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Category: antenna Theory
Antenna Theory – Yagi-Uda Antenna Yagi-Uda antenna is the most commonly used type of antenna for TV reception over the last few decades. It is the most popular and easy-to-use type of antenna with better performance, which is famous for its high gain and directivity Frequency range The frequency range in which the Yagi-Uda antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands. Construction of Yagi-Uda Antenna A Yagi-Uda antenna was seen on top of almost every house during the past decades. The parasitic elements and the dipole together form this Yagi-Uda antenna. The figure shows a Yagi-Uda antenna. It is seen that there are many directors placed to increase the directivity of the antenna. The feeder is the folded dipole. The reflector is the lengthy element, which is at the end of the structure. The figure depicts a clear form of the Yagi-Uda antenna. The center rod like structure on which the elements are mounted is called as boom. The element to which a thick black head is connected is the driven element to which the transmission line is connected internally, through that black stud. The single element present at the back of the driven element is the reflector, which reflects all the energy towards the direction of the radiation pattern. The other elements, before the driven element, are the directors, which direct the beam towards the desired angle. Designing For this antenna to be designed, the following design specifications should be followed. They are − ELEMENT SPECIFICATION Length of the Driven Element 0.458λ to 0.5λ Length of the Reflector 0.55λ to 0.58λ Length of the Director 1 0.45λ Length of the Director 2 0.40λ Length of the Director 3 0.35λ Spacing between Directors 0.2λ Reflector to dipole spacing 0.35λ Dipole to Director spacing 0.125λ If the specifications given above are followed, one can design an Yagi-Uda antenna. Radiation Pattern The directional pattern of the Yagi-Uda antenna is highly directive as shown in the figure given below. The minor lobes are suppressed and the directivity of the major lobe is increased by the addition of directors to the antenna. Advantages The following are the advantages of Yagi-Uda antennas − High gain is achieved. High directivity is achieved. Ease of handling and maintenance. Less amount of power is wasted. Broader coverage of frequencies. Disadvantages The following are the disadvantages of Yagi-Uda antennas − Prone to noise. Prone to atmospheric effects. Applications The following are the applications of Yagi-Uda antennas − Mostly used for TV reception. Used where a single-frequency application is needed. Learning working make money
Antenna Theory – Lonosphere and its Layers Earth’s atmosphere has several layers. These layers play an important role in the wireless communication. These are mainly classified into three layers. Troposphere This is the layer of the earth, which lies just above the ground. We, the flora and fauna live in this layer. The ground wave propagation and LOS propagation take place here. Stratosphere This is the layer of the earth, which lies above Troposphere. The birds fly in this region. The airplanes travel in this region. Ozone layer is also present in this region. The ground wave propagation and LOS propagation takes place here. Ionosphere This is the upper layer of the Earth’s atmosphere, where ionization is appreciable. The energy radiated by the Sun, not only heats this region, but also produces positive and negative ions. Since the Sun constantly radiates UV rays and air pressure is low, this layer encourages ionization of particles. Importance of Ionosphere The ionosphere layer is a very important consideration in the phase of wave propagation because of the following reasons − The layer below ionosphere has higher amount of air particles and lower UV radiation. Due to this, more collisions occur and ionization of particles is minimum and not constant. The layer above ionosphere has very low amount of air particles and density of ionization is also quite low. Hence, ionization is not proper. The ionosphere has good composition of UV radiation and average air density that does not affect the ionization. Hence, this layer has most influence on the Sky wave propagation. The ionosphere has different gases with different pressures. Different ionizing agents ionize these at different heights. As various levels of ionization are done at each level, having different gases, few layers with different properties are formed in the ionosphere. The layers of ionosphere can be studied from the following figure. The number of layers, their heights, the amount of sky wave that can be bent will vary from day to day, month to month and year to year. For each such layer, there is a frequency, above which if the wave is sent upward vertically, it penetrates through the layer. The function of these layers depends upon the time of the day, i.e., day time and night time. There are three principal layers- E, F1 and F2 during day time. There is another layer called D layer, which lies below E layer. This layer is at 50 to 90kms above the troposphere. The following figure depicts the layers present in both day time and night time in the earth’s atmosphere. This D layer is responsible for the day time attenuation of HF waves. During night time, this D layer almost vanishes out and the F1 and F2 layers combine together to form F layer. Hence, there are only two layers E and F present at the night time. Learning working make money
