PowerPoint Presentation: SPACE WAVE PROPAGATION INTRODUCTION(above 30MHz): INTRODUCTION(above 30MHz) The EM wave that propagates from the transmitter to the receiver in the earth’s troposphere is called space wave or tropospheric wave. Troposphere is the region of the atmosphere within 16km above the surface of the earth. The radio waves having high frequencies are basically called as space waves. These waves have the ability to propagate through atmosphere, from transmitter antenna to receiver antenna. These waves can travel directly or can travel after reflecting from earth’s surface to the troposphere surface of earth. So, it is also called as Tropospherical Propagation. Basically the technique of space wave propagation is used in bands having very high frequencies. E.g. V.H.F. band, U.H.F band etc. At such higher frequencies the other wave propagation techniques like sky wave propagation, ground wave propagation can’t work. Direct and Indirect waves: Direct and Indirect waves Direct wave : Direct wave Direct waves, are radio waves that travel directly from the transmitting antenna to the receiving antenna. In order for this to occur, the two antennas must be able to “see” each other; that is there must be a line of sight path between them. The maximum line of sight distance between two antennas depends on the height of each antenna. Indirect wave: Indirect wave Indirect or ground reflected wave. The wave reaches antenna after reflection from the ground. The phase change of 180 ° is introduced due to reflection at the ground. Both the waves (direct & indirect) leave the transmitting antenna at the same time with the same phase but may reach the receiving antenna in phase or out of phase. Factors contributing to field strength at the receiver:: Factors contributing to field strength at the receiver: In space wave propagation, the field strenth at the receiver is contributed by Direct ray from transmitter Ground reflected ray Reflected and refracted rays from the troposphere Diffracted rays around the curvature of the earth, hills and so on. Considerations in space wave propagation: Considerations in space wave propagation The space wave field strength is affected by the following: Curvature of the earth. Earth’s imperfections and roughness. Hills,tall buildings and other obstacles. Height above the earth. Transition between ground and space wave. Polarisation of the wave. Effect of curvature of earth: Effect of curvature of earth When the distance between the transmitting and receiving antennas is large,curvature of earth has considerable effect on SWP. The field strenth at the receiver becomes small as the direct ray may not be able to reach the receiving antenna. The earth reflected rays diverge after their incidence on the earth. The curvature of earth creates shadow zones. Where no signal reaches. Effect of earth curvature: Effect of earth curvature Shadow zones: Shadow zones Effect of earth’s imperfections and roughness: Effect of earth’s imperfections and roughness Earth is basically imperfect and electrically rough. When a wave is reflected from perfect earth, its phase change is 180̊ .But actual earth makes the phase change different from 180 ̊̊. The amplitude of ground reflected ray is smaller then that of direct ray. The field at the receiving point due to space is reduced by earth’s imperfection and roughness. Effect of hills,buildings and other obstacles.: Effect of hills,buildings and other obstacles. Hills, buildings and other obstacles create shadow zones. As a result possible distance of transmission is reduced Effect of the height above the earth: Effect of the height above the earth The field varies with the height above the earth. The field variation is characterised by the presence of maximas,minimas and nulls. Atmospheric effect: Atmospheric effect The atmosphere consist of gas molecules and water vapour . This causes the dielectric constant to be slightly greater than unity . The density of air and water vapour vary with height. As a result , the dielectric constant and hence refractive index of air depends on the height. The variation of refractive index with height gives rise to different phenomena like refraction,reflection , scattering , duct propagation and fading. Applications: Applications It is useful for FM,TV and radar applications. It is also used in VHF,UHF and higher frequency. Important at VHF band (between 30MHz to 300MHz). Microwave , communication,frequency modulation etc..