Category: antenna Theory

Antenna Theory – Collinear Array A Collinear array consists of two or more half-wave dipoles, which are placed end to end. These antennas are placed on a common line or axis, being parallel or collinear. The maximum radiation in these arrays is broad side and perpendicular to the line of array. These arrays are also called as broad cast or Omni-directional arrays. Frequency range The frequency range in which the collinear array antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands. Construction of Array These collinear arrays are uni-directional antennas having high gain. The main purpose of this array is to increase the power radiated and to provide high directional beam, by avoiding power loss in other directions. The above images show the pictures of collinear arrays. In figure 1, it is seen that collinear array is formed using folded dipoles, while in figure 2, the collinear array is formed by normal dipoles. Both types are half-wave dipoles used commonly. Radiation Pattern The radiation pattern of these collinear arrays is similar to that of a single dipole, but the array pattern of increasing number of dipoles, makes the difference. The radiation pattern of collinear array when made using two elements, three elements and four elements respectively are shown in the figure given above. The broad side array also has the same pattern, in which the direction of maximum radiation is perpendicular to the line of antenna. Advantages The following are the advantages of collinear array antennas − Use of array reduces the broad ends and increases the directivity Minor lobes are minimised Wastage of power is reduced Disadvantages The following are the disadvantages of collinear array antennas − Displacement of these antennas is a difficult task Used only in outdoor areas Applications The following are the applications of collinear array antennas − Used for VHF and UHF bands Used in two-way communications Used also for broadcasting purposes Learning working make money

Antenna Theory – Parasitic Array The antenna arrays as seen above, are used for the improvement of gain and directivity. A parasitic element is an element, which depends on other’s feed. It does not have its own feed. Hence, in this type of arrays we employ such elements, which help in increasing the radiation indirectly. These parasitic elements are not directly connected to the feed. The above image shows an example of a parasitic array. The mesh structure seen in the picture, is nothing but a set of reflectors. These reflectors are not electrically connected. They increase the signal strength by increasing the directivity of the beam. Construction & Working of Parasitic Array Let us look at the important parts of a Parasitic array and how they work. The main parts are − Driven element Parasitic elements Reflector Director Boom Driven element The antennas radiate individually and while in array, the radiation of all the elements sum up to form the radiation beam. All the elements of the array need not be connected to the feed. The dipole that is connected to the feed is known as a driven element. Parasitic Elements The elements, which are added do not possess an electrical connection between them to the driven element or the feed. They are positioned so that they lie in the induction field of the driven element. Hence, they are known as parasitic elements. Reflector If one of the parasitic element, which is 5% longer than driven element, is placed close to the driven element is longer, then it acts as a concave mirror, which reflects the energy in the direction of the radiation pattern rather than its own direction and hence is known as a reflector. Director A parasitic element, which is 5% shorter than the driven element, from which it receives energy, tends to increase radiation in its own direction and therefore, behaves like convergent convex lens. This element is called as a director. A number of directors are placed to increase the directivity. Boom The element on which all these are placed is callled a boom. It is a non-metallic structure which provides insulation, so that there will not be any short circuit between the other elements of the array. These are all the main elements, which contribute the radiation. This can be better understood with the help of a diagram The image shown above is that of a parasitic array, which shows the parts of parsitic array such as the driven element, the directors and the reflector. The feed is given through the feeder. The arrays are used at frequencies ranging from 2MHz to several GHz. These are especially used to get high directivity, and better forward gain with a uni-directional. The most common example of this type of array is the Yagi-Uda antenna. Quad antenna may also be quoted as another example. Learning working make money

Antenna Theory – Quick Guide 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 Antenna Theory – Basic Parameters The basic communication parameters are discussed in this chapter to have a better idea about the wireless communication using antennas. The wireless communication is done in the form of waves. Hence, we need to have a look at the properties of waves in the communications. In this chapter, we are going to discuss about the following parameters − Frequency Wavelength Impedance matching VSWR & reflected power Bandwidth Percentage bandwidth Radiation intensity Now, let us learn them in detail. Frequency According to the standard definition, “The rate of repetition of a wave over a particular period of time, is called as frequency.” Simply, frequency refers to the process of how often an event occurs. A periodic wave repeats itself after every ‘T’ seconds (time period). Frequency of periodic wave is nothing but the reciprocal of time period (T). Mathematical Expression Mathematically, it is written as shown below. $$f = frac{1}{T}$$ Where f is the frequency of periodic wave. T is the time period at which the wave repeats. Units The unit of frequency is Hertz, abbreviated as Hz. The figure given above represents a sine wave, which is plotted here for Voltage in millivolts against time in milliseconds. This wave repeats after every 2t milliseconds. So, time period, T=2t milliseconds and frequency, $f = frac{1}{2T}KHz$ Wavelength According to the standard definition, “The distance between two consecutive maximum points (crests) or between two consecutive minimum points (troughs) is known as the wavelength.” Simply, the distance between two immediate positive peaks

