High Frequency bands comprise of X-rays and Gamma rays. Let us look at the pattern of energy radiation. It is sometimes referred to as a point source . The power from the transmission line travels through the waveguide which has an aperture, to radiate the energy. The following figure shows a periodic waveform. The above diagram represents a waveguide, which acts as an antenna. It may be a dipole and horn or any other type of antenna. Fig 2 shows the dipole with negative charges induced. The working of Slot Antenna can be easily understood through Babinet’s principle of optics. In terms of dB, it can be called as 0dB gain (zero loss). The following image shows a Parabolic Reflector Antenna. This is the upper layer of the Earth’s atmosphere, where ionization is appreciable. We need to employ an amplifier cum transmitter here to amplify the signal and transmit again. Definition − “Poynting vector gives the rate of energy transfer per unit area”, “The energy that a wave carries per unit time per unit area is given by the Poynting vector.”, The quantity that is used to describe the power associated with the electromagnetic waves is the instantaneous Poynting vector, which is defined as. According to the standard definition, “The amount of power that an isotropical antenna radiates to produce the peak power density observed in the direction of maximum antenna gain, is called as Equivalent Isotropic Radiated Power.”. The colors range from: Green – Indicates a Strong Signal. That means, the radiated power from the antenna when plotted, is expressed in terms of electric field, E (v/m). If the specifications given above are followed, one can design an Yagi-Uda antenna. This antenna works in HF and VHF ranges. The maximum transmission ranges of these waves are of the order of a few hundred kilometers. The lens antennas use the curved surface for both transmission and reception. The function of radial distance is also indicated as Φ. The absolute bandwidth is given by- fH - fL. It consists of several layers of charged particles ranging in altitude from 30- 250 miles above the surface of the earth. For the frequencies above 30 MHz, the sky wave propagation exists. This clearly indicates the three co-ordinates (x, y, z). The image of a helix antenna detailing the antenna parts is shown above. Usually a wave guide horn antenna is used as a feed radiator for the paraboloid reflector antenna. If a Parabolic Reflector antenna is used for transmitting a signal, the signal from the feed, comes out of a dipole or a horn antenna, to focus the wave on to the parabola. A Horn antenna may be considered as a flared out wave guide, by which the directivity is improved and the diffraction is reduced. The radiation power is 3dB below the maximum radiation of a half wave dipole radiating the same power. The predominant modes of operation of a helical antenna are −. Dispersion is the phenomenon of spreading of a burst of electromagnetic energy during propagation. Hence, the electrons again move towards the charge. Small loop antennas are mainly of two types −. But only a small portion of the energy is radiated while a large portion of it gets reflected back in the open circuit. 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), Metallic or artificial dielectric type of lens, The following are the advantages of Lens antenna −. Hence, the antenna is said to have its directivity in that particular direction. When employed with others, used for wide variety of applications. Due to resonance, both the arrays display narrower beam and high directivity. Definition − “Effective area is the area of the receiving antenna, which absorbs most of the power from the incoming wave front, to the total area of the antenna, which is exposed to the wave front.”. The power when radiated from the antenna has its effect in the near and far field regions. These dipoles are excited 90° out of phase with each other. 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. Displacement and adjustment of antenna is a hassle. The typical figure-ofeight patterns were combined to produce a nearly circular pattern. During night time, this D layer almost vanishes out and the F1 and F2 layers combine together to form F layer. If the radiation is equal in all directions, then it is known as isotropic radiation. These parameters help us to know about the antenna’s performance. The polarization of these turnstile antennas depend upon their mode of operation. It is seen that two half-wave dipoles are joined to make a full-wave dipole. Impedance matching may not be always good, Used in Aircraft receivers for direction finding, Antenna is larger and requires more space, Efficiency decreases with number of turns, Radiation is greater than two-wire transmission line, Designing of flare angle, decides the directivity, Flare angle and length of the flare should not be very small, It can be fabricated and concealed within metallic objects, It can provide covert communications with a small transmitter, Use of array reduces the broad ends and increases the directivity, Displacement of these antennas is a difficult task. It is a measure of depth to which an EM wave can penetrate. In the above figure, the source at the focal point, at a focal distance from the lens, gets collimated in the plane wave front. One can simply understand the function and directivity of an antenna by having a look at its radiation pattern. The frequency range of operation