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Subelement A
VHF-DSC Equipment & Operation:
Section 7
Transmitter Faults-1:
What condition may be indicated if a VHF radio is unable to contact nearby stations and an inline wattmeter indicates power outputs of 1 watt (LOW power) mode and 8.5 watts (HIGH power) mode with +12.8 vdc applied to the radio's DC line?
  • The AF Power Amplifier (PA) may be malfunctioning when the transmitter operates in the high power mode.
  • Negative feedback in the PA stage may be preventing high amplitude oscillations in the high power mode.
  • Correct Answer
    The transmitter output is low in the high power mode but it is still functioning within FCC specifications.
  • The transmitter output will increase in the high power mode with modulation between 75 and 100 percent.
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Antenna system matching can be analyzed by determining the Standing Wave Ratio (SWR). Which of the following is the best test procedure to measure the SWR of a marine VHF antenna system?
  • Insert an in-line RF Power meter between the VHF radio and an artificial antenna, set the radio to any channel, key the microphone, and read the measured value.
  • Insert a SWR meter between the VHF transmitter and the coaxial transmission line connected to the antenna, set the radio to channel 16, and conduct a "RADIO CHECK" with a nearby U.S. Coast Guard station or vessel.
  • Insert an in-line RF Power meter between the VHF radio and the coaxial transmission line disconnected from the antenna, activate the Push-To-Talk (PTT) circuit, and read the forward and reflected values.
  • Correct Answer
    Insert a SWR meter between the VHF radio and the coaxial transmission line connected to the antenna, set the radio to an unused channel, activate the Push-To-Talk (PTT) circuit, and read the measured value.
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A mismatched antenna system can degrade marine VHF ­radio performance. Antenna system matching can be analyzed by determining the Standing Wave Ratio (SWR). Which of the following is the best test procedure to determine the VHF antenna system SWR?
  • Correct Answer
    Insert a directional wattmeter between the VHF transmitter and the coaxial transmission line connected to the antenna, activate the Push-To-Talk (PTT) circuit, observe the measured values of forward and reflected power.
  • Insert a SWR meter between the VHF transmitter and the coaxial transmission line disconnected from the antenna, set the radio to an unused channel, activate the Push-To-Talk circuit, and read the measured value.
  • Insert a directional wattmeter between the VHF radio and an artificial antenna, set the radio to any channel, key the microphone, and read the values of forward and reflected power.
  • Insert an in-line RF Power meter between the VHF transmitter and the coaxial transmission line connected to a 50 ohm load, activate the Push-To-Talk (PTT) circuit, observe the measured value of reflected power.
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What may be indicated if a VHF radio operates normally but the transmitter produces nearly zero RF output in the high power mode on all marine channels?
  • The antenna system is presenting a Standing Wave Ratio (SWR) of 1.5 to 1.0 on the desired frequency.
  • Correct Answer
    The SWR protection circuit in the transmitter has engaged to prevent damage to the Power Amplifier.
  • A "crowbar" circuit in the power supply has prevented an over current condition that might damage the radio.
  • A 50-ohm impedance in the antenna system is preventing the transmitter from achieving full power output.
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What condition may indicate voltage controlled oscillator failure in the synthesizer of a VHF transmitter?
  • Low transmitter power output in the high power mode.
  • Correct Answer
    An out-of-lock condition in the Phased Locked-Loop circuit.
  • Excessive deviation in the frequency modulated output signal.
  • A constant transmitter power output at various modulation levels.
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What condition may exist if a VHF radio is operational in receive mode but unable to transmit?
  • Excessive bias is being applied to switching transistors in the transmitter output stage.
  • The solid-state antenna relay is no longer functioning properly.
  • Correct Answer
    A component in the Push-To-Talk (PTT) circuit has failed.
  • The SWR protection circuit has engaged to prevent transmission.
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