Antenna Theory – Rhombic The Rhombic Antenna is an equilateral parallelogram shaped antenna. Generally, it has two opposite acute angles. The tilt angle, θ is approximately equal to 90° minus the angle of major lobe. Rhombic antenna works under the principle of travelling wave radiator. It is arranged in the form of a rhombus or diamond shape and suspended horizontally above the surface of the earth. Frequency Range The frequency range of operation of a Rhombic antenna is around 3MHz to 300MHz. This antenna works in HF and VHF ranges. Construction of Rhombic Antenna Rhombic antenna can be regarded as two V-shaped antennas connected end-to-end to form obtuse angles. Due to its simplicity and ease of construction, it has many uses − In HF transmission and reception Commercial point-to-point communication The construction of the rhombic antenna is in the form a rhombus, as shown in the figure. The two sides of rhombus are considered as the conductors of a two-wire transmission line. When this system is properly designed, there is a concentration of radiation along the main axis of radiation. In practice, half of the power is dissipated in the terminating resistance of the antenna. The rest of the power is radiated. The wasted power contributes to the minor lobes. Figure 1 shows the construction of rhombic antenna for point-to-point communication in olden days. Figure 2 shows the rhombic UHF antenna for TV reception, used these days. The maximum gain from a rhombic antenna is along the direction of the main axis, which passes through the feed point to terminate in free space. The polarization obtained from a horizontal rhombic antenna is in the plane of rhombus, which is horizontal. Radiation Pattern The radiation pattern of the rhombic antenna is shown in the following figure. The resultant pattern is the cumulative effect of the radiation at all four legs of the antenna. This pattern is uni-directional, while it can be made bi-directional by removing the terminating resistance. The main disadvantage of rhombic antenna is that the portions of the radiation, which do not combine with the main lobe, result in considerable side lobes having both horizontal and vertical polarization. Advantages The following are the advantages of Rhombic antenna − Input impedance and radiation pattern are relatively constant Multiple rhombic antennas can be connected Simple and effective transmission Disadvantages The following are the disadvantages of Rhombic antenna − Wastage of power in terminating resistor Requirement of large space Redued transmission efficiency Applications The following are the applications of Rhombic antenna − Used in HF communications Used in Long distance sky wave propagations Used in point-to-point communications Another method of using long wire is by bending and making the wire into a loop shaped pattern and observing its radiational parameters. This type of antennas are termed as loop antennas. Learning working make money
Antenna Theory – Parabolic Reflector Parabolic Reflectors are Microwave antennas. For better understanding of these antennas, the concept of parabolic reflector has to be discussed. Frequency Range The frequency range used for the application of Parabolic reflector antennas is above 1MHz. These antennas are widely used for radio and wireless applications. Principle of Operation 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. The following figure shows the geometry of parabolic reflector. The point F is the focus (feed is given) and V is the vertex. The line joining F and V is the axis of symmetry. PQ are the reflected rays where L represents the line directrix on which the reflected points lie (to say that they are being collinear). Hence, as per the above definition, the distance between F and L lie constant with respect to the waves being focussed. The reflected wave forms a colllimated wave front, out of the parabolic shape. The ratio of focal length to aperture size (ie., f/D) known as “f over D ratio” is an important parameter of parabolic reflector. Its value varies from 0.25 to 0.50. The law of reflection states that the angle of incidence and the angle of reflection are equal. This law when used along with a parabola, helps the beam focus. The shape of the parabola when used for the purpose of reflection of waves, exhibits some properties of the parabola, which are helpful for building an antenna, using the waves reflected. Properties of Parabola All the waves originating from focus, reflects back to the parabolic axis. Hence, all the waves reaching the aperture are in phase. As the waves are in phase, the beam of radiation along the parabolic axis will be strong and concentrated. Following these points, the parabolic reflectors help in producing high directivity with narrower beam width. Construction & Working of a Parabolic Reflector 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. It means that, the waves come out of the focal point and strike the Paraboloidal reflector. This wave now gets reflected as collimated wave front, as discussed previously, to get transmitted. The same antenna is used as a receiver. When the electromagnetic wave hits the shape of the parabola, the wave gets reflected onto the feed point. 