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Subelement J
Antennas
Section 63
Antenna Theory
Which of the following could cause a high standing wave ratio on a transmission line?
  • Excessive modulation.
  • An increase in output power.
  • Correct Answer
    A detuned antenna coupler.
  • Low power from the transmitter.
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Why is the value of the radiation resistance of an antenna important?
  • Correct Answer
    Knowing the radiation resistance makes it possible to match impedances for maximum power transfer.
  • Knowing the radiation resistance makes it possible to measure the near-field radiation density from transmitting antenna.
  • The value of the radiation resistance represents the front-to-side ratio of the antenna.
  • The value of the radiation resistance represents the front-to-back ratio of the antenna.
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A radio frequency device that allows RF energy to pass through in one direction with very little loss but absorbs RF power in the opposite direction is a:
  • Circulator.
  • Wave trap.
  • Multiplexer.
  • Correct Answer
    Isolator.
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What is an advantage of using a trap antenna?
  • Correct Answer
    It may be used for multiband operation.
  • It has high directivity in the high-frequency bands.
  • It has high gain.
  • It minimizes harmonic radiation.
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What is meant by the term radiation resistance of an antenna?
  • Losses in the antenna elements and feed line.
  • The specific impedance of the antenna.
  • The resistance in the trap coils to received signals.
  • Correct Answer
    An equivalent resistance that would dissipate the same amount of power as that radiated from an antenna.
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What is meant by the term antenna bandwidth?
  • Antenna length divided by the number of elements.
  • Correct Answer
    The frequency range over which an antenna can be expected to perform well.
  • The angle between the half-power radiation points.
  • The angle formed between two imaginary lines drawn through the ends of the elements.

Antenna bandwidth is the frequency range over which an antenna can be expected to perform well.

Some antennas have a wider bandwidth than others. When an antenna has narrow bandwidth, this doesn't necessarily mean that the antenna won't work, but it does mean that it will have a harder time working on the frequencies that a wideband antenna would be able to handle.

A good example of a wideband antenna is the Log-periodic antenna

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