These slot antennas can be formed by simply making a cut on the surface, where they are mounted on. Frequency Range The frequency range used for the application of Slot antenna is 300 MHz to 30 GHz. It works in UHF and SHF frequency ranges. Construction & Working of Slot Antennas The use of slot antennas is well understood through its working principle. Let us have a look at the structure of a slot antenna. When an infinite conducting sheet is made a rectangular cut and the fields are excited in the aperture (which is called as a slot), it is termed as Slot antenna. This can be understood by observing the image of a slot antenna. The following image shows the model of a Slot antenna. The working of Slot Antenna can be easily understood through Babinet’s principle of optics. This concept gives an introduction to the slot antennas. Babinet’s Principle Babinet’s principle states that- “When the field behind a screen with an opening is added to the field of a complementary structure, the sum is equal to the field when there is no screen”. The above images clearly explain the principle. In all the regions, which are non-collinear with the beam, the above two screens, in figures 1 & 2, produce the same diffraction pattern. Case 1 − Consider a light source and a conducting plane (field) with an aperture before a screen. The light does not pass through the opaque area, but passes through the aperture. Case 2 − Consider the light source and a conducting plane of the size of the aperture in the previous case, being held against the screen. The light does not pass through the plane but through the remaining portion. Case 3 − Combine these two conducting planes of both the cases and put before the light source. The screen is not placed to observe the resultant combination. The effect of screen gets nullified. Working of Slot Antenna This principle of optics is applied to electromagnetic waves for the wave to get radiated. It is true that when a HF field exists across a narrow slot in a conducting plane, the energy is radiated. The image shows a slot antenna, which explains well about its working. Consider an infinite plane conducting screen is taken and pierced with apertures of desired shape and size and this will be the screen of slot antenna. Another screen is considered interchanging the places of aperture and screen area which is the complementary screen. These two screens are said to be complementary as they result in complete infinte metal screen. Now, this becomes the slot antenna. The terminal impedance is quite desirable for the radiation. Radiation Pattern The radiation pattern of the Slot antenna is Omni-directional, just like a half-wave dipole antenna. Take a look at the following illustration. It shows the radiation pattern of Slot antenna drawn in Horizontal and Vertical planes respectively Advantages The following are the advantages of Slot antenna − It can be fabricated and concealed within metallic objects It can provide covert communications with a small transmitter Disadvantages The following are the disadvantages of Slot antenna − Higher cross-polarization levels Lower radiation efficiency Applications The following are the applications of Slot antenna − Usually for radar navigational purposes Used as an array fed by a wave guide Learning working make money

Antenna Theory – Short Dipole 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 $$L < frac{lambda}{10}$$ 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 similar to it, but uses a capacitor plate. Infinitesimal Dipole A dipole whose length is far less than wavelength is infitesimal dipole. This antenna is actually impractical. Here, the length of the dipole is less than even fiftith part of the wavelength. The length of the dipole, Δl << λ. Where, λ is the wavelength. $$Delta l = frac{lambda}{50}$$ Hence, this is the infinitely small dipole, as the name implies. As the length of these dipoles is very small, the current flow in the wire will be dI. These wires are generally used with capacitor plates on both sides, where low mutual coupling is needed. Because of the capacitor plates, we can say that uniform distribution of current is present. Hence the current is not zero here. The capacitor plates can be simply conductors or the wire equivalents. The fields radiated by the radial currents tend to cancel each other in the far field so that the far fields of the capacitor plate antenna can be approximated by the infinitesimal dipole. Radiation Pattern The radiation pattern of a short dipole and infinitesimal dipole is similar to a half wave dipole. If the dipole is vertical, the pattern will be circular. The radiation pattern is in the shape of “figure of eight” pattern, when viewed in two-dimensional pattern. The following figure shows the radiation pattern of a short dipole antenna, which is in omni-directional pattern. Advantages The following are the advantages of short dipole antenna − Ease of construction, due to small size Power dissipation efficiency is higher Disadvantages The following are the disadvantages of short dipole antenna − High resistive losses High power dissipation Low Signal-to-noise ratio Radiation is low Not so efficient Applications The following are the applications of short dipole antenna − Used in narrow band applications. Used as an antenna for tuner circuits. In this chapter, the popular and most widely used short-wire antennas were discussed. We will discuss the Long-wire antennas in the coming chapters. Learning working make money