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. The following image shows a Parabolic Reflector Antenna. The gain of the paraboloid is a function of aperture ratio (D/λ). The Effective Radiated Power (ERP) of an antenna is the multiplication of the input power fed to the antenna and its power gain. Usually a wave guide horn antenna is used as a feed radiator for the paraboloid reflector antenna. Along with this technique, we have another type of feed given to the paraboloid reflector antenna, called as Cassegrain feed. Cassegrain Feed Casse grain is another type of feed given to the reflector antenna. In this type, the feed is located at the vertex of the paraboloid, unlike in the parabolic reflector. A convex shaped reflector, which acts as a hyperboloid is placed opposite to the feed of the antenna. It is also known as secondary hyperboloid reflector or sub-reflector. It is placed such that its one of the foci coincides with the focus of the paraboloid. Thus, the wave gets reflected twice. The above figure shows the working model of cassegrain feed. Working of a Cassegrain Antenna When the antenna acts as a transmitting antenna, the energy from the feed radiates through a horn antenna onto the hyperboloid concave reflector, which again reflects back on to the parabolic reflector. The signal gets reflected into the space from there. Hence, wastage of power is controlled and the directivity gets improved. When the same antenna is used for reception, the electromagnetic waves strike the reflector, gets reflected on to the concave hyperboloid and from there, it reaches to the feed. A wave guide horn antenna presents there to receive this signal and sends to the receiver circuitry for amplification. Take a look at the following image. It shows a paraboloid reflector with cassegrain feed. Advantages The following are the advantages of Parabolic reflector antenna − Reduction of minor lobes Wastage of power is reduced Equivalent focal length is achieved Feed can be placed in any location, according to our convenience Adjustment of beam (narrowing or widening) is done by adjusting the reflecting surfaces Disadvantage The following is the disadvantage of a Parabolic reflector antenna − Some of the power that gets reflected from the parabolic reflector is obstructed. This becomes a problem with small dimension paraboloid. Applications The following are the applications of Parabolic reflector antenna − The cassegrain feed parabolic reflector is mainly used in satellite communications. Also used in wireless telecommunication systems. Let us look at the other type of feed called as Gregorian feed for the parabolic reflectors. Gregorian Feed This is another type of feed used. A pair of certain configurations are there, where the feed beamwidth is progressively increased while antenna dimensions are held fixed. Such a type of feed is known as Gregorian feed. Here, the convex shaped hyperboloid of casssegrain is replaced with a concave shaped paraboloid reflector, which is of course, smaller in size These Gregorian feed type reflectors can be used in four ways − Gregorian systems using reflector ellipsoidal sub-reflector at foci F1. Gregorian systems using reflector ellipsoidal sub-reflector at foci F2. Cassegrain systems using hyperboloid sub-reflector (convex). Cassegrain systems using hyperboloid sub-reflector (concave but the feed being very near to it.) These are all just to mention because
Antenna Theory – Horn 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. So that all the energy in the forward direction gets radiated. This can be termed as Flaring. Now, this can be done using a horn antenna. Frequency Range The operational frequency range of a horn antenna is around 300MHz to 30GHz. This antenna works in UHF and SHF frequency ranges. Construction & Working of Horn Antenna The energy of the beam when slowly transform into radiation, the losses are reduced and the focussing of the beam improves. A Horn antenna may be considered as a flared out wave guide, by which the directivity is improved and the diffraction is reduced. The above image shows the model of a horn antenna. The flaring of the horn is clearly shown. There are several horn configurations out of which, three configurations are most commonly used. Sectoral horn This type of horn antenna, flares out in only one direction. Flaring in the direction of Electric vector produces the sectorial E-plane horn. Similarly, flaring in the direction of Magnetic vector, produces the sectorial H-plane horn. Pyramidal horn This type of horn antenna has flaring on both sides. If flaring is done on both the E & H walls of a rectangular waveguide, then pyramidal horn antenna is produced. This antenna has the shape of a truncated pyramid. Conical horn When the walls of a circular wave guide are flared, it is known as a conical horn. This is a logical termination of a circular wave guide. The above figures show the types of horn configurations, which were discussed earlier. Flaring helps to match the antenna impedance with the free space impedance for better radiation. It avoids standing wave ratio and provides greater directivity and narrower beam width. The flared wave guide can be technically termed as Electromagnetic Horn Radiator. Flare angle, Φ of the horn antenna is an important factor to be considered. If this is too small, then the resulting wave will be spherical instead of plane and the radiated beam will not be directive. Hence, the flare angle should have an optimum value and is closely related to its length. Combinations Horn antennas, may also be combined with parabolic reflector antennas to form special type of horn antennas. These are − Cass-horn antenna Hog-horn or triply folded horn reflector In Cass-horn antenna, radio waves are collected by the large bottom surface, which is parabolically curved and reflected upward at 45° angle. After hitting top surface, they are reflected to the focal point. The gain and beam width of these are just like parabolic reflectors. In hog-horn antenna, a parabolic cylinder is joined to pyramidal horn, where the beam reaches apex of the horn. It forms a low-noise microwave antenna. The main advantage of hog-horn antenna is that its receiving point does not move, though the antenna is rotated about its axis. Radiation Pattern The radiation pattern of a horn antenna is a Spherical Wave front. The following figure shows the radiation pattern of horn antenna. The wave radiates from the aperture, minimizing the diffraction of waves. The flaring keeps the beam focussed. The radiated beam has high directivity. Advantages The following are the advantages of Horn antenna − Small minor lobes are formed Impedance matching is good Greater directivity Narrower beam width Standing waves are avoided Disadvantages The following are the disadvantages of Horn antenna − Designing of flare angle, decides the directivity Flare angle and length of the flare should not be very small Applications The following are the applications of Horn antenna − Used for astronomical studies Used in microwave applications Learning working make money
