Category: antenna Theory

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 – Isotropic Radiation In the previous chapter, we have gone through the radiation pattern. To have a better analysis regarding the radiation of an antenna, a referential point is necessary. The radiation of an isotropic antenna, fills this space. Definition Isotropic radiation is the radiation from a point source, radiating uniformly in all directions, with same intensity regardless of the direction of measurement. The improvement of radiation pattern of an antenna is always assessed using the isotropic radiation of that antenna. If the radiation is equal in all directions, then it is known as isotropic radiation. The point source is an example of isotropic radiator. However, this isotropic radiation is practically impossible, because every antenna radiates its energy with some directivity. The isotropic radiation is nothing but Omni-directional radiation. It has a doughnut-shaped pattern when viewed in 3D and a figure-of-eight pattern when viewed in 2D. The figures given above show the radiation pattern of an isotropic or Omni-directional pattern. Figure 1 illustrates the doughnut shaped pattern in 3D and Figure 2 illustrates the figure-of-eight pattern in 2D. Gain The isotropic radiator has unity gain, which means having a gain factor of 1 in all directions. In terms of dB, it can be called as 0dB gain (zero loss). Equivalent Isotropic Radiated Power According to the standard definition, “The amount of power that an isotropical antenna radiates to produce the peak power density observed in the direction of maximum antenna gain, is called as Equivalent Isotropic Radiated Power.” If the radiated energy of an antenna is made to concentrate on one side or a particular direction, where the radiation is equivalent to that antenna’s isotropic radiated power, such a radiation would be termed as EIRP i.e. Equivalent Isotropic Radiated Power. Gain Though isotropic radiation is an imaginary one, it is the best an antenna can give. The gain of such antenna will be 3dBi where 3dB is a factor of 2 and ‘i’ represents factor of isotropic condition. If the radiation is focused in certain angle, then EIRP increases along with the antenna gain. Gain of the antenna is best achieved by focusing the antenna in certain direction. Effective Radiated Power If the radiated power is calculated by taking half-wave dipole as the reference, rather than an isotropic antenna, then it can be termed as ERP (Effective Radiated Power). $$ERP(dBW) = EIRP(dBW) – 2.15dBi$$ If EIRP is known, then ERP can be calculated from formula given above. Learning working make money

Antenna Theory – Beam and Polarization This chapter deals with the parameters of radiated beam of the antenna. These parameters help us to know about the beam specifications. Beam Area According to the standard definition, “Beam area is the solid angle through which all the power radiated by the antenna would stream if P (θ, Ø) maintained its maximum value over ΩA and was zero elsewhere.” The radiated beam of the antenna comes out from an angle at the antenna, known as solid angle, where the power radiation intensity is maximum. This solid beam angle is termed as the beam area. It is represented by ΩA. The radiation intensity P (θ, Ø) should be maintained constant and maximum throughout the solid beam angle ΩA, its value being zero elsewhere. $$Power radiated = P(theta,Phi)Omega_{A} :watts$$ Beam angle is a set of angles between the half power points of the main lobe. Mathematical Expression The mathematical expression for beam area is $$Omega_{A} =int_{0}^{2pi}int_{0}^{pi}P_{pi}(theta,Phi)dOmega wattts$$ $$dOmega = sintheta dtheta dPhi watts$$ Where $Omega_{A}$ is the solid beam angle. $theta$ is the function of angular position. $Phi$ is the function of radial distance. Units The unit of beam area is watts. Beam Efficiency According to the standard definition, “The beam efficiency states the ratio of the beam area of the main beam to the total beam area radiated.” The energy when radiated from an antenna, is projected according to the antenna’s directivity. The direction in which an antenna radiates more power has maximum efficiency, while some of the energy is lost in side lobes. The maximum energy radiated by the beam, with minimum losses can be termed as beam efficiency. Mathematical Expression The mathematical expression for beam efficiency is − $$eta_{B} = frac{Omega_{MB}}{Omega_{A}}$$ Where, $eta_{B}$ is the beam efficiency. $Omega_{MB}$ is beam area of the main beam. $Omega_{A}$ is total solid beam angle (beam area). Antenna Polarization An Antenna can be polarized depending upon our requirement. It can be linearly polarized or circularly polarized. The type of antenna polarization decides the pattern of the beam and polarization at the reception or transmission. Linear polarization When a wave is transmitted or received, it may be done in different directions. The linear polarizationof the antenna helps in maintaining the wave in a particular direction, avoiding all the other directions. Though this linear polarization is used, the electric field vector stays in the same plane. Hence, we use this linear polarization to improve the directivity of the antenna. Circular polarization When a wave is circularly polarized, the electric field vector appears to be rotated with all its components loosing orientation. The mode of rotation may also be different at times. However, by using circular polarization, the effect of multi-path gets reduced and hence it is used in satellite communications such as GPS. Horizontal polarization Horizontal polarization makes the wave weak, as the reflections from the earth surface affect it. They are usually weak at low frequencies below 1GHz. Horizontal polarization is used in the transmission of TV signals to achieve a better signal to noise ratio. Vertical polarization The low frequency vertically polarized waves are advantageous for ground wave transmission. These are not affected by the surface reflections like the horizontally polarized ones. Hence, the vertical polarization is used for mobile communications. Each type of polarization has its own advantages and disadvantages. A RF system designer is free to select the type of polarization, according to the system requirements. Learning working make money

Antenna Theory – Long-Wire We have gone through different types of short wire antennas. Now, lets us look at the long wire antennas. The long wire antennas are formed by using a number of dipoles. The length of the wire in these type of antennas is n times λ/2 $$L = n lambda/2$$ Where, L is the length of the antenna, n is the number of elements, λ is the wavelength As ‘n’ increases, the directional properties also increase. Types of Long-wire Antennas Long wire antennas are divided into two types namely − Resonant Antennas and Non-resonant Antennas. Resonant Antennas Resonant Antennas are those for which a sharp peak in the radiated power is intercepted by the antenna at certain frequency, to form a standing wave. The radiation pattern of the radiated wave is not matched with the load impedance in this type of antenna. The resonant antennas are periodic in nature. They are also called as bi-directional travelling wave antennas, as the radiated wave moves in two directions, which means both incident and reflected waves occur here. In these antennas, the length of the antenna and frequency are proportional to each other. Non-resonant Antennas Non-resonant Antennas are those for which resonant frequency does not occur. The wave moves in forward direction and hence do not form a standing wave. The radiation pattern of the radiated wave matches with the load impedance in the non-resonant antennas. These non-resonant antennas are non-periodic in nature. They are also called as Unidirectional travelling wave antennas, as the radiated wave moves in forward direction only, which means that only incident wave is present. As the frequency increases, the length of the antenna decreases and vice versa. Hence, the frequency and length are inversely proportional to each other. These long-wire antennas are the basic elements for the construction of V-shaped antennas or the Rhombic antennas. Learning working make money