TELECOMMUNICATIONS SYSTEM

  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

  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 phas

  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

  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

  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 s

  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 wil