Antenna Theory – Broad-side Array The antenna array in its simplest form, having a number of elements of equal size, equally spaced along a straight line or axis, forming collinear points, with all dipoles in the same phase, from the same source together form the broad side array. 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 & Working of Broad-side Array 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. Hence, the radiation pattern of the antenna is perpendicular to the axis on which the array exists. The following diagram shows the broad side array, in front view and side view, respectively. The broad side array is strongly directional at right angles to the plane of the array. However, the radiation in the plane will be very less because of the cancellation in the direction joining the center. The figure of broad side array with λ/4 spacing is shown below. Typical antenna lengths in the broad side array are from 2 to 10 wavelengths. Typical spacings are λ/2 or λ. The feed points of the dipoles are joined as shown in the figure. Radiation Pattern The radiation pattern of this antenna is bi-directional and right angles to the plane. The beam is very narrow with high gain. The above figure shows the radiation pattern of the broad side array. The beam is a bit wider and minor lobes are much reduced in this. Learning working make money
Antenna Theory – Half-Wave Folded Dipole A folded dipole is an antenna, with two conductors connected on both sides, and folded to form a cylindrical closed shape, to which feed is given at the center. The length of the dipole is half of the wavelength. Hence, it is called as half wave folded dipole antenna. Frequency range The range of frequency in which half wave folded dipole operates is around 3KHz to 300GHz. This is mostly used in television receivers. Construction & Working of Half-wave Folded Dipole This antenna is commonly used with the array type antennas to increase the feed resistance. The most commonly used one is with Yagi-Uda antenna. The following figure shows a half-wave folded dipole antenna. This antenna uses an extra conducting element (a wire or a rod) when compared with previous dipole antenna. This is continued by placing few conducting elements in parallel, with insulation in-between, in array type of antennas. The following figure explains the working of a half-wave folded dipole antenna, when it is provided with excitation. If the diameter of the main conductor and the folded dipole are same, then there will be four folded (two times of squared one) increase in the feed impedance of the antenna. This increase in feed impedance is the main reason for the popular usage of this folded dipole antenna. Due of the twin-lead, the impedance will be around 300Ω. Radiation Pattern The radiation pattern of half-wave folded dipoles is the same as that of the half-wave dipole antennas. The following figure shows the radiation pattern of half-wave folded dipole antenna, which is Omni-directional pattern. Half-wave folded dipole antennas are used where optimum power transfer is needed and where large impedances are needed. This folded dipole is the main element in Yagi-Uda antenna. The following figure shows a Yagi-Uda antenna, which we will study later. The main element used here is this folded dipole, to which the antenna feed is given. This antenna has been used extensively for television reception over the last few decades. Advantages The following are the advantages of half-wave folded dipole antenna − Reception of balanced signals. Receives a particular signal from a band of frequencies without losing the quality. A folded dipole maximizes the signal strength. Disadvantages The following are the disadvantages of half-wave folded dipole antenna − Displacement and adjustment of antenna is a hassle. Outdoor management can be difficult when antenna size increases. Applications The following are the applications of half-wave folded dipole antenna − Mainly used as a feeder element in Yagi antenna, Parabolic antenna, turnstile antenna, log periodic antenna, phased and reflector arrays, etc. Generally used in radio receivers. Most commonly used in TV receiver antennas. Learning working make money
