Unguided Transmission Lines (Radio and Microwave)

• Radio wave propagation


  • Understand principles and parameters
  • Describe characteristics of ground waves, sky waves, space waves, and other propagation modes

  
• Microwave communication system


  • Describe principles and parameters involved

  
• Satellite communication system


  • Describe principles and parameters involved
  • Calculate for link budget and system gain
• Transverse electromagnetic wave

  The electric field, the magnetic field and the direction of propagation of the wave are all mutually perpendicular

  
• Essential properties of radio waves

  • Power density
  • Polarization
  • Characteristic impedance

  
• It is important to consider the effects of the environment and the optical properties of radio waves
• Free-space propagation of electromagnetic waves

  Radio-frequency (RF) propagation or simply radio propagation

  
• The energy level of the signal decreases rapidly with distance from the transmitting antenna
• Characteristic impedance of a lossless transmission medium

  Equal to the square root of the ration of its magnetic permeability to its electric permittivity
• Relationship between Electric and Magnetic Field Strength

  ξ = 120πH
• Power density for isotropic source

  Total power (transmitted power) divided by the surface area of the sphere
• Power densities and distances

  PD2 = PD1*(d1/d2)^2
• Calculating power density


  • 23.49 nW/m^2 at 25 km from a 90% efficient half-dipole with 125 W transmit power
  • 128 μW/m^2 at 25 km from a source with 200 μW/m^2 at 20 km
• Relationship between power density and electric field intensity


  PD = ξ^2/Z
• Electric field intensity in terms of output power and distance


  ξ = 30*√(Pt/r) or ξ = 30*√(Pt*Gt/d)
• Electric field strength for VHF Propagation (line-of-sight)


  ξ = ξ0*4πhtxhrx/(λ*d^2)
• Calculating electric field strength

  • 7 mV/m at 10 km from a 100 W half-wave dipole transmitter
  • 41.786 μV/m at 30 km from a 100 W base station transmitter at 160 MHz using a half-wave dipole antenna 20 m above ground, with receiving antenna at 4 m height
• The dielectric strength of air is about 3 MV/m
• Polarization of a plane electromagnetic wave

  The orientation of the electric field vector in respect to the surface of the earth (looking at the horizon)

  
• Types of polarization


  • Linear
  • Circular
  • Elliptical
• Attenuation


  The reduction in power density with distance due to spreading
• Absorption


  The reduction in the intensity of radiated energy within a medium caused by converting some of all of the energy into another form
• Refraction


  The changing of direction of an electromagnetic ray as it passes obliquely from one medium into another with different velocities of propagation

  
• Snell's law


  n1*sin(θi) = n2*sin(θr)
• Critical angle


  The angle of incidence that results in an angle of refraction of exactly 90°
• Specular reflection


  Reflection phenomenon where the wave hits a smooth surface and obeys the law of reflection θi = θr

  
• Diffuse reflection


  The reflected signal are scattered in different direction after it hits a rough surface

  
• Rayleigh criterion


  A semi-rough surface will reflect as if it were a smooth surface whenever the cosine of angle of incidence is greater than λ/8d, where d is the depth of irregularity (m)
• Diffraction


  The modulation or redistribution of energy within a wavefront when it passes near the edge of an opaque object

  
• Diffraction is the phenomenon that allows light or radio waves to propagate (peek) around corners

  
• Huygen's principle


  Every point on a given spherical wavefront can be considered as a secondary point source of electromagnetic waves from which the other secondary waves (wavelets) are reradiated

  
• Interference


  Phenomena that occurs when two radio waves that left one source and traveled different path arrive at a point

  
• Fading


  A general term applied to the reduction in signal strength at the input to a receiver
• Causes of fading


  • Variation in distance between transmitter and receiver
  • Changes in the environmental characteristics of the signal path
  • The presence of multiple signal paths
  • Relative motion between the transmitter and receiver
• Fading is also caused by objects coming between the transmitter and receiver known as shadow fading
• Free space path loss

  A fabricated engineering quantity that evolved from manipulating communications system link budget equations, defined as the loss incurred by an electromagnetic wave as it propagates in a straight line through vacuum with no absorption
• Fading


  Reduction in signal strength at the input to a receiver
• Fading


  • Applies to propagation variables in the physical radio path that affect changes in the path loss between transmit and receive antennas
  • Typically makes the received signal smaller
• Factors causing fading


  • Variation in distance between transmitter and receiver
  • Changes in the environmental characteristics of the signal path
  • The presence of multiple signal paths
  • Relative motion between the transmitter and receiver
• Shadow fading


  Fading caused by objects coming between the transmitter and receiver