of helical antenna is around 30MHz to 3GHz. Thus, the wave gets reflected twice. The radiation of energy when done through such a bent wire, the end of such transmission line is termed as dipole or dipole antenna. The dipole or the horn antenna, which acts as the receiver antenna at its feed, receives this signal, to convert it into electric signal and forwards it to the receiver circuitry. This can be well understood with the help of the following diagram. Outdoor management can be difficult when antenna size increases. An Antenna is a transducer, which converts electrical power into electromagnetic waves and vice versa. After hitting top surface, they are reflected to the focal point. If the radiated energy of an antenna is made to concentrate on one side or a particular direction, where the radiation is equivalent to that antenna’s isotropic radiated power, such a radiation would be termed as EIRP i.e. The feeder is the folded dipole. There is no radiation in the right angles to the plane of the array because of cancellation. The image of the model of a lens antenna is shown. A normal glass lens works on the principle of refraction. Both the positive and negative peaks of the wave induce positive and negative voltages respectively. These wires are generally used with capacitor plates on both sides, where low mutual coupling is needed. This phenomenon is called convergence. Because of the capacitor plates, we can say that uniform distribution of current is present. The basic communication parameters are discussed in this chapter to have a better idea about the wireless communication using antennas. Axial or end-fire or beam mode of radiation. Both types are half-wave dipoles used commonly. Such a type of feed is known as Gregorian feed. This can be better understood with the help of a diagram. 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.”. The above images show circular and square loop antennas. We will discuss about them in the next chapter. It can be termed as the Wavelength. As the length of these wires increases, the gain and directivity also increases. The following figure shows the radiation pattern of half-wave folded dipole antenna, which is Omni-directional pattern. A considerable amount of energy is wasted even here. This antenna works in UHF range. HELIX ANTENNA A conducting wire in the form of screw thread fed by power source. This process would obviously be easier than laying a wiring system throughout the area. These parameters help us to know about the beam specifications. The standard definition of a parabola is - Locus of a point, which moves in such a way that its distance from the fixed point (called focus) plus its distance from a straight line (called directrix) is constant. This opening which is an aperture, makes it an Aperture antenna. When a wave is transmitted or received, it may be done in different directions. To have a greater directivity, an array can be formed by using these patch antennas. Though the area is away from the antenna, it is considered effective, as the radiation intensity is still high there. All of the rays are sent in parallel to the plane wave front. Where, ‘i’ represents the isotropic radiation. Due to this, more collisions occur and ionization of particles is minimum and not constant. An antenna works effectively at its resonant frequency, which occurs at its resonant length. To have a better analysis regarding the radiation of an antenna, a referential point is necessary. If another V-antenna is added to this antenna and fed with 90˚ phase difference, then the resultant pattern would be end-fire, doubling the power gain. The field patterns are plotted as a function of electric and magnetic fields. The array consists of dipoles of different lengths and spacing, which are fed from a two-wire transmission line. Such an antenna cannot physically be built, but it serves as a useful mathematical reference, as such an antenna has no gain, or a gain of 0 dBi. The major part of the radiated field, which covers a larger area, is the main lobe or major lobe. Along with this technique, we have another type of feed given to the paraboloid reflector antenna, called as Cassegrain feed. The dipole antenna is cut and bent for effective radiation. Although an isotropic antenna cannot be constructed, it is sometimes useful to compare actual antennas to it. Shaped beam pattern − The beam, which is non-uniform and patternless is known as shaped beam. The emission of radiation to a maximum possible extent is nothing but the radiation intensity. The antenna arrays as seen above, are used for the improvement of gain and directivity. The mathematical expression for beam efficiency is −. Here, communication takes place through sound waves. A rectangular slot is made on the conducting sheet. This is better understood with the help of the following figure. The individual national governments allocate spectrum for applications such as AM/FM radio broadcasting, television broadcasting, mobile telephony, military communication, and government usage. Hence, ionization is not proper. Neither of them is used for reception, because the necessity of covering a range of frequencies is needed for any kind of reception. The angle made by one of the legs with the axis of the antenna, is known as the tilt angle and is denoted by θ. Waveguide is an example of aperture antenna. Now the electrons tend to move towards the charge. Figure 1 is the radiation pattern for a single array, while figures 2, 3, and 4 represent the radiation pattern for multiple arrays. Used where a single-frequency application is needed. Another method of using long wire is by bending and making the wire into a loop shaped pattern and observing its radiational parameters. Antenna can also be termed as an Aerial. Its radiation intensity is focused in a particular direction, while it is transmitting or receiving. These non-resonant antennas are non-periodic in nature. According to the standard definition, “The approximate value of impedance of a transmitter, when equals the approximate value of the impedance of a receiver, or vice versa, it is termed as Impedance matching.”