Ducting::
· A duct is something that will confine whatever is traveling along it into a narrow
‘pipe’.
· The atmosphere can assume a structure that will produce a similar effect on radio
waves. When a radio wave enters a duct it can travel with low loss over great
distances. The atmosphere will then act in the manner of a giant optical fiber,
trapping the radio wave within the layer of high refractive index.
· A wave trapped in a duct can travel beyond the radio horizon with very little loss,
producing signal levels within a few dB of the free-space level.
Scattering::
· When an electromagnetic wave is incident on a rough surface, the wave is not so
much reflected as “scattered”.
· Scattering is the process by which small particles suspended in a medium of a
different index of refraction diffuse a portion of the incident radiation in all directions.
· Scattering occurs when incoming signal hits an object whose size in the order of the
wavelength of the signal or less.
Reflection::
· Reflection occurs when signal encounters a surface that is large relative to the
wavelength of the signal
· Radio waves may be reflected from various substances or objects they meet during
travel between the transmitting and receiving sites.
· The amount of reflection depends on the reflecting material.
o Smooth metal surfaces of good electrical conductivity are efficient reflectors of
radio waves.
o The surface of the Earth itself is a fairly good reflector.
· The radio wave is not reflected from a single point on the reflector but rather from an
area on its surface. The size of the area required for reflection to take place depends
on the wavelength of the radio wave and the angle at which the wave strikes the
reflecting substance.
· When radio waves are reflected from flat surfaces, a phase shift in the alternations of
the wave occurs
· The shifting in the phase relationships of reflected radio waves is one of the major
reasons for fading.
Refraction
· Refraction it is the bending of the waves as they move from one medium into
another in which the velocity of propagation is different.
· This bending, or change of direction, is always toward the medium that has the lower
velocity of propagation.
Difraction::
· Diffraction is the name given to the mechanism by which waves enter into the
shadow of an obstacle.
· Diffraction occurs at the edge of an impenetrable body that is large compared to
wavelength of radio wave.
· A radio wave that meets an obstacle has a natural tendency to bend around the
obstacle. The bending, called diffraction, results in a change of direction of part of
the wave energy from the normal line-of-sight path. This change makes it possible to
receive energy around the edges of an obstacle.
· The ratio of the signal strengths without and with the obstacle is referred to as the
diffraction loss. The diffraction loss is affected by the path geometry and the
frequency of operation. The signal strength will fall by 6 dB as the receiver
approaches the shadow boundary, but before it enters into the shadow region.
· Deep in the shadow of an obstacle, the diffraction loss increases with
10*log(frequency). So, if double the frequency, deep in the shadow of an obstacle
the loss will increase by 3 dB. This establishes a general truth, namely that radio
waves of longer wavelength will penetrate more deeply into the shadow of an
obstacle