Category: antenna Theory

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

Antenna Theory – Near and Far Fields 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 radiation intensity when measured nearer to the antenna, differs from what is away from the antenna. Though the area is away from the antenna, it is considered effective, as the radiation intensity is still high there. Near Field The field, which is nearer to the antenna, is called as near-field. It has an inductive effect and hence it is also known as inductive field, though it has some radiation components. Far field The field, which is far from the antenna, is called as far-field. It is also called as radiation field, as the radiation effect is high in this area. Many of the antenna parameters along with the antenna directivity and the radiation pattern of the antenna are considered in this region only. Field Pattern The field distribution can be quantifying in terms of field intensity is referred to as field pattern. That means, the radiated power from the antenna when plotted, is expressed in terms of electric field, E (v/m). Hence, it is known as field pattern. If it is quantified in terms of power (W), then it is known as power pattern. The graphical distribution of radiated field or power will be as a function of spatial angles (θ, Ø) for far-field. spatial angles (θ, Ø) and radial distance(r) for near-field. The distribution of near and far field regions can be well understood with the help of a diagram. The field pattern can be classified as − Reactive near-field region and Radiating near-field region – both termed as nearfield. Radiating far-field region – simply called as far-field. The field, which is very near to the antenna is reactive near field or non-radiative field where the radiation is not pre-dominant. The region next to it can be termed as radiating near field or Fresnel’s field as the radiation predominates and the angular field distribution, depends on the physical distance from the antenna. The region next to it is radiating far-field region. In this region, field distribution is independent of the distance from antenna. The effective radiation pattern is observed in this region. Learning working make money

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 or two immediate negative peaks is nothing but the length of that wave. It can be termed as the Wavelength. The following figure shows a periodic waveform. The wavelength (λ) and amplitude are denoted in the figure. The higher the frequency, the lesser will be the wavelength and vice versa. Mathematical Expression The formula for wavelength is, $$lambda = frac{c}{f}$$ Where λ is the wavelength c is the speed of light ($3 * 10^{8}$ meters/second) f is the frequency Units The wavelength λ is expressed in the units of length such as meters, feet or inches. The commonly used term is meters. Impedance Matching 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.” Impedance matching is necessary between the antenna and the circuitry. The impedance of the antenna, the transmission line, and the circuitry should match so that maximum power transfer takes place between the antenna and the receiver or the transmitter. Necessity of Matching A resonant device is one, which gives better output at certain narrow band of frequencies. Antennas are such resonant devices whose impedance if matched, delivers a better output. The power radiated by an antenna, will be effectively radiated, if the antenna impedance matches the free space impedance. For a receiver antenna, antenna’s output impedance should match with the input impedance of the receiver amplifier circuit. For a transmitter antenna, antenna’s input impedance should match with transmitter amplifier’s output impedance, along with the transmission line impedance. Units The unit of impedance (Z) is Ohms. VSWR & Reflected Power According to the standard definition, “The ratio of the maximum voltage to the minimum voltage in a standing wave is known as Voltage Standing Wave Ratio.” If the impedance of the antenna, the transmission line and the circuitry do not match with each other, then the power will not be radiated effectively. Instead, some of the power is reflected back. The key features are − The term, which indicates the impedance mismatch is VSWR. VSWR stands for Voltage Standing Wave Ratio. It is also called as SWR. The higher the impedance mismatch, the higher will be the value of VSWR. The ideal value of VSWR should be 1:1 for effective radiation. Reflected power is the power wasted out of the forward power. Both reflected power and VSWR indicate the same thing. Bandwidth According to the standard definition, “A band of frequencies in a wavelength, specified for the particular communication, is known as bandwidth.” The signal when transmitted or received, is done over a range of frequencies. This particular range of frequencies are allotted to a particular signal, so that other signals may not interfere in its transmission. Bandwidth is the band of frequencies between the higher and lower frequencies over which a signal is transmitted. The bandwidth once allotted, cannot be used by others. The whole spectrum is divided into bandwidths to allot to different transmitters. The bandwidth, which we just discussed can also be called as Absolute Bandwidth. Percentage Bandwidth According to the standard definition, “The ratio of absolute bandwidth to the center frequency of that bandwidth can be termed as percentage bandwidth.” The particular frequency within a frequency band, at which the signal strength is maximum, is called as resonant frequency. It is also called as center frequency (fC) of the band. The higher and lower frequencies are denoted as fH and fL respectively. The absolute bandwidth is given by- fH – fL. To know how wider the bandwidth is, either fractional bandwidth or percentage bandwidth has to be calculated. Mathematical Expression The Percentage bandwidth is calculated to know how much frequency variation either a component or a system can handle. $$Percentage bandwidth = frac{absolute bandwidth}{center frequency} = frac{f_{H} – f_{L}}{f_{c}}$$ Where ${f_{H}}$ is higher frequency ${f_{L}}$ is lower frequency ${f_{c}}$ is center frequency The higher the percentage bandwidth, the wider will be the bandwidth of the channel. Radiation Intensity “Radiation intensity is defined as the power per unit solid angle” Radiation emitted from an antenna which is more intense in a particular direction, indicates the maximum intensity of that antenna. The emission of radiation to a maximum possible extent is nothing but the radiation intensity. Mathematical Expression Radiation Intensity is obtained by multiplying the power radiated with the square of the radial distance. $$U = r^{2} times W_{rad}$$ Where U is the radiation intensity r is the

