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Subel. E
6 questions
Subelement E
Antenna Systems
Section 40
Transmission Lines
The VSWR of a microwave transmission line device might be measured using:
  • A dual directional coupler and a power meter.
  • A network analyzer.
  • A spectrum analyzer.
  • Correct Answer
    A dual directional coupler, a power meter, and a network analyzer.

Answer D. would be more correct if it read: "A dual directional coupler and a power meter OR a network analyzer" as using one precludes having to use the other.

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The impedance total (ZO) of a transmission line can be calculated by ZO = √L/C when L and C are known. When a section of transmission line contains 250 microhenries of L and 1000 picofarads of C, its impedance total (ZO) will be:
  • 50 ohms.
  • 250 ohms.
  • Correct Answer
    500 ohms.
  • 1,000 ohms.

Z = sqrt ( L / C )

Z = sqrt ( 250µH / 1000pF )

Z = sqrt (250x10^-6H / 1000x10^-12F)

Z = sqrt (250x10^-6H / 1000x10^-12F)

Z = sqrt (250000)

Z = 500Ω

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If long-length transmission lines are not properly shielded and terminated:
  • The silicon crystals can be damaged.
  • Correct Answer
    Communications receiver interference might result.
  • Overmodulation might result.
  • Minimal RF loss can result.

Long improperly-shielded transmission lines can and will act as antennae, so Answer B is the right one.

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A certain length of transmission line has a characteristic impedance of 72 ohms. If the line is cut at its center, each half of the transmission line will have a ZO of:
  • 36 ohms.
  • 144 ohms.
  • Correct Answer
    72 ohms.
  • The exact length must be known to determine Zo.

The impedance of a transmission line is independent of its length.

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Standing waves on a transmission line may be an indication that:
  • All energy is being delivered to the load.
  • Source and surge impedances are equal to ZO and ZL.
  • The line is terminated in impedance equal to ZO.
  • Correct Answer
    Some of the energy is not absorbed by the load.

Some of the energy is not absorbed by the load... and is being reflected with a non-zero phase difference back to the transmitter.

The superposition of the RF wave going to the transmitter and the wave being reflected back will cause a voltage standing wave pattern to develop along the transmission line.

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What precautions should be taken with horizontal waveguide runs?
  • Correct Answer
    They should be sloped slightly downwards at the elbow and a small drain hole drilled in the elbow.
  • They should be absolutely level.
  • They should not exceed 10 feet in length.
  • None of the above.

Water infiltration is the bane of trnsmillion lines, both coax and waveguides.

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