Antenna Theory – Lens The antennas, which we have discussed till now, used the plane surface. The lens antennas use the curved surface for both transmission and reception. Lens antennas are made up of glass, where the converging and diverging properties of lens are followed. The lens antennas are used for higher frequency applications. Frequency Range The frequency range of usage of lens antenna starts at 1000 MHz but its use is greater at 3000 MHz and above. To have a better understanding of the lens antenna, the working principle of a lens has to be known. A normal glass lens works on the principle of refraction. Construction & Working of Lens Antenna If a light source is assumed to be present at a focal point of a lens, which is at a focal distance from the lens, then the rays get through the lens as collimated or parallel rays on the plane wavefront. The rays that pass through the centre of the lens are less refracted than the rays that pass through the edges of the lens. All of the rays are sent in parallel to the plane wave front. This phenomenon of lens is called as divergence. The same procedure gets reversed if a light beam is sent from right side to left of the same lens. Then the beam gets refracted and meets at a point called focal point, at a focal distance from the lens. This phenomenon is called convergence. The same can be better understood by observing the following diagram − The ray diagram represents the focal point and focal length from the source to the lens. The parallel rays obtained are also called as collimated rays. In the above figure, the source at the focal point, at a focal distance from the lens, gets collimated in the plane wave front. This phenomenon can be reversed which means the light if sent from the left side, gets converged at the right side of the lens. It is because of this reciprocity, the lens can be used as an antenna, as the same phenomenon helps in utilizing the same antenna for both transmission and reception. The image of the model of a lens antenna is shown. To achieve the focusing properties at higher frequencies, the refractive index should be less than unity. Whatever may be the refractive index, the purpose of lens is to straighten the waveform. Based on this, the E-plane and H-plane lens are developed, which also delay or speed up the wave front. Types of Lens Antennas The following types of Lens Antennas are available − Di-electric lens or H-plane metal plate lens or Delay lens (Travelling waves are delayed by lens media) E-plane metal plate lens Non-metallic di-electric type lens Metallic or artificial dielectric type of lens Advantages The following are the advantages of Lens antenna − In lens antennas, feed and feed support, do not obstruct the aperture. It has greater design tolerance. Larger amount of wave, than a parabolic reflector, can be handled. Beam can be moved angularly with espect to the axis. Disadvantages The following are the disadvantages of Lens antenna − Lenses are heavy and bulky, especially at lower frequencies Complexity in design Costlier compared to reflectors, for the same specifications Applications The following are the applications of Lens antenna − Used as wide band antenna Especially used for Microwave frequency applications The converging properties of lens antennas can be used for developing higher level of antennas known as Parabolic reflector antennas, which are widely used in satellite communications. We will discuss about them in the next chapter. Learning working make money

Antenna Theory – Helical Helical antenna is an example of wire antenna and itself forms the shape of a helix. This is a broadband VHF and UHF antenna. Frequency Range The frequency range of operation of helical antenna is around 30MHz to 3GHz. This antenna works in VHF and UHF ranges. Construction & Working of Helical Antenna Helical antenna or helix antenna is the antenna in which the conducting wire is wound in helical shape and connected to the ground plate with a feeder line. It is the simplest antenna, which provides circularly polarized waves. It is used in extra-terrestrial communications in which satellite relays etc., are involved. The above image shows a helical antenna system, which is used for satellite communications. These antennas require wider outdoor space. It consists of a helix of thick copper wire or tubing wound in the shape of a screw thread used as an antenna in conjunction with a flat metal plate called a ground plate. One end of the helix is connected to the center conductor of the cable