. This can be understood by observing the image of a slot antenna. Length of the antenna matches with size and directivity. The radiation intensity P (θ, Ø) should be maintained constant and maximum throughout the solid beam angle ΩA, its value being zero elsewhere. Hence, in this type of arrays we employ such elements, which help in increasing the radiation indirectly. a) Half power beam width (HPBW) Figure C shows the resultant combined pattern of four bays showing better directivity. Hence, the frequency and length are inversely proportional to each other. If multi-turn ferrite core constructions are used, then high radiation resistance can be achieved. This is mostly used in radio receivers. If the virtual height is known, the angle of incidence can be found. With the change in operation frequency, the active region shifts among the elements and hence all the elements will not be active only on a single frequency. The layer below ionosphere has higher amount of air particles and lower UV radiation. This inverted V-antenna is easily installed on a non-conducting mast. According to the standard definition, “An arrangement in which the principal direction of radiation is perpendicular to the array axis and also to the plane containing the array element” is termed as the broad side array. The higher and lower frequencies are denoted as fH and fL respectively. If loop antenna have to act as transmitting antenna, then this impedance mis-match would definitely be a problem. As the signal can travel only to lesser distances in this mode, this transmission is used for infrared or microwave transmissions. If flaring is done on both the E & H walls of a rectangular waveguide, then pyramidal horn antenna is produced. 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. For example, a wire, which is connected to an antenna. They are positioned so that they lie in the induction field of the driven element. This antenna is a bi-directional resonant antenna. To improve the radiation efficiency and directivity of the beam, the wave guide should be provided with an extended aperture to make the abrupt discontinuity of the wave into a gradual transformation. The functioning of an antenna depends upon the radiation mechanism of a transmission line. The open end, which has high impedance, matches with the impedance of free space to provide better radiation. Beam can be moved angularly with espect to the axis. In hog-horn antenna, a parabolic cylinder is joined to pyramidal horn, where the beam reaches apex of the horn. The figures given above show the radiation pattern of an isotropic or Omni-directional pattern. The energy of the beam when slowly transform into radiation, the losses are reduced and the focussing of the beam improves. Due to the long transmission range of the radio waves, interference between transmissions is a problem that needs to be addressed. This type of horn antenna, flares out in only one direction. Low voltage and high current. The physical structure and electrical characteristics, when observed, are repetitive in nature. Reactive near-field region and Radiating near-field region – both termed as nearfield. This acts as a half-wave dipole antenna. The frequency range of operation of V-antenna is around 3 to 30 MHz. All the information you need to select and point your antenna. It has a doughnut-shaped pattern when viewed in 3D and a figure-of-eight pattern when viewed in 2D. The figure illustrates the radiation pattern of an inverted V-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 radiation pattern of the radiated wave is not matched with the load impedance in this type of antenna. A point source has negligible extent, distinguishing it from other source geometries. The length of the dipole is half of the wavelength. This is mostly used in low frequency receivers. Similarly, flaring in the direction of Magnetic vector, produces the sectorial H-plane horn. The ionosphere has good composition of UV radiation and average air density that does not affect the ionization. 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). If the radiation is equal in all directions, then it is known as isotropic radiation. It is especially prevalent in wireline transmissions such as an optical fiber. These paths include a number of hops. Hence, the property of reciprocity is followed. The improvement of radiation pattern of an antenna is always assessed using the isotropic radiation of that antenna. The point source is an example of isotropic radiator. The following are the advantages of Turnstile antennas −, Super-turnstile produces high-gain output, The following is the disadvantage of Turnstile antennas −. The broad side array is strongly directional at right angles to the plane of the array. the directivities are same whether calculated from transmitting antenna’s power or receiving antenna’s power. Equality in the lengths of both transmitting and receiving antennas is maintained according to the value of the wavelength. It is not a good idea to meander the loop so as to reduce the size, as that increases capacitive effects and results in low efficiency. The field distribution can be quantifying in terms of field intensity is referred to as field pattern. Take a look at the following image. 