Antenna Theory – Aperture 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 Let us now have a look at these types of aperture antennas. Waveguide Antenna A Waveguide is capable of radiating energy when excited at one end and opened at the other end. The radiation in wave guide is greater than a two-wire transmission line. Frequency Range The operational frequency range of a wave guide is around 300MHz to 300GHz. This antenna works in UHF and EHF frequency ranges. The following image shows a waveguide. This waveguide with terminated end, acts as an antenna. But only a small portion of the energy is radiated while a large portion of it gets reflected back in the open circuit. It means VSWR (voltage standing wave ratio, discussed in basic parameters chapter) value increases. The diffraction around the waveguide provides poor radiation and non-directive radiation pattern. Radiation Pattern The radiation of waveguide antenna is poor and the pattern is non-directive, which means omni-directional. An omni-directional pattern is the one which has no certain directivity but radiates in all directions, hence it is called as non-directive radiation pattern. The above figure shows a top section view of an omni-directional pattern, which is also called as non-directional pattern. The two-dimensional view is a figure-of-eight pattern, as we already know. Advantages The following are the advantages of Aperture antenna − Radiation is greater than two-wire transmission line Radiation is Omni-directional Disadvantages The following are the disadvantages of Aperture antenna − VSWR increases Poor radiation Applications The following are the applications of Aperture antenna − Micro wave applications Surface search radar applications The waveguide antenna has to be further modified to achieve better performance, which results in the formation of Horn antenna. Learning working make money

Antenna Theory – Types of Propagation In this chapter, let us go through different interesting topics such as the properties of radio waves, the propagation of radio waves and their types. Radio Waves 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 key features are − Since radio transmission is Omni directional in nature, the need to physically align the transmitter and receiver does not arise. The frequency of the radio wave determines many of the characteristics of the transmission. At low frequencies, the waves can pass through obstacles easily. However, their power falls with an inverse-squared relation with respect to the distance. The higher frequency waves are more prone to absorption by rain drops and they get reflected by obstacles. Due to the long transmission range of the radio waves, interference between transmissions is a problem that needs to be addressed. In the VLF, LF and MF bands the propagation of waves, also called as ground waves follow the curvature of the earth. The maximum transmission ranges of these waves are of the order of a few hundred kilometers. They are used for low bandwidth transmissions such as Amplitude Modulation (AM) radio broadcasting. The HF and VHF band transmissions are absorbed by the atmosphere, near the Earth”s surface. However, a portion of the radiation, called the sky wave, is radiated outward and upward to the ionosphere in the upper atmosphere. The ionosphere contains ionized particles formed due to the Sun”s radiation. These ionized particles reflect the sky waves back to the Earth. A powerful sky wave may be reflected several times between the Earth and the ionosphere. Sky waves are used by amateur ham radio operators and for military communication. Radio Wave Propagation In Radio communication systems, we use wireless electromagnetic waves as the channel. The antennas of different specifications can be used for these purposes. The sizes of these antennas depend upon the bandwidth and frequency of the signal to be transmitted. The mode of propagation of electromagnetic waves in the atmosphere and in free space may be divided in to the following three categories − Line of sight (LOS) propagation Ground wave propagation Sky wave propagation In ELF (Extremely low frequency) and VLF (Very low frequency) frequency bands, the Earth, and the ionosphere act as a wave guide for electromagnetic wave propagation. In these frequency ranges, communication signals practically propagate around the world. The channel band widths are small. Therefore, the information is transmitted through these channels has slow speed and confined to digital transmission. Line of Sight (LOS) Propagation Among the modes of propagation, this line-of-sight propagation is the one, which we commonly notice. In the line-of-sight communication, as the name implies, the wave travels a minimum distance of sight. Which means it travels to the distance up to which a naked eye can see. Now what happens after that? We need to employ an amplifier cum transmitter here to amplify the signal and transmit again. This is better understood with the help of the following diagram. The figure depicts this mode of propagation very clearly. The line-of-sight propagation will not be smooth if there occurs any obstacle in its transmission path. As the signal can travel only to lesser distances in this mode, this transmission is used for infrared or microwave transmissions. Ground Wave Propagation Ground wave propagation of the wave follows the contour of earth. Such a wave is called as direct wave. The wave sometimes bends due to the Earth’s magnetic field and gets reflected to the receiver. Such a wave can be termed as reflected wave. The above figure depicts ground wave propagation. The wave when propagates through the Earth’s atmosphere is known as ground wave. The direct wave and reflected wave together contribute the signal at the receiver station. When the wave finally reaches the receiver, the lags are cancelled out. In addition, the signal is filtered to avoid distortion and amplified for clear output. Sky Wave Propagation Sky wave propagation is preferred when the wave has to travel a longer distance. Here the wave is projected onto the sky and it is again reflected back onto the earth. The sky wave propagation is well depicted in the above picture. Here the waves are shown to be transmitted from one place and where it is received by many receivers. Hence, it is an example of broadcasting. The waves, which are transmitted from the transmitter antenna, are reflected from the ionosphere. It consists of several layers of charged particles ranging in altitude from 30- 250 miles above the surface of the earth. Such a travel of the wave from transmitter to the ionosphere and from there to the receiver on Earth is known as Sky Wave Propagation. Ionosphere is the ionized layer around the Earth’s atmosphere, which is suitable for sky wave propagation. Learning working make money