and the outer conductor is connected to the ground plate. The image of a helix antenna detailing the antenna parts is shown above. The radiation of helical antenna depends on the diameter of helix, the turn spacing and the pitch angle. Pitch angle is the angle between a line tangent to the helix wire and plane normal to the helix axis. $$alpha = tan^{-1}(frac{S}{pi D})$$ where, D is the diameter of helix. S is the turn spacing (centre to centre). α is the pitch angle. Modes of Operation The predominant modes of operation of a helical antenna are − Normal or perpendicular mode of radiation. Axial or end-fire or beam mode of radiation. Let us discuss them in detail. Normal mode In normal mode of radiation, the radiation field is normal to the helix axis. The radiated waves are circularly polarized. This mode of radiation is obtained if the dimensions of helix are small compared to the wavelength. The radiation pattern of this helical antenna is a combination of short dipole and loop antenna. The above figure shows the radiation pattern for normal mode of radiation in helical antenna. It depends upon the values of diameter of helix, D and its turn spacing, S. Drawbacks of this mode of operation are low radiation efficiency and narrow bandwidth. Hence, it is hardly used. Axial mode In axial mode of radiation, the radiation is in the end-fire direction along the helical axis and the waves are circularly or nearly circularly polarized. This mode of operation is obtained by raising the circumference to the order of one wavelength (λ) and spacing of approximately λ/4. The radiation pattern is broad and directional along the axial beam producing minor lobes at oblique angles. The figure shows the radiation pattern for axial mode of radiation in helical antenna. If this antenna is designed for right-handed circularly polarized waves, then it will not receive left-handed circularly polarized waves and vice versa. This mode of operation is generated with great ease and is more practically used. Advantages The following are the advantages of Helical antenna − Simple design Highest directivity Wider bandwidth Can achieve circular polarization Can be used at HF & VHF bands also Disadvantages The following are the disadvantages of Helical antenna − Antenna is larger and requires more space Efficiency decreases with number of turns Applications The following are the applications of Helical antenna − A single helical antenna or its array is used to transmit and receive VHF signals Frequently used for satellite and space probe communications Used for telemetry links with ballastic missiles and satellites at Earth stations Used to establish communications between the moon and the Earth Applications in radio astronomy Learning working make money

Antenna Theory – Half-Wave Dipole 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-wave dipole. Fig 1 shows the dipole when the charges induced are in positive half cycle. Now the electrons tend to move towards the charge. Fig 2 shows the dipole with negative charges induced. The electrons here tend to move away from the dipole. Fig 3 shows the dipole with next positive half cycle. Hence, the electrons again move towards the charge. The cumulative effect of this produces a varying field effect which gets radiated in the same pattern produced on it. Hence, the output would be an effective radiation following the cycles of the output voltage pattern. Thus, a half-wave dipole radiates effectively. The above figure shows the current distribution in half wave dipole. The directivity of half wave dipole is 2.15dBi, which is reasonably good. Where, ‘i’ represents the isotropic radiation. Radiation Pattern The radiation pattern of this half-wave dipole is Omni-directional in the H-plane. It is desirable for many applications such as mobile communications, radio receivers etc. The above figure indicates the radiation pattern of a half wave dipole in both H-plane and V-plane. The radius of the dipole does not affect its input impedance in this half wave dipole, because the length of this dipole is half wave and it is the first resonant length. An antenna works effectively at its resonant frequency, which occurs at its resonant length. Advantages The following are the advantages of half-wave dipole antenna − Input impedance is not sensitive. Matches well with transmission line impedance. Has reasonable length. Length of the antenna matches with size and directivity. Disadvantages The following are the disadvantages of half-wave dipole antenna − Not much effective due to single element. It can work better only with a combination. Applications The following are the applications of half-wave dipole antenna − Used in radio receivers. Used in television receivers. When employed with others, used for wide variety of applications. Learning working make money