Discuss Antenna Theory This tutorial is meant to provide the readers a detailed description of the antennas used in communication systems. After completing this tutorial, you will be able to calculate the parameters of an antenna and decide which antenna suits for which type of application and why. Learning working make money

Antenna Theory – Spectrum & Transmission In the Earth’s atmosphere, the propagation of wave depends not only on the properties of the wave, but also on environment effects and the layers of earth’s atmosphere. All of these have to be studied in order to form an idea of how a wave propagates in the environment. Let us look at the frequency spectrum over which the signal transmission or reception takes place. Different types of antennas are manufactured depending upon the frequency range in which they are operated. Electromagnetic Spectrum Wireless communication is based on the principle of broadcast and reception of electromagnetic waves. These waves can be characterized by their frequency (f) and their wavelength (λ) lambda. A pictorial representation of the electromagnetic spectrum is given in the following figure. Low Frequency bands Low Frequency bands comprise of the radio, microwave, infrared and visible portions of the spectrum. They can be used for information transmission by modulating the amplitude, frequency or phase of the waves. High Frequency bands High Frequency bands comprise of X-rays and Gamma rays. Theoretically, these waves are better for information propagation. However, these waves are not used practically because of difficulty in modulation and the waves are harmful to living beings. In addition, high frequency waves do not propagate well through buildings. Frequency Bands and their Uses The following table depicts the frequency bands and its uses − Band Name Frequency Wavelength Applications Extremely Low Frequency (ELF) 30 Hz to 300 Hz 10,000 to 1,000 KM Power line frequencies Voice Frequency (VF) 300 Hz to 3 KHz 1,000 to 100 KM Telephone Communications Very Low Frequency (VLF) 3 KHz to 30 KHz 100 to 10 KM Marine Communications Low Frequency (LF) 30 KHz to 300 KHz 10 to 1 KM Marine Communications Medium Frequency (MF) 300 KHz to 3 MHz 1000 to 100 m AM Broadcasting High Frequency (HF) 3 MHz to 30 MHz 100 to 10 m Long distance aircraft/ship Communications Very High Frequency(VHF) 30 MHz to 300 MHz 10 to 1 m FM Broadcasting Ultra High Frequency (UHF) 300 MHz to 3 GHz 100 to 10 cm Cellular Telephone Super High Frequency (SHF) 3 GHz to 30 GHz 10 to 1 cm Satellite Communications, Microwave links Extremely High Frequency (EHF) 30 GHz to 300 GHz 10 to 1 mm Wireless local loop Infrared 300 GHz to 400 THz 1 mm to 770 nm Consumer Electronics Visible Light 400 THz to 900 THz 770 nm to 330 nm Optical Communications Spectrum Allocation Since the electromagnetic spectrum is a common resource, which is open for access by anyone, several national and international agreements have been drawn regarding the usage of the different frequency bands within the spectrum. The individual national governments allocate spectrum for applications such as AM/FM radio broadcasting, television broadcasting, mobile telephony, military communication, and government usage. Worldwide, an agency of the International Telecommunications Union Radio Communication (ITU-R) Bureau called World Administrative Radio Conference (WARC) tries to coordinate the spectrum allocation by the various national governments, so that communication devices that can work in multiple countries can be manufactured. Transmission Limitations Four types of limitations that affect electromagnetic wave transmissions are − Attenuation According to the standard definition, “The decrease in the quality and the strength of the signal is known as attenuation.” The strength of a signal falls with distance over transmission medium. The extent of attenuation is a function of distance, transmission medium, as well as the frequency of the underlying transmission. Even in free space, with no other impairment, the transmitted signal attenuates over distance, simply because the signal is being spread over a larger and larger area. Distortion According to the standard definition, “Any change that alters the basic relation between the frequency components of a signal or the amplitude levels of a signal is known as distortion.” Distortion of a signal is the process, which causes disturbance to the properties of signal, adding some unwanted components, which affects the quality of the signal. This is usually in FM receiver, where the received signal, sometimes gets completely disturbed giving a buzzing sound as the output. Dispersion According to the standard definition, “Dispersion is the phenomenon, in which the velocity of propagation of an Electromagnetic wave is wavelength dependent.” Dispersion is the phenomenon of spreading of a burst of electromagnetic energy during propagation. It is especially prevalent in wireline transmissions such as an optical fiber. Bursts of data sent in rapid succession tend to merge due to dispersion. The longer the length of the wire, the more severe is the effect of dispersion. The effect of dispersion is to limit the product of R and L. Where ‘R’ is the data rate and ‘L’ is distance. Noise According to the standard definition, “Any unwanted form of energy tending to interfere with the proper and easy reception and reproduction of wanted signals is known as Noise.” The most pervasive form of noise is thermal noise. It is often modeled using an additive Gaussian model. Thermal noise is due to the thermal agitation of electrons and is uniformly distributed across the frequency spectrum. Other forms of noise include − Inter modulation noise − Caused by signals produced at frequencies that are sums or differences of carrier frequencies. Crosstalk − Interference between two signals. Impulse noise − Irregular pulses of high energy caused by external electromagnetic disturbances. An impulse noise may not have a significant impact on analog data. However, it has a noticeable effect on digital data, causing burst errors. Learning working make money