Antenna Theory – Antenna Arrays An antenna, when individually can radiate an amount of energy, in a particular direction, resulting in better transmission, how it would be if few more elements are added it, to produce more efficient output. It is exactly this idea, which led to the invention of Antenna arrays. An antenna array can be better understood by observing the following images. Observe how the antenna arrays are connected. An antenna array is a radiating system, which consists of individual radiators and elements. Each of this radiator, while functioning has its own induction field. The elements are placed so closely that each one lies in the neighbouring one’s induction field. Therefore, the radiation pattern produced by them, would be the vector sum of the individual ones. The following image shows another example of an antenna array. The spacing between the elements and the length of the elements according to the wavelength are also to be kept in mind while designing these antennas. 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. Advantages The following are the advantages of using antenna arrays − The signal strength increases High directivity is obtained Minor lobes are reduced much High Signal-to-noise ratio is achieved High gain is obtained Power wastage is reduced Better performance is obtained Disadvantages The following are the disadvantages of array antennas − Resistive losses are increased Mounting and maintenance is difficult Huge external space is required Applications The following are the applications of array antennas − Used in satellite communications Used in wireless communications Used in military radar communications Used in the astronomical study Types of Arrays The basic types of arrays are − Collinear array Broad side array End fire array Parasitic array Yagi-Uda array Log-peroidic array Turnstile array Super-turnstile array We will discuss these arrays in the coming chapters. Learning working make money

Antenna Theory – Inverted V-Antenna In the previous chapter, we have studied V-antenna. Its operating frequency is limited. This can be modified by using another antenna, which is a non-resonant antenna or a travelling wave antenna. A travelling wave antenna produces no standing wave, as discussed previously. Frequency Range The frequency range of operation of an inverted vee antenna (or V-antenna) is around 3 to 30 MHz. This antenna works in high frequency range. Construction & Working of Inverted V-Antenna A travelling wave antenna, used in high-frequency band is an inverted V-antenna. This inverted V-antenna is easily installed on a non-conducting mast. Take a look at the following image. It shows an inverted V-antenna mounted on a roof top. The maximum radiation for an inverted V-antenna is at its center. It is similar to a halfwave dipole antenna. The antenna is placed in the shape of an inverted V, with its two transmission lines or legs bent towards the ground making 120° or 90° angle between them. The center of the antenna should not be higher than λ/4. 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. Radiation Pattern The radiation pattern of inverted V-antenna is uni-directional pattern, as no standing waves are formed here. It can be clearly understood by the radiation pattern shown below. The figure illustrates the radiation pattern of an inverted V-antenna. Primary radiated field is shown along with the fields when the tilt angles are 120˚ and 90˚ in the figure given above. The gain and directivity are improved by having an array of antennas. Advantages The following are the advantages of inverted V-antenna − Occupies less horizontal place No standing waves are formed High gain Disadvantages The following are the disadvantages of inverted V-antenna − It has considerable undesired minor lobes Minor lobes create horizontally polarized waves Applications The following are the applications of inverted V-antenna − Used in tuned circuit applications Used in radio communications Used in commercial applications After the V-antenna and inverted V-antenna, another important long wire antenna is the Rhombic antenna. It is a combination of two V-antennas. This is discussed in the next chapter. Learning working make money