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 ΩA and was zero elsewhere.”. The center rod like structure on which the elements are mounted is called as boom. This pattern is uni-directional, while it can be made bi-directional by removing the terminating resistance. In this chapter, we shall discuss about another important factor in the radiation pattern of an antenna, known as beam width. Pencil-beam pattern − The beam has a sharp directional pencil shaped pattern. This type of antennas are termed as loop antennas. QUE > WHAT IS PATTERN MULTIPLICATION PRINCIPLE ? In these antennas, the length of the antenna and frequency are proportional to each other. However, if two people want to communicate who are at longer distances, then we have to convert these sound waves into electromagnetic waves. The following image shows another example of an antenna array. In all the regions, which are non-collinear with the beam, the above two screens, in figures 1 & 2, produce the same diffraction pattern. the total wire, equals the full wavelength λ, then it is called as full wave dipole. Gain of the antenna is best achieved by focusing the antenna in certain direction. Such a movement of signal can be termed as multipath. It has low radiation efficiency due to high losses. Enter the main menu “Settings” > System > Satellites. Parabolic Reflectors are Microwave antennas. The figure given above shows radiation pattern of a dipole antenna. 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.”. It is proportional to the wavelength. Among the antenna parameters, the effective length and effective area are also important. A Log-periodic antenna is that whose impedance is a logarithamically periodic function of frequency. Flaring in the direction of Electric vector produces the sectorial E-plane horn. It is arranged in the form of a rhombus or diamond shape and suspended horizontally above the surface of the earth. This antenna is commonly used with the array type antennas to increase the feed resistance. The linear polarizationof the antenna helps in maintaining the wave in a particular direction, avoiding all the other directions. When we first described a parabolic dish antenna, we put a point source at the focus, so that energy would radiate uniformly in all directions both in magnitude and phase. After the antenna parameters discussed in the previous chapter, another important topic of consideration is the near field and the far field regions of the antenna. The sky wave propagation is well depicted in the above picture. This antenna works in high frequency range. Use the map line-of-sight to tower line to check for potential ground interference (hills, valleys, obstructions). It is placed such that its one of the foci coincides with the focus of the paraboloid. The highest frequency, which is reflected from the ionosphere to the receiver is called as critical frequency, fc. This is mostly used in television receivers. Other types (rectangular, delta, elliptical etc.) Antennas play an important role in the operation of all radio equipment. These antennas are widely used for radio and wireless applications. The Turnstile antenna is another type of array antenna. The ratio of focal length to aperture size (ie., f/D) known as “f over D ratio” is an important parameter of parabolic reflector. Following are the types of antennas according to the modes of applications −. However, these waves are not used practically because of difficulty in modulation and the waves are harmful to living beings. It has a doughnut-shaped pattern when viewed in 3D and a figure-of-eight pattern when viewed in 2D. Resonant Antennas are those for which a sharp peak in the radiated power is intercepted by the antenna at certain frequency, to form a standing wave. In this chapter, the popular and most widely used short-wire antennas were discussed. Antenna Effective length is used to determine the polarization efficiency of the antenna. Thermal noise is due to the thermal agitation of electrons and is uniformly distributed across the frequency spectrum. The broad side array also has the same pattern, in which the direction of maximum radiation is perpendicular to the line of antenna. Hence, we use this linear polarization to improve the directivity of the antenna. It is true that when a HF field exists across a narrow slot in a conducting plane, the energy is radiated. Red – Indicates No Signal. R is the distance between the feed and the dipole. Dipole antenna, Monopole antenna, Helix antenna, Loop antenna, Personal applications, buildings, ships, automobiles, space crafts, Flush-mounted applications, air-craft, space craft, Microwave communication, satellite tracking, radio astronomy, Convex-plane, Concave-plane, Convex-convex, Concaveconcave lenses, Circular-shaped, Rectangularshaped metallic patch above the ground plane. The region next to it is radiating far-field region. This folded dipole is the main element in Yagi-Uda antenna. There can be a number of hops for the signal as it may move back and forth from the ionosphere and earth surface many times. The effect of screen gets nullified. To have a better understanding, consider the following figure, which represents the radiation pattern of a dipole antenna. Quad antenna may also be quoted as another example. In this chapter, we are going to discuss about the following parameters −, According to the standard definition, “The rate of repetition of a wave over a particular period of time, is called as frequency.”