Antenna Theory – Radiation Pattern 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 above shows radiation pattern of a dipole antenna. The energy being radiated is represented by the patterns drawn in a particular direction. The arrows represent directions of radiation. The radiation patterns can be field patterns or power patterns. The field patterns are plotted as a function of electric and magnetic fields. They are plotted on logarithmic scale. The power patterns are plotted as a function of square of the magnitude of electric and magnetic fields. They are plotted on logarithmic or commonly on dB scale. Radiation Pattern in 3D The radiation pattern is a three-dimensional figure and represented in spherical coordinates (r, θ, Φ) assuming its origin at the center of spherical coordinate system. It looks like the following figure − The given figure is a three dimensional radiation pattern for an Omni directional pattern. This clearly indicates the three co-ordinates (x, y, z). Radiation Pattern in 2D Two-dimensional pattern can be obtained from three-dimensional pattern by dividing it into horizontal and vertical planes. These resultant patterns are known as Horizontal pattern and Vertical pattern respectively. The figures show the Omni directional radiation pattern in H and V planes as explained above. H-plane represents the Horizontal pattern, whereas V-plane represents the Vertical pattern. Lobe Formation In the representation of radiation pattern, we often come across different shapes, which indicate the major and minor radiation areas, by which the radiation efficiency of the antenna is known. To have a better understanding, consider the following figure, which represents the radiation pattern of a dipole antenna. Here, the radiation pattern has main lobe, side lobes and back lobe. The major part of the radiated field, which covers a larger area, is the main lobe or major lobe. This is the portion where maximum radiated energy exists. The direction of this lobe indicates the directivity of the antenna. The other parts of the pattern where the radiation is distributed side wards are known as side lobes or minor lobes. These are the areas where the power is wasted. There is other lobe, which is exactly opposite to the direction of main lobe. It is known as back lobe, which is also a minor lobe. A considerable amount of energy is wasted even here. Example If the antennas used in radar systems produce side lobes, target tracing becomes very difficult. This is because, false targets are indicated by these side lobes. It is messy to trace out the real ones and to identify the fake ones. Hence, elimination of these side lobes is must, in order to improve the performance and save the energy. Remedy The radiated energy, which is being wasted in such forms needs to be utilized. If these minor lobes are eliminated and this energy is diverted into one direction (that is towards the major lobe), then the directivity of the antenna gets increased which leads to antenna’s better performance. Types of Radiation patterns The common types of Radiation patterns are − Omni-directional pattern (also called non-directional pattern): The pattern usually has a doughnut shape in three-dimensional view. However, in two-dimensional view, it forms a figure-of-eight pattern. Pencil-beam pattern − The beam has a sharp directional pencil shaped pattern. Fan-beam pattern − The beam has a fan-shaped pattern. Shaped beam pattern − The beam, which is non-uniform and patternless is known as shaped beam. A referential point for all these types of radiation is the isotropic radiation. It is important to consider the isotropic radiation even though it is impractical. Learning working make money

Antenna Theory – Poynting Vector 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 Poynting vector.” Poynting vector is represented by Ŝ. Units The SI unit of Poynting vector is W/m2. Mathematical Expression The quantity that is used to describe the power associated with the electromagnetic waves is the instantaneous Poynting vector, which is defined as $$hat{S} = hat{E} times hat{H}$$ Where $hat{S}$ is the instantaneous Poynting vector (W/m2). $hat{E}$ is the instantaneous electric field intensity (V/m). $hat{H}$ is the instantaneous magnetic field intensity (A/m). The important point to be noted here is that the magnitude of E is greater than H within an EM wave. However, both of them contribute the same amount of energy. Ŝ is the vector, which has both direction and magnitude. The direction of Ŝ is same as the velocity of the wave. Its magnitude depends upon the E and H. Derivation of Poynting Vector To have a clear idea on Poynting vector, let us go through the derivation of this Poynting vector, in a step-by-step process. Let us imagine that an EM Wave, passes an area (A) perpendicular to the X-axis along which the wave travels. While passing through A, in infinitesimal time (dt), the wave travels a distance (dx). $$dx = C dt$$ Where $$C = velocity of light = 3times 10^{8}m/s$$ $$volume, dv = Adx = AC dt$$ $$dmu = mu dv = (epsilon_{0}E^{2})(AC dt)$$ $$= epsilon_{0} AC E^{2} dt$$ Therefore, Energy transferred in time (dt) per area (A) is − $$S = frac{Energy}{Timetimes Area} = frac{dW}{dt A} = frac{epsilon_{0}ACE^{2} dt}{dt A} = epsilon_{0}C:E^{2}$$ Since $$frac{E}{H} = sqrt{frac{mu_{0}}{epsilon_{0}}} then S= frac{CB^{2}}{mu_{0}}$$ Since $$C = frac{E}{H} then S = frac{EB}{mu_{0}}$$ $$= hat{S} = frac{1}{mu_{0}}(hat{E}hat{H})$$ Ŝ denotes the Poynting vector. The above equation gives us the energy per unit time, per unit area at any given instant of time, which is called as Poynting vector. Learning working make money