Antenna Theory – Short Dipole A short dipole is a simple wire antenna. One end of it is open-circuited and the other end is fed with AC source. This dipole got its name because of its length. Frequency range The range of frequency in which short dipole operates is around 3KHz to 30MHz. This is mostly used in low frequency receivers. Construction & Working of Short Dipole The Short dipole is the dipole antenna having the length of its wire shorter than the wavelength. A voltage source is connected at one end while a dipole shape is made, i.e., the lines are terminated at the other end. The circuit diagram of a short dipole with length L is shown. The actual size of the antenna does not matter. The wire that leads to the antenna must be less than one-tenth of the wavelength. That is $$L < frac{lambda}{10}$$ Where L is the length of the wire of the short dipole. λ is the wavelength. Another type of short dipole is infinitesimal dipole, whose length is far less than its wave length. Its constructiion is similar to it, but uses a capacitor plate. Infinitesimal Dipole A dipole whose length is far less than wavelength is infitesimal dipole. This antenna is actually impractical. Here, the length of the dipole is less than even fiftith part of the wavelength. The length of the dipole, Δl << λ. Where, λ is the wavelength. $$Delta l = frac{lambda}{50}$$ Hence, this is the infinitely small dipole, as the name implies. As the length of these dipoles is very small, the current flow in the wire will be dI. These wires are generally used with capacitor plates on both sides, where low mutual coupling is needed. Because of the capacitor plates, we can say that uniform distribution of current is present. Hence the current is not zero here. The capacitor plates can be simply conductors or the wire equivalents. The fields radiated by the radial currents tend to cancel each other in the far field so that the far fields of the capacitor plate antenna can be approximated by the infinitesimal dipole. Radiation Pattern The radiation pattern of a short dipole and infinitesimal dipole is similar to a half wave dipole. If the dipole is vertical, the pattern will be circular. The radiation pattern is in the shape of “figure of eight” pattern, when viewed in two-dimensional pattern. The following figure shows the radiation pattern of a short dipole antenna, which is in omni-directional pattern. Advantages The following are the advantages of short dipole antenna − Ease of construction, due to small size Power dissipation efficiency is higher Disadvantages The following are the disadvantages of short dipole antenna − High resistive losses High power dissipation Low Signal-to-noise ratio Radiation is low Not so efficient Applications The following are the applications of short dipole antenna − Used in narrow band applications. Used as an antenna for tuner circuits. In this chapter, the popular and most widely used short-wire antennas were discussed. We will discuss the Long-wire antennas in the coming chapters. Learning working make money
Antenna Theory – Lens The antennas, which we have discussed till now, used the plane surface. The lens antennas use the curved surface for both transmission and reception. Lens antennas are made up of glass, where the converging and diverging properties of lens are followed. The lens antennas are used for higher frequency applications. Frequency Range The frequency range of usage of lens antenna starts at 1000 MHz but its use is greater at 3000 MHz and above. To have a better understanding of the lens antenna, the working principle of a lens has to be known. A normal glass lens works on the principle of refraction. Construction & Working of Lens Antenna If a light source is assumed to be present at a focal point of a lens, which is at a focal distance from the lens, then the rays get through the lens as collimated or parallel rays on the plane wavefront. The rays that pass through the centre of the lens are less refracted than the rays that pass through the edges of the lens. All of the rays are sent in parallel to the plane wave front. This phenomenon of lens is called as divergence. The same procedure gets reversed if a light beam is sent from right side to left of the same lens. Then the beam gets refracted and meets at a point called focal point, at a focal distance from the lens. This phenomenon is called convergence. The same can be better understood by observing the following diagram − The ray diagram represents the focal point and focal length from the source to the lens. The parallel rays obtained are also called as collimated rays. In the above figure, the source at the focal point, at a focal distance from the lens, gets collimated in the plane wave front. This phenomenon can be reversed which means the light if sent from the left side, gets converged at the right side of the lens. It is because of this reciprocity, the lens can be used as an antenna, as the same phenomenon helps in utilizing the same antenna for both transmission and reception. The image of the model of a lens antenna is shown. To achieve the focusing properties at higher frequencies, the refractive index should be less than unity. Whatever may be the refractive index, the purpose of lens is to straighten the waveform. Based on this, the E-plane and H-plane lens are developed, which also delay or speed up the wave front. Types of Lens Antennas The following types of Lens Antennas are available − Di-electric lens or H-plane metal plate lens or Delay lens (Travelling waves are delayed by lens media) E-plane metal plate lens Non-metallic di-electric type lens Metallic or artificial dielectric type of lens Advantages The following are the advantages of Lens antenna − In lens antennas, feed and feed support, do not obstruct the aperture. It has greater design tolerance. Larger amount of wave, than a parabolic reflector, can be handled. Beam can be moved angularly with espect to the axis. Disadvantages The following are the disadvantages of Lens antenna − Lenses are heavy and bulky, especially at lower frequencies Complexity in design Costlier compared to reflectors, for the same specifications Applications The following are the applications of Lens antenna − Used as wide band antenna Especially used for Microwave frequency applications The converging properties of lens antennas can be used for developing higher level of antennas known as Parabolic reflector antennas, which are widely used in satellite communications. We will discuss about them in the next chapter. Learning working make money