Chia

• Transmission line


  Metallic conductor system used to transfer electrical energy from one point to another using electrical current flow
• Transverse electromagnetic wave


  Waves where the direction of displacement is perpendicular to the direction of propagation
• Wave velocity


  The distance travelled by a wave per unit time
• Frequency


  The rate at which the periodic wave repeats
• Wavelength


  The distance of one cycle occurring in space
• Balanced transmission line


  Has two conductor wires and a ground wire
• Unbalanced transmission line


  Has one conductor and a ground wire
• Open wire transmission line

  Two wire parallel conductors separated by air
• Twin lead transmission line

  Has spacers between the two conductors that are continuous solid dielectric ensuring uniform spacing
• Twisted pair transmission line

  Formed by twisting the insulated conductors around each other
• Shielded cable transmission line

  Parallel two wire transmission line with a conductive metal sleeve around the wire and dielectric
• Coaxial cable


  Consists of a center conductor surrounded by a dielectric material, then a concentric shielding, and finally a rubber environmental protection outer jacket
• Electrical properties of a transmission line


  • Characteristic impedance
  • Propagation constant
  • Attenuation constant
  • Phase constant
• Physical properties of a transmission line

  • Conductor material
  • Dielectric material
• Four primary constants of a transmission line

  • Series resistance
  • Shunt conductance
  • Series inductance
  • Shunt capacitance
• Characteristic impedance


  A complex quantity expressed in ohms, ideally independent of line length, and cannot be directly measured
• Propagation constant


  Used to express the attenuation and the phase shift per unit length of a transmission line
• Velocity factor


  The ratio of the actual velocity of propagation of an electromagnetic wave through a given medium to the velocity of propagation through vacuum
• Dielectric constant


  The relative permittivity of a material
• Electrical length


  At low frequencies the voltage along the line remains relatively constant, but at high frequencies several wavelengths of the signal may be present on the line at the same time
• Types of transmission line losses

  • Conductor losses
  • Dielectric heating losses
  • Radiation loss
  • Coupling losses
  • Corona
• Incident wave


  Electromagnetic waves that travel along a transmission line from the source to the load
• Reflected wave


  Electromagnetic waves that travel from the load back toward the source
• Resonant transmission line


  Has standing waves of current and voltage
• Non-resonant transmission line


  Has no standing waves of current and voltage
• Reflection coefficient


  A vector quantity that represents the ratio of reflected voltage or reflected current to incident current
• Standing waves


  Formed by the superposition of two travelling waves of the same frequency (with the same polarization and the same amplitude) travelling in opposite directions
• Standing wave ratio (SWR)


  The ratio of the maximum voltage to the minimum voltage or the maximum current to the minimum current of a standing wave on a transmission line
• Standing waves are waves of voltage and current which do not propagate, but are the result of interference between incident and reflected waves along a transmission line
• In a shorted line, the incident voltage and current waves are reflected back in the opposite manner
• Input impedance


  The impedance seen by any signal entering a transmission line, caused by the physical dimensions of the transmission line and its downstream circuit elements
• Quarter wavelength sections of transmission line play an important role in many systems at radio and optical frequencies, as they can act as impedance inverters
• Open transmission lines can behave as resistors, insulators, or capacitors depending on their electrical length