Antenna Theory – Parameters Radiation intensity of an antenna is closely related to the direction of the beam focused and the efficiency of the beam towards that direction. In this chapter, let us have a look at the terms that deal with these topics. Directivity According to the standard definition, “The ratio of maximum radiation intensity of the subject antenna to the radiation intensity of an isotropic or reference antenna, radiating the same total power is called the directivity.” An Antenna radiates power, but the direction in which it radiates matters much. The antenna, whose performance is being observed, is termed as subject antenna. Its radiation intensity is focused in a particular direction, while it is transmitting or receiving. Hence, the antenna is said to have its directivity in that particular direction. The ratio of radiation intensity in a given direction from an antenna to the radiation intensity averaged over all directions, is termed as directivity. If that particular direction is not specified, then the direction in which maximum intensity is observed, can be taken as the directivity of that antenna. The directivity of a non-isotropic antenna is equal to the ratio of the radiation intensity in a given direction to the radiation intensity of the isotropic source. Mathematical Expression The radiated power is a function of the angular position and the radial distance from the circuit. Hence, it is expressed by considering both the terms θ and Ø. $$Directivity = frac{Maximum radiation intensity of subject antenna}{Radiation intensity of an isotropic antenna}$$ $$D = frac{phi(theta,phi)_{max}(from subject antenna)}{phi_{0}(from an isotropic antenna) }$$ Where ${phi(theta,phi)_{max}}$ is the maximum radiation intensity of subject antenna. ${phi_{0}}$ is the radiation intensity of an isotropic antenna (antenna with zero losses). Aperture Efficiency According to the standard definition, “Aperture efficiency of an antenna, is the ratio of the effective radiating area (or effective area) to the physical area of the aperture.” An antenna has an aperture through which the power is radiated. This radiation should be effective with minimum losses. The physical area of the aperture should also be taken into consideration, as the effectiveness of the radiation depends upon the area of the aperture, physically on the antenna. Mathematical Expression The mathematical expression for aperture efficiency is as follows − $$varepsilon_{A} = frac{A_{eff}}{A_{p}}$$ where $varepsilon_{A}$ is Aperture Efficiency. ${A_{eff}}$ is effective area. ${A_{p}}$ is physical area. Antenna Efficiency According to the standard definition, “Antenna Efficiency is the ratio of the radiated power of the antenna to the input power accepted by the antenna.” Simply, an Antenna is meant to radiate power given at its input, with minimum losses. The efficiency of an antenna explains how much an antenna is able to deliver its output effectively with minimum losses in the transmission line. This is otherwise called as Radiation Efficiency Factor of the antenna. Mathematical Expression The mathematical expression for antenna efficiency is given below − $$eta_{e} = frac{P_{rad}}{P_{input}}$$ Where $eta_{e}$is the antenna efficiency. ${P_{rad}}$ is the power radiated. ${P_{input}}$ is the input power for the antenna. Gain According to the standard definition, “Gain of an antenna is the ratio of the radiation intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically.” Simply, gain of an antenna takes the directivity of antenna into account along with its effective performance. If the power accepted by the antenna was radiated isotropically (that means in all directions), then the radiation intensity we get can be taken as a referential. The term antenna gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Gain is usually measured in dB. Unlike directivity, antenna gain takes the losses that occur also into account and hence focuses on the efficiency. Mathematical Expression The equation of gain, G is as shown below. $$G = eta_{e}D$$ Where G is gain of the antenna. $eta_{e}$is the antenna’s efficiency. D is the directivity of the antenna. Units The unit of gain is decibels or simply dB. Learning working make money

Antenna Theory – Full-Wave Dipole 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 transmission dipole. Radiation Pattern As there is no radiation pattern, no directivity and no gain, the Full wave dipole is seldom used as an antenna. Which means, though the antenna radiates, it is just some heat dissipation, which is a wastage of power. Disadvantages The following are the disadvantages of full-wave dipole antenna. Heat dissipation Wastage of power No radiation pattern No directivity and no gain Due to these drawbacks, the full-wave dipole is seldom used. Learning working make money