Antenna Theory – Terms in Wave Propagation In the process of propagation of a wave, there are few terms which we come across quite often. Let us discuss about these terms one by one. Virtual Height When a wave is refracted, it is bent down gradually, but not sharply. However, the path of incident wave and reflected wave are same if it is reflected from a surface located at a greater height of this layer. Such a greater height is termed as virtual height. The figure clearly distinguishes the virtual height (height of wave, supposed to be reflected) and actual height (the refracted height). If the virtual height is known, the angle of incidence can be found. Critical Frequency Critical frequency for a layer determines the highest frequency that will be returned down to the earth by that layer, after having been beamed by the transmitter, straight up into the sky. The rate of ionization density, when changed conveninetly through the layers, the wave will be bent downwards. The maximum frequency that gets bent and reaches the receiver station with minimum attenuation, can be termed as critical frequency. This is denoted by fc. Multi-path For the frequencies above 30 MHz, the sky wave propagation exists. Signal multipath is the common problem for the propagation of electromagnetic waves going through Sky wave. The wave, which is reflected from the ionosphere, can be called as a hop or skip. 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. Such a movement of signal can be termed as multipath. The above figure shows an example of multi-path propagation. Multipath propagation is a term, which describes the multiple paths a signal travels to reach the destination. These paths include a number of hops. The paths may be the results of reflection, refraction or even diffraction. Finally, when the signal from such different paths gets to the receiver, it carries propagation delay, additional noise, phase differences etc., which decrease the quality of the received output. Fading The decrease in the quality of the signal can be termed as fading. This happens because of atmospheric effects or reflections due to multipath. Fading refers to the variation of the signal strength with respect to time/distance. It is widely prevalent in wireless transmissions. The most common causes of fading in the wireless environment are multipath propagation and mobility (of objects as well as the communicating devices). Skip Distance The measurable distance on the surface of the Earth from transmitter to receiver, where the signal reflected from the ionosphere can reach the receiver with minimum hops or skips, is known as skip distance. Maximum Usable Frequency (MUF) The Maximum Usable Frequency (MUF) is the highest frequency delivered by the transmitter regardless of the power of the transmitter. The highest frequency, which is reflected from the ionosphere to the receiver is called as critical frequency, fc. $$MUF = frac{Critical frequency}{costheta} = f_{c}sectheta$$ Optimum Working Frequency (OWF) The frequency, which is being used mostly for a particular transmission and which has been predicted to be used over a particular period of time, over a path, is termed as Optimum Working Frequency (OWF). Inter Symbol Interference Inter symbol interference (ISI) occurs more commonly in communication system. This is the main reason for signal multipath also. When signals arrive at the receiving stations via different propagation paths, they cancel out each other, which is known as the phenomenon of signal fading. Here, it should be remembered that the signals cancel out themselves in vector way. Skin Depth Electromagnetic waves are not suitable for underwater propagations. However, they can propagate under water provided we make the frequency of propagation extremely low. The attenuation of electromagnetic waves under water is expressed in terms of skin depth. Skin depth is defined as the distance at which the signal is attenuated by 1/e. It is a measure of depth to which an EM wave can penetrate. Skin depth is represented as δ (delta). Duct Propagation At a height of around 50 mts from the troposphere, a phenomenon exists; the temperature increases with the height. In this region of troposphere, the higher frequencies or microwave frequencies tend to refract back into the Earth’s atmosphere, instead of shooting into ionosphere, to reflect. These waves propagate around the curvature of the earth even up to a distance of 1000km. This refraction goes on continuing in this region of troposphere. This can be termed as Super refraction or Duct propagation. The above image shows the process of Duct Propagation. The main requirement for the duct formation is the temperature inversion. The increase of temperature with height, rather than the decrease in the temperature is known as the phenomenon of temperature inversion. We have discussed the important parameters, which we come across in wave propagation. The waves of higher frequencies are transmitted and received using this wave propagation technique. Learning working make money

