UNIT-5
Ground Wave Propagation
● Reflection occurs when a wave impinges on an object with large dimensions relative to a wavelength. The surface of the earth, and large buildings are examples.
● In considering reflections introduced by the environment, the most significant source of reflections in terrestrial links is the earth’s surface itself. Consider the link show below:
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Figure 5.1. Plane Earth Reflection
● Sample E-field vectors are shown for the parallel and perpendicular polarization cases. Note that there are two paths from the transmitter (TX) to the receiver (RX): a line-of-sight (LOS) path and a reflected path.
● The polarization of the LOS path, regardless of the TE/TM polarization case, remains constant; however, for the reflected path, the polarization of the reflected components are shown 180 degrees out of phase with the LOS path.
● This results when θi → 90◦ (grazing incidence) at an interface with lossy dielectric (not a PEC). The grazing angle assumption is valid since in many cases the antenna heights h1, h2 are very small compared to the TX-RX separation.
Key takeaways:
● Reflection occurs when a wave impinges on an object with large dimensions relative to a wavelength.
● The surface of the earth, and large buildings are examples.
● Space wave propagation is defined for the radio waves that occur within the 20km of the atmosphere i.e. troposphere, comprising of a direct and reflected waves.
● These waves are also known as tropospheric propagation as they can travel directly from the earth’s surface to the troposphere surface of the earth. It is also known as a line of sight propagation as the signals are sent in a straight line from the transmitter to the receiver.
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Figure 5.2. Space Wave Propagation
Space wave propagation depends on three components:
- Direct Wave- The radio waves that transmitted from the transmitting antenna, reach the receiving antenna directly.
- Ground Reflected Wave - The radio waves that reach the receiving antenna after reflection from the ground.
- Tropospheric Wave – The radio waves that reach the receiving antenna after reflection from the troposphere.
● In order to prevent attenuation and loss of signal strength, the height of the antennas and distance between them can be given as:
Dm = (2RHt)-½ + (2RHr)-½
Where,
Dm: distance between the two antennas
R: radius of the earth
Ht: height of transmission antenna
Hr: height of receiver antenna
5.2.1. Applications
It is used in various communication systems like
● A line of sight communication and satellite communication
● Radar communication
● Microwave linking
5.2.2. Limitations
● These waves are affected by the curvature of the earth.
● The propagation of these waves happens along the line of sight distance which is defined as the distance between the transmitting antenna and the receiving antenna which is also known as the range of communication.
Key takeaways:
● Space wave propagation is defined for the radio waves that occur within the 20km of the atmosphere i.e. troposphere, comprising of a direct and reflected waves.
● In order to prevent attenuation and loss of signal strength, the height of the antennas and distance between them can be given as: Dm = (2RHt)-½ + (2RHr)-½)
● Surface wave is a mechanical wave that propagates along the interface between differing media.
● Surface waves are typically generated when the source of the earthquake is close to the Earth’s surface. As their name suggests, surface waves travel just below the surface of the ground.
● Although they move even more slowly than S-waves, they can be much larger in amplitude and are often the most destructive type of seismic wave. There are several types of surface wave, but the two most common varieties are Rayleigh waves and Love waves.
5.3.1. Rayleigh waves
● Rayleigh waves, also known as ground roll, spread through the ground as ripples, similar to rolling waves on the ocean. Like rolling ocean waves, Rayleigh waves move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.
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Figure 5.3.1. Rayleigh waves
● Rayleigh waves propagate through the ground as ripples.
● Eyewitnesses have claimed to observe Rayleigh waves in large open spaces, such as car parks, where they described the vehicles moving up and down like corks floating on the ocean.
● Rayleigh waves are slower than body waves and typically travel at a speed that is 10% slower than S-waves.
● Love waves have the same motion as S-waves but without the vertical displacement. They move the ground from side to side in a horizontal plane but at right angles to the direction of propagation.
● Love waves are particularly damaging to the foundations of structures because of the horizontal ground motion they generate. Love waves can also cause horizontal shearing of the ground.
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● They usually travel slightly faster than Rayleigh waves, at a speed that is usually about 10% slower than S-waves, but like S-waves, they cannot spread through water.
● Love waves are particularly damaging to the foundations of structures.
Key takeaways:
● Surface waves are typically generated when the source of the earthquake is close to the Earth’s surface.
● As their name suggests, surface waves travel just below the surface of the ground.
● Rayleigh waves propagate through the ground as ripples.Love waves are particularly damaging to the foundations of structures.)
5.4.1. Introduction
● These waves occur within the lower 20 km of the atmosphere, and are comprised of a direct and reflected wave. 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.
● Only space wave propagation is left which can handle frequency waves of higher frequencies. The other name of space wave propagation is line of sight propagation.
5.4.2. Principle used in space wave propagation
● 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.
● 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 line-of-sight or natural horizon.
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Figure 5.4.2. Principle used in space wave propagation
● Although space waves suffer little ground attenuation, they nevertheless are susceptible to fading. This is because space waves actually follow two paths of different lengths (direct path and ground reflected path) to the receiving site and, therefore, may arrive in or out of phase.
● If these two component waves are received in phase, the result is a reinforced or stronger signal. Likewise, if they are received out of phase, they tend to cancel one another, which results in a weak or fading signal.
5.4.3. Field strength
● In principle, well removed from the earth’s surface, the field strength of the space wave is given by the inverse distance law. However, most services operate in the vicinity of the earth and propagation paths other than the direct ray from the transmitting to the receiving antennas are possible, most notably involving reflection from the ground.
● As expected, the difference in path lengths between the direct and ground reflected rays will lead to a phase difference on reception; that will cause interference between the two rays.
● In addition, there is a further phase change introduced into the ground reflected ray at the point of reflection that adds to the interference.
● We will now analyse this situation to derive an expression for space wave field strength. Consider the geometry of Fig. below, from which we can see that the path lengths for the two rays are given by the following expressions, noting the simplifications possible since generally d >> ht, hr.
dd = 
d 
dr = 
d 
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Figure 5.4.3. Geometry of space wave propagation
● The difference in path lengths is
- 
= 
● From which the phase difference (calculated as 2π times the fractional difference in wavelength) is seen to be
● This is one element of the phase delay of the reflected ray compared with the direct ray. As noted above, there is a further phase delay introduced at the point of reflection with the ground.
● The electric field just after reflection compared with that just before is described by the reflection coefficient ρ = |ρ|
, a complex quantity that describes a change in amplitude and phase.
5.4.4. 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 strength 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.
5.4.5. Effect of Imperfection of earth
● 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 from180̊
● The amplitude of ground reflected ray is smaller than that of direct ray.
● The field at the receiving point due to space is reduced by earth’s imperfection and roughness
5.4.6. Limitations of space wave Propagation
● As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering.
● There are some limitations of space wave propagation: These waves are limited to the curvature of the earth. These waves have line of sight propagation, means their propagation is along the line of sight distance.
Key takeaways:
● These waves occur within the lower 20 km of the atmosphere, and are comprised of a direct and reflected wave. The radio waves having high frequencies are basically called as 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.
● 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.
● The earth reflected rays diverge after their incidence on the earth. The curvature of earth creates shadow zones.
● When a wave is reflected from perfect earth, its phase change is 180̊ . But actual earth makes the phase change different from180̊.
References
● “Antennas and Wave Propagation” by A R Harish and M Sachidananda.
● G. S. N. Raju, “Antennas and Wave Propagation”, Pearson Education.
● K. D. Prasad, “Antenna and Wave Propagation”, Satya Prakashan.