. A transmission line conducting current with uniform velocity, and the line being a straight one with infinite extent, radiates no power. 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 mathematical expression for aperture efficiency is as follows −. Cassegrain systems using hyperboloid sub-reflector (concave but the feed being very near to it.). 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. The above figure indicates the radiation pattern of a half wave dipole in both H-plane and V-plane. Inter symbol interference (ISI) occurs more commonly in communication system. However, this isotropic radiation is practically impossible, because every antenna radiates its energy with some directivity. 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). These resultant patterns are known as Horizontal pattern and Vertical pattern respectively. Because the second Fresnel zone begins where the first Fresnel zone ends, the radius of the second Fresnel zone is larger than the radius of the first Fresnel zone. The power patterns are plotted as a function of square of the magnitude of electric and magnetic fields. Antennas are such resonant devices whose impedance if matched, delivers a better output. Where, λ is the wavelength. The radiation pattern of the rhombic antenna is shown in the following figure. It is often modeled using an additive Gaussian model. The cassegrain feed parabolic reflector is mainly used in satellite communications. Gain and radiation pattern are varied according to the requirements. The antennas radiate individually and while in array, the radiation of all the elements sum up, to form the radiation beam, which has high gain, high directivity and better performance, with minimum losses. Get angle and range to tower, signal strength, and frequency band (VHF or UHF). The frequency range in which the turnstile antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands. According to the standard definition, “A band of frequencies in a wavelength, specified for the particular communication, is known as bandwidth.”. These collinear arrays are uni-directional antennas having high gain. The particular frequency within a frequency band, at which the signal strength is maximum, is called as resonant frequency. PQ are the reflected rays where L represents the line directrix on which the reflected points lie (to say that they are being collinear). The figure shows half-power points on the major lobe and HPBW. In Axial mode of operation, the antenna radiates circularly polarized waves along its axis i.e. The frequency range of operation of an inverted vee antenna (or V-antenna) is around 3 to 30 MHz. To know how wider the bandwidth is, either fractional bandwidth or percentage bandwidth has to be calculated. A short dipole is a simple wire antenna. The parasitic elements and the dipole together form this Yagi-Uda antenna. In normal mode of radiation, the radiation field is normal to the helix axis. Isotropic Point Source Radiation Pattern for Isotropic Point Source Antenna radiates equally in all directions Antenna only exists theoretically Therefore, the radiation pattern produced by them, would be the vector sum of the individual ones. It is also called as SWR. The maximum frequency that gets bent and reaches the receiver station with minimum attenuation, can be termed as critical frequency. The radiation intensity when measured nearer to the antenna, differs from what is away from the antenna. As there is no radiation pattern, no directivity and no gain, the Full wave dipole is seldom used as an antenna. They are usually weak at low frequencies below 1GHz. The mathematical expression for half power beam width is −. Multiple Choice Questions and Answers on Antenna & Wave Propagation.Objective Questions and Answers on Antenna & Wave Propagation . The diagram of log-periodic array is given above. The reactance of the input impedance is a function of the radius and length of the dipole. When the electromagnetic wave hits the shape of the parabola, the wave gets reflected onto the feed point. The radiation pattern of this helical antenna is a combination of short dipole and loop antenna. 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. Radio waves are easy to generate and are widely used for both indoor and outdoor communications because of their ability to pass through buildings and travel long distances. The parallel rays obtained are also called as collimated rays. A Waveguide is capable of radiating energy when excited at one end and opened at the other end. The arrows represent directions of radiation. The following figure shows the geometry of parabolic reflector. The point source is an example of isotropic radiator. This D layer is responsible for the day time attenuation of HF waves. 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.

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