Antenna Theory – Log-periodic Antenna The Yagi-Uda antenna is mostly used for domestic purpose. However, for commercial purpose and to tune over a range of frequencies, we need to have another antenna known as the Log-periodic antenna. A Log-periodic antenna is that whose impedance is a logarithamically periodic function of frequency. Frequency range The frequency range, in which the log-periodic antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands. Construction & Working of Log-periodic Antenna The construction and operation of a log-periodic antenna is similar to that of a Yagi-Uda antenna. The main advantage of this antenna is that it exhibits constant characteristics over a desired frequency range of operation. It has the same radiation resistance and therefore the same SWR. The gain and front-to-back ratio are also the same. The image shows a log-periodic antenna. 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. This is its special characteristic. There are several type of log-periodic antennas such as the planar, trapezoidal, zig-zag, V-type, slot and the dipole. The mostly used one is log-periodic dipole array, in short, LPDA. The diagram of log-periodic array is given above. The physical structure and electrical characteristics, when observed, are repetitive in nature. The array consists of dipoles of different lengths and spacing, which are fed from a two-wire transmission line. This line is transposed between each adjacent pair of dipoles. The dipole lengths and seperations are related by the formula − $$frac{R_{1}}{R_{2}} = frac{R_{2}}{R_{3}} = frac{R_{3}}{R_{4}} = T = frac{l_{1}}{l_{2}} = frac{l_{2}}{l_{3}} = frac{l_{3}}{l_{4}}$$ Where т is the design ratio and т<1 R is the distance between the feed and the dipole l is the length of the dipole. The directive gains obtained are low to moderate. The radiational patterns may be Unidirectional or Bi-directional. Radiation Pattern The Radiation pattern of log-periodic antenna can be of uni-directional or bi-directional, depending upon the log periodic structures. For uni-directional Log-periodic antenna, the radiation towards shorter element is of considerable amount, whereas in forward direction, it is small or zero. The radiational pattern for uni-directional log-periodic antenna is given above. For bi-directional Log-periodic antenna, the maximum radiation is in broad side, which is normal to the surface of the antenna. The figure given above shows the radiational pattern for a bi-directional log-periodic antenna. Advantages The following are the advantages of Log-periodic antennas − The antenna design is compact. Gain and radiation pattern are varied according to the requirements. Disadvantages The following are the disadvantages of Log-periodic antennas − External mount. Installation cost is high. Applications The following are the applications of Log-periodic antennas − Used for HF communications. Used for particular sort of TV receptions. Used for all round monitoring in higher frequency bands. Learning working make money

Antenna Theory – End-fire Array The physical arrangement of end-fire array is same as that of the broad side array. The magnitude of currents in each element is same, but there is a phase difference between these currents. This induction of energy differs in each element, which can be understood by the following diagram. The above figure shows the end-fire array in top and side views respectively. There is no radiation in the right angles to the plane of the array because of cancellation. The first and third elements are fed out of phase and therefore cancel each other’s radiation. Similarly, second and fourth are fed out of phase, to get cancelled. The usual dipole spacing will be λ/4 or 3λ/4. This arrangement not only helps to avoid the radiation perpendicular to the antenna plane, but also helps the radiated energy get diverted to the direction of radiation of the whole array. Hence, the minor lobes are avoided and the directivity is increased. The beam becomes narrower with the increased elements. Radiation Pattern The Radiation pattern of end-fire array is uni-directional. A major lobe occurs at one end, where maximum radiation is present, while the minor lobes represent the losses. The figure explains the radiation pattern of an end-fire array. Figure 1 is the radiation pattern for a single array, while figures 2, 3, and 4 represent the radiation pattern for multiple arrays. End-fire Array Vs Broad Side Array We have studied both the arrays. Let us try to compare the end-fire and broad side arrays, along with their characteristics. The figure illustrates the radiation pattern of end-fire array and broad side array. Both, the end fire array and broad side array, are linear and are resonant, as they consist of resonant elements. Due to resonance, both the arrays display narrower beam and high directivity. Both of these arrays are used in transmission purposes. Neither of them is used for reception, because the necessity of covering a range of frequencies is needed for any kind of reception. Learning working make money

