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Subel. ZLH
20 questions
Subelement ZLH

From Transmitter to Receiver

Section ZLH26

Transmission Lines

Any length of transmission line may be made to appear as an infinitely long line by

  • shorting the line at the end
  • leaving the line open at the end
  • Correct Answer
    terminating the line in its characteristic impedance
  • increasing the standing wave ratio above unity
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The characteristic impedance of a transmission line is determined by the

  • length of the line
  • load placed on the line
  • Correct Answer
    physical dimensions and relative positions of the conductors
  • frequency at which the line is operated
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The characteristic impedance of a 20 metre length of transmission line is 52 ohm. If 10 metres is cut off, the impedance will be

  • 13 ohm
  • 26 ohm
  • 39 ohm
  • Correct Answer
    52 ohm
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The following feeder is the best match to the base of a quarter wave ground plane antenna

  • 300 ohm balanced feedline
  • Correct Answer
    50 ohm coaxial cable
  • 75 ohm balanced feedline
  • 300 ohm coaxial cable
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The designed output impedance of the antenna socket of most modern transmitters is nominally

  • 25 ohm
  • Correct Answer
    50 ohm
  • 75 ohm
  • 100 ohm
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To obtain efficient transfer of power from a transmitter to an antenna, it is important that there is a

  • high load impedance
  • low load impedance
  • Correct Answer
    correct impedance match between transmitter and antenna
  • high standing wave ratio
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A coaxial feedline is constructed from

  • a single conductor
  • two parallel conductors separated by spacers
  • Correct Answer
    braid and insulation around a central conductor
  • braid and insulation twisted together
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An RF transmission line should be matched at the transmitter end to

  • prevent frequency drift
  • overcome fading of the transmitted signal
  • ensure that the radiated signal has the intended polarisation
  • Correct Answer
    transfer maximum power to the antenna
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A damaged antenna or feedline attached to the output of a transmitter will present an incorrect load resulting in

  • the driver stage not delivering power to the final
  • the output tuned circuit breaking down
  • Correct Answer
    excessive heat being produced in the transmitter output stage
  • loss of modulation in the transmitted signal
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A result of mismatch between the power amplifier of a transmitter and the antenna is

  • Correct Answer
    reduced antenna radiation
  • radiation of key clicks
  • lower modulation percentage
  • smaller DC current drain
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Losses occurring on a transmission line between a transmitter and antenna result in

  • Correct Answer
    less RF power being radiated
  • a SWR of 1:1
  • reflections occurring in the line
  • improved transfer of RF energy to the antenna
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If the characteristic impedance of a feedline does not match the antenna input impedance then

  • Correct Answer
    standing waves are produced in the feedline
  • heat is produced at the junction
  • the SWR drops to 1:1
  • the antenna will not radiate any signal
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A result of standing waves on a non-resonant transmission line is

  • maximum transfer of energy to the antenna from the transmitter
  • perfect impedance match between transmitter and feedline
  • Correct Answer
    reduced transfer of RF energy to the antenna
  • lack of radiation from the transmission line
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A quarter-wave length of 50-ohm coaxial line is shorted at one end. The impedance seen at the other end of the line is

  • zero
  • 5 ohm
  • 150 ohm
  • Correct Answer
    infinite
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A switching system to use a single antenna for a separate transmitter and receiver should also

  • Correct Answer
    disable the unit not being used
  • disconnect the antenna tuner
  • ground the antenna on receive
  • switch between power supplies
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An instrument to check whether RF power in the transmission line is transferred to the antenna is

  • Correct Answer
    a standing wave ratio meter
  • an antenna tuner
  • a dummy load
  • a keying monitor
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This type of transmission line will exhibit the lowest loss

  • twisted flex
  • coaxial cable
  • Correct Answer
    open-wire feeder
  • mains cable
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The velocity factor of a coaxial cable with solid polythene dielectric is about

  • Correct Answer
    0.66
  • 0.1
  • 0.8
  • 1.0

A radio wave in free space travels with the speed of light. When a wave travels on a transmission line, it travels slower, travelling through a dielectric/insulation. The speed at which it travels on a line compared to the free-space velocity is known as the "velocity factor". Typical figures are: Twin line 0.82, Coaxial cable 0.66, (free space 1.0). So a wave in a coaxial cable travels at about 66% of the speed of light (as an example). In practice this means that if you have to cut a length of coaxial transmission line to be a half-wavelength long (for, say, some antenna application), the length of line you cut off will have to be 0.66 of the free-space length that you calculated.

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This commonly available antenna feedline can be buried directly in the ground for some distance without adverse effects

  • 75 ohm twinlead
  • 300 ohm twinlead
  • 600 ohm open-wire
  • Correct Answer
    coaxial cable
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If an antenna feedline must pass near grounded metal objects, the following type should be used

  • 75 ohm twinlead
  • 300 ohm twinlead
  • 600 ohm open-wire
  • Correct Answer
    coaxial cable
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