Antenna Theory – Fundamentals A person, who needs to convey a thought, an idea or a doubt, can do so by voice communication. The following illustration shows two individuals communicating with each other. Here, communication takes place through sound waves. However, if two people want to communicate who are at longer distances, then we have to convert these sound waves into electromagnetic waves. The device, which converts the required information signal into electromagnetic waves, is known as an Antenna. What is an Antenna ? An Antenna is a transducer, which converts electrical power into electromagnetic waves and vice versa. An Antenna can be used either as a transmitting antenna or a receiving antenna. A transmitting antenna is one, which converts electrical signals into electromagnetic waves and radiates them. A receiving antenna is one, which converts electromagnetic waves from the received beam into electrical signals. In two-way communication, the same antenna can be used for both transmission and reception. Antenna can also be termed as an Aerial. Plural of it is, antennae or antennas. Now-adays, antennas have undergone many changes, in accordance with their size and shape. There are many types of antennas depending upon their wide variety of applications. Following pictures are examples of different types of Antennas. In this chapter, you are going to learn the basic concepts of antenna, specifications and different types of antennas. Need of Antenna In the field of communication systems, whenever the need for wireless communication arises, there occurs the necessity of an antenna. Antenna has the capability of sending or receiving the electromagnetic waves for the sake of communication, where you cannot expect to lay down a wiring system. The following scenario explains this. Scenario In order to contact a remote area, the wiring has to be laid down throughout the whole route along the valleys, the mountains, the tedious paths, the tunnels etc., to reach the remote location. The evolution of wireless technology has made this whole process very simple. Antenna is the key element of this wireless technology. In the above image, the antennas help the communication to be established in the whole area, including the valleys and mountains. This process would obviously be easier than laying a wiring system throughout the area. Radiation Mechanism The sole functionality of an antenna is power radiation or reception. Antenna (whether it transmits or receives or does both) can be connected to the circuitry at the station through a transmission line. The functioning of an antenna depends upon the radiation mechanism of a transmission line. A conductor, which is designed to carry current over large distances with minimum losses, is termed as a transmission line. For example, a wire, which is connected to an antenna. A transmission line conducting current with uniform velocity, and the line being a straight one with infinite extent, radiates no power. For a transmission line, to become a waveguide or to radiate power, has to be processed as such. If the power has to be radiated, though the current conduction is with uniform velocity, the wire or transmission line should be bent, truncated or terminated. If this transmission line has current, which accelerates or decelerates with a timevarying constant, then it radiates the power even though the wire is straight. The device or tube, if bent or terminated to radiate energy, then it is called as waveguide. These are especially used for the microwave transmission or reception. This can be well understood by observing the following diagram − The above diagram represents a waveguide, which acts as an antenna. The power from the transmission line travels through the waveguide which has an aperture, to radiate the energy. Basic Types of Antennas Antennas may be divided into various types depending upon − The physical structure of the antenna. The frequency ranges of operation. The mode of applications etc. Physical structure Following are the types of antennas according to the physical structure. You will learn about these antennas in later chapters. Wire antennas Aperture antennas Reflector antennas Lens antennas Micro strip antennas Array antennas Frequency of operation Following are the types of antennas according to the frequency of operation. Very Low Frequency (VLF) Low Frequency (LF) Medium Frequency (MF) High Frequency (HF) Very High Frequency (VHF) Ultra High Frequency (UHF) Super High Frequency (SHF) Micro wave Radio wave Mode of Applications Following are the types of antennas according to the modes of applications − Point-to-point communications Broadcasting applications Radar communications Satellite communications Learning working make money