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RADIO WAVE PROPAGATION (Topics) 15EC304- ANTENNA AND WAVE PROPAGATION Presented By : Dr. T.Deepa Associate Professor, Department of Telecommunication Engineering SRM Institute of Science and Technology- 603203 UNIT V- RADIO WAVE PROPAGATION (Topics) • Basics of propagation: Definition and General Classification • Different Modes of propagation • Structure of ionosphere • Refraction and Reflection of sky wave by ionosphere • Ray path, Critical frequency, MUF, LUF , OF, Virtual Height and skip distance • Relation between MUF & Skip distance for flat Earth & Curved Earth • Ionospheric abnormalities, Impact of solar activity • Multi-Hop propagation and • Radio wave characteristics 01-05-2020 Prepared by Dr. T. Deepa 2 RADIO WAVE PROPAGATION INTRODUCTION : RADIO WAVE PROPAGATION 01-05-2020 Prepared by Dr. T. Deepa 3 What is Electromagnetic Wave? • Electromagnetic Waves also called Electro- magnetic Radiations are basically defined as superimposed oscillations of an Electric and a Magnetic Field in space with their direction of propagation perpendicular to both of them. • In simple words, electromagnetic waves are oscillations produced due to crossing over of an electric and a magnetic field. The direction of the propagation of such waves is perpendicular to the direction of the force of either of these fields as seen in the above figure. 01-05-2020 Prepared by Dr. T. Deepa 4 Properties of Electromagnetic(EM) Wave Propagation • EM waves travel at the speed of light. • EM waves do not require any medium for propagation. • EM waves travel in a transverse form. • EM waves are not deflected by electric or magnetic field. • EM waves can be polarized. • EM waves undergo interference and diffraction. 01-05-2020 Prepared by Dr. T. Deepa 5 Wavelength & Frequency of the EM waves • The wavelength(λ) and frequency (f) of the EM waves can be related as: c = f.λ where c = velocity of the wave. Now let us study some MODES of propagation of EM waves: Mesosphere, Troposphere, Ionosphere are the different layers. 01-05-2020 Prepared by Dr. T. Deepa 6 Classification : Radio Wave Propagation • These layers are used for propagation of EM waves and that EM waves travel basically in any one of the three methods given below: • Ground Wave • Space Wave • Sky Wave 01-05-2020 Prepared by Dr. T. Deepa 7 What is Ground Wave /Surface Wave Propagation ? • To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4). • At longer wavelengths (i.e., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground. • In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong influence on the propagation of the signal. • The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth. 01-05-2020 Prepared by Dr. T. Deepa 8 Properties of Ground Wave Propagation . Used for a low-frequency range transmission, mostly less than 2MHz. Employs the use of large antennas order of which is equivalent to the wavelength of the EM waves and uses the ground or Troposphere for its propagation. Signals over large distances are not sent using this method. It causes severe attenuation which increases with increased frequency of the waves. The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz). 01-05-2020 Prepared by Dr. T. Deepa 9 2. Sky Wave Propagation 01-05-2020 Prepared by Dr. T. Deepa 10 What is Sky Wave Propagation? • In the Frequency range 300KHz–30MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth. • The sky wave, often called the ionospheric wave, is radiated in an upward direction and returned to Earth at some distant location because of refraction from the ionosphere. • This form of propagation is relatively unaffected by the Earth's surface and can propagate signals over great distances. Usually the high frequency (HF) band is used for sky wave propagation. Prepared by Dr. T. Deepa 01-05-2020 11 Properties of Sky Wave Propagation • Used for the propagation of EM waves with a frequency range of 300KHz – 30MHz. • Make use of the ionosphere so called due to the presence of charged ions in the region of about 60 to 300 km from the Earth surface. These ions provide a reflecting medium to the radio or communication waves within a particular frequency range. • Use of the property of the ionosphere for long-distance transmission of the waves without much attenuation and loss of signal strength. • Examples • o Amateur radio , Citizens Band (CB) radio , International broadcasts 01-05-2020 Prepared by Dr. T. Deepa 12 3. Space Wave Propagation • Another mode of radio wave propagation is by space waves. • The space wave follows two distinct paths from the transmitting antenna to the receiving antenna : . one through the air directly to the receiving antenna, . the other reflected from the ground to the receiving antenna. 