Antenna Theory – V-Antennas A better version of long-wire antennas is the V-Antenna. This antenna is formed by arranging the long wire in a V-shaped pattern. The end wires are called as legs. This antenna is a bi-directional resonant antenna. Frequency Range The frequency range of operation of V-antenna is around 3 to 30 MHz. This antenna works in high frequency range. Construction & Working of V-Antennas Two long wires are connected in the shape of V to make a V-antenna. The two long wires are excited with 180˚ out of phase. As the length of these wires increases, the gain and directivity also increases. The following figure shows a V-antenna with the transmission line impedance z and the lengh of the wire λ/2, making an angle Φm with the axis, which is called as apex angle. The gain achieved by V-antenna is higher than normal single long wire antenna. The gain in this V-formation is nearly twice compared to the single long wire antenna, which has a length equal to the legs of V-antenna. If wide range of radiation is to be achieved, the apex angle should have an average value between higher and lower frequencies in terms of the number of λ/2 in each leg. Radiation Pattern The radiation pattern of a V-antenna is bi-directional. The radiation obtained on each transmission line is added to obtain the resultant radiation pattern. This is well explained in the following figure − The figure shows the radiation pattern of V-antenna. The two transmission lines forming V-pattern are AA’ and BB’. The patterns of individual transmission lines and the resultant pattern are shown in the figure. The resultant pattern is shown along the axis. This pattern resembles the broad-side array. 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 directivity is further increased by adding the array of V-antennas. Advantages The following are the advantages of V-antenna − Construction is simple High gain Low manufacturing cost Disadvantages The following are the disadvantages of V-antenna − Standing waves are formed The minor lobes occurred are also strong Used only for fixed frequency operations Applications The following are the applications of V-antenna − Used for commercial purposes Used in radio communications Learning working make money

Antenna Theory – Beam Width 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 radiation pattern’s origin and the half power points on the major lobe, on both the sides, the angle between those two vectors is termed as HPBW, half power beam width. This can be well understood with the help of the following diagram. The figure shows half-power points on the major lobe and HPBW. Mathematical Expression The mathematical expression for half power beam width is − $$Half: power: Beam :with=70lambda_{/D} $$ Where $lambda$ is wavelength (λ = 0.3/frequency). D is Diameter. Units The unit of HPBW is radians or degrees. First Null Beam Width According to the standard definition, “The angular span between the first pattern nulls adjacent to the main lobe, is called as the First Null Beam Width.” Simply, FNBW is the angular separation, quoted away from the main beam, which is drawn between the null points of radiation pattern, on its major lobe. Indication of FNBW Draw tangents on both sides starting from the origin of the radiation pattern, tangential to the main beam. The angle between those two tangents is known as First Null Beam Width (FNBW). This can be better understood with the help of the following diagram. The above image shows the half power beam width and first null beam width, marked in a radiation pattern along with minor and major lobes. Mathematical Expression The mathematical expression of First Null Beam Width is $$FNBW = 2 HPBW$$ $$FNBW:2left ( 70lambda/D right ):=140lambda/D$$ Where $lambda$ is wavelength (λ = 0.3/frequency). D is Diameter. Units The unit of FNBW is radians or degrees. Effective Length & Effective Area Among the antenna parameters, the effective length and effective area are also important. These parameters help us to know about the antenna’s performance. Effective length Antenna Effective length is used to determine the polarization efficiency of the antenna. Definition− “The Effective length is the ratio of the magnitude of voltage at the open terminals of the receiving antenna to the magnitude of the field strength of the incident wave front, in the same direction of antenna polarization.” When an incident wave arrives at the antenna’s input terminals, this wave has some field strength, whose magnitude depends upon the antenna’s polarization. This polarization should match with the magnitude of the voltage at receiver terminals. Mathematical Expression The mathematical expression for effective length is − $$l_{e} = frac{V_{oc}}{E_{i}}$$ Where $l_{e}$ is the effective length. $V_{oc}$ is open-circuit voltage. $E_{i}$ is the field strength of the incident wave. Effective area 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 whole area of an antenna while receiving, confronts the incoming electromagnetic waves, whereas only some portion of the antenna, receives the signal, known as the effective area. Only some portion of the received wave front is utilized because some portion of the wave gets scattered while some gets dissipated as heat. Hence, without considering the losses, the area, which utilizes the maximum power obtained to the actual area, can be termed as effective area. Effective area is represented by $A_{eff}$. Learning working make money

Antenna Theory – Reciprocity 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 directivities are same whether calculated from transmitting antenna’s power or receiving antenna’s power. Equality of Effective lengths The value of maximum effective aperture is same for both transmitting and receiving antennas. Equality in the lengths of both transmitting and receiving antennas is maintained according to the value of the wavelength. Equality in Antenna Impedances The output impedance of a transmitting antenna and the input impedance of a receiving antenna are equal in an effective communication. These properties will not change though the same antenna is operated as a transmitter or as a receiver. Hence, the property of reciprocity is followed. Learning working make money