Antenna Theory – Micro Strip Micro strip antennas are low-profile antennas. A metal patch mounted at a ground level with a di-electric material in-between constitutes a Micro strip or Patch Antenna. These are very low size antennas having low radiation. Frequency Range The patch antennas are popular for low profile applications at frequencies above 100MHz. Construction & Working of Micro strip Antennas Micro strip antenna consists of a very thin metallic strip placed on a ground plane with a di-electric material in-between. The radiating element and feed lines are placed by the process of photo-etching on the di-electric material. Usually, the patch or micro-strip is choosen to be square, circular or rectangular in shape for the ease of analysis and fabrication. The following image shows a micro-strip or patch antenna. The length of the metal patch is λ/2. When the antenna is excited, the waves generated within the di-electric undergo reflections and the energyis radiated from the edges of the metal patch,which is very low. Radiation Pattern The radiation pattern of microstrip or patch antenna is broad. It has low radiation power and narrow frequency bandwidth. The radiation pattern of a microstrip or patch antenna is shown above. It has lesser directivity. To have a greater directivity, an array can be formed by using these patch antennas. Advantages The following are the advantages of Micro strip antenna − Lighteweight Low cost Ease of installation Disadvantages The following are the disadvantages of Micro strip antenna − Inefficient radiation Narrow frequency bandwidth Applications The following are the applications of Micro strip antenna − Used in Space craft applications Used in Air craft applications Used in Low profile antenna applications Learning working make money

Antenna Theory – Turnstile Antenna The Turnstile antenna is another type of array antenna. The shape of this array symbolizes the turnstile, which is used at the entrances of few places. This antenna has a wide variety of military applications. Frequency range The frequency range in which the turnstile antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands. Construction & Working of Turnstile Antenna Two identical half-wave dipoles are placed at right angles to each other and are fed inphase. These dipoles are excited 90° out of phase with each other. Turnstile array can also be termed as crossed dipoles array. The above images illustrate turnstile antennas. To provide high directivity, several turnstiles may be stacked along a vertical axis, and are phased as shown in the figure given above. The polarization of these turnstile antennas depend upon their mode of operation. The pair of such dipoles frequently stacked, is known as BAY. In the figures shown above, two bays are spaced half wavelength (λ/2) apart and the corresponding elements are fed in phase. The radiation produce by the combination of bays results in better directivity. Modes of Operation The following are the modes of operation of a Turnstile antenna. Normal mode In Normal mode of operation, the antenna radiates horizontally polarized waves which are perpendicular to its axis. Axial mode In Axial mode of operation, the antenna radiates circularly polarized waves along its axis i.e. parallel to its axis. For circular polarization, the transmitter radiating with right-circular polarization should have a receiver with same right-circular polarization and vice versa. If it is left-circular polarized one, unlike the transmitter, there will be a severe loss of gain. Super Turnstile Antenna For a turnstile antenna, the radiation power is 3dB below the maximum radiation of a halfwave dipole radiating the same power. Therefore, to overcome this disadvantage, the Super-turnstile antenna is built. The simple dipole elements in turnstile are replaced by four flat sheets in Super-turnstile. The design of Super-turnstile array is such that 1 to 8 bays can be constructed on a single mast. The other name for Super-turnstile antenna is the Batwing Antenna. The above images show super-turnstile antenna. Figure 1 shows the arrangement of superturnstile array with the red dots being the feed points. Figure 2 shows the stacked turnstile array used in satellite communications. Radiation Pattern The radiation pattern will be similar to the radiation pattern of two super imposed dipoles. Though it is close to omni-directional pattern, it leaves a cloveleaf shaped pattern. The above figure shows the radiational pattern of a turnstile array. The typical figure-ofeight patterns were combined to produce a nearly circular pattern. Figure A shows the individual patterns being combined. Figure B shows the vertical pattern of single bay and also the combined pattern of four bays. Figure C shows the resultant combined pattern of four bays showing better directivity. Advantages The following are the advantages of Turnstile antennas − High-gain is achieved by stacking Super-turnstile produces high-gain output Better directivity is achieved Disadvantage The following is the disadvantage of Turnstile antennas − The radiation power is 3dB below the maximum radiation of a half wave dipole radiating the same power. Applications The following are the applications of Turnstile antennas − Used for VHF communications Used for FM and TV broadcasting Used in military communications Used in satellite communications Learning working make money