01-05-2020 Prepared by Dr. T. Deepa 13 Direct Wave Propagation . The primary path of the space wave is directly from the transmitting antenna to the receiving antenna. So, the receiving antenna must be located within the radio horizon of the transmitting antenna. Because space waves are refracted slightly, even when propagated through the troposphere, the radio horizon is actually about one-third farther than the LOS or natural horizon. 01-05-2020 Prepared by Dr. T. Deepa 14 Direct Wave Propagation (Cont’d) • At frequencies above 40 MHz, communication is essentially limited to LOS paths. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths above the ground. Because of LOS nature of propagation, direct waves get blocked at some point by the curvature of the earth as illustrated in Fig. • If the signal is to be received beyond the horizon then the receiving antenna must be high enough to intercept the LOS waves. Prepared by Dr. T. Deepa 01-05-2020 15 Direct Wave Propagation (Cont’d) • If the transmitting antenna is at a height hT, then the distance to the horizon dT is given as dT 2RhT where R is the radius of the Earth (approximately 6400 km). dT is also called the radio horizon of the transmitting antenna. • The maximum LOS distance dM between the two antennas having heights hT and hR above the earth is given by dM 2RhT 2RhR where hR is the height of receiving antenna. 01-05-2020 Prepared by Dr. T. Deepa 16 Space wave propagation (Satellite) • Space satellite communication and very high-frequency waves use this propagation method. • For very large distances, the height of the tower used for transmission is high enough to prevent waves from touching the earth curvature thus preventing attenuation and loss of signal strength. 01-05-2020 Prepared by Dr. T. Deepa 17 Properties of Space Wave Propagation • Space waves are used for LOS communication as well as satellite communication. • Space satellite communication and very high-frequency waves use this propagation method. 01-05-2020 Prepared by Dr. T. Deepa 18 Structure of Ionosphere • In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere. • The ionosphere is a region of the upper atmosphere, from about 80 km to 1000 km in altitude, where neutral air is ionized by solar photons and cosmic rays. • Ionization occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules. 01-05-2020 Prepared by Dr. T. Deepa 19 Ionospheric Effects on Radio Wave Propagation • Each ionized layer has a central region of relatively dense ionization, which tapers off in intensity both above and below the maximum region. As the wave enters into the upper part of the layer of DECREASING IONIZATION, the velocity of the radio wave decreases, and the wave is bent away from the Earth. As the wave is in the highly dense centre portion of the layer, however, refraction occurs more slowly because the DENSITY OF IONIZATION IS ALMOST UNIFORM. As a radio wave enters a region of INCREASING IONIZATION, the increase in velocity of the upper part of the wave causes it to be bent back towards the Earth. The relationship between radio waves 01-05-2020 and ionization density. Prepared by Dr. T. Deepa 20 Ionisation in the Ionospheric Layers . The ionosphere is further RADIATION CAUSING IONISATION IN THE IONOSPHERIC subdivided into several layers, LAYERS the details of which are given LAYERS PRIMARY IONISING in Table. RADIATION FORMS . Ionospheric Layers: C, D, E, C Cosmic F1, F2, Regions D Lyman alpha, Hard X-Rays E Soft X-Rays and some Extreme Ultra-Violet F1 Extreme Ultra-violet, and some Ultra-Violet F2 Ultra-Violet 01-05-2020 Prepared by Dr. T. Deepa 21 Electron density as a function of altitude • Layers of high electrons densities : D, E, and F layers, as shown in Figure. • During the day the F layer splits into two layers called the F1 and F2 layers. • The D layer vanishes completely at night. Electron density as a function of altitude, and various ionospheric layers 01-05-2020 Prepared by Dr. T. Deepa 22 Refraction by the Ionosphere . When a radio wave is transmitted into an ionized layer, refraction, or bending of the wave, occurs. Refraction is caused by an abrupt change in the velocity of the upper part of a radio wave as it strikes or enters a new medium. The amount of refraction that occurs depends on three main factors: the density of ionization of the layer, the frequency of the radio wave, and the angle at which the wave enters the layer 01-05-2020 Prepared by Dr.
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