Login or Register for FREE!
Subelement G
VHF-DSC Equipment & Communications:
Section 49
VHF Controls, Power and Range:
Which of the following control selections may result in limited receiving range?
  • Correct Answer
    Setting the squelch control to its maximum level.
  • Setting the squelch control to its minimum level.
  • The power switch is set to the "high" output position, resulting in receiver overloading.
  • Setting the channel selection switch midway between channels 6 and 16.

Which of the following control selections may result in limited receiving range?

(A). Setting the squelch control to its maximum level.

Squelch is a setting which filters out background noise being received.

Squelch is turned down such that the background noise is just audible, and then down one additional level.

But you can have too much of a good thing, and if you increase squelch, you will blank out receiving some transmission. Same works for hand-helds and CB radios.

For more information about squelch control, please see Wikipedia's article on Squelch

Last edited by markadlerdallas. Register to edit

Tags: none

At mid-day, what would be the best choice in attempting to communicate with a shore station 15 miles distant?
  • 16 MHz band
  • Correct Answer
    156-164 MHz band
  • 12 MHz band
  • 22 MHz band

At mid-day, what would be the best choice in attempting to communicate with a shore station 15 miles distant?

(B). 156-164 MHz band

US Coast Guards has defined channels within the 156.050 MHz to 162.025 MHz. The only answer of 156-164 MHz bands fits the defined channels.

US Coast Guard Table of Marine Frequencies

Last edited by markadlerdallas. Register to edit

Tags: none

Which of the following factors does not normally affect the range of VHF transmissions?
  • Salt water ingress into the antenna coaxial cable.
  • Power level setting.
  • Correct Answer
    Ionospheric refraction.
  • Vessel antenna height.

Which of the following factors does not normally affect the range of VHF transmissions?

(C). Ionospheric refraction.

The ionosphere lets the VHF propagation waves to advance further into space. It does not reflect these higher frequency waves as it does with HF waves. Because there is bouncing back from the ionosphere, there is no far propagation.

If there is an inversion, cold air close to ground, and warm air on top of the cold air layer, then there is reflective propagation. Right weather condition occur randomly so the greater propagation cannot be expected.

For more information about the radio wave focused atmospheric layers composition, please see Wikipedia's article on Ionosphere

Last edited by markadlerdallas. Register to edit

Tags: none

Much longer than normal VHF communications distances are typically caused by:
  • Changing power from 1W to 25 W.
  • Skywave reflections from the D layer.
  • Ionospheric activity in layers F1/F2.
  • Correct Answer
    Atmospheric ducting or tropospheric propagation.

Much longer than normal VHF communications distances are typically caused by:

(D). Atmospheric ducting or tropospheric propagation.

When there is a temperature inversion, or a thermal inversion, the warmer air is above the cooler air. Normally, the air gets colder as it rises.

This condition significantly affects radio frequencies and their propagation. The VHF frequencies change direction (angle) by a kind of bounce, and so can be heard much further than normally expected.

Also, some waves instead of continuing into space are bounced back to Earth, increasing the listening coverage area.

For more information about the radio wave focused atmospheric layers composition, please see Wikipedia's article on Ionosphere

Last edited by markadlerdallas. Register to edit

Tags: none

Describing VHF transmissions as "line of sight" does not mean:
  • Correct Answer
    VHF communications are effective only with nearby stations within visual range of the bridge.
  • Vessel antenna height will not affect the radius of propagation.
  • The normal transmission range to a coast station is approximately is 10 NM.
  • Coast station antenna height has no effect on the radius of transmission.

Describing VHF transmissions as "line of sight" does not mean:

(A). VHF communications are effective only with nearby stations within visual range of the bridge.

The VHF frequencies propagate well when there are no obstructions between the transmitter and receiver. The line-of-sight transmission is limited to a distance due to curvature of the Earth. However, it tends to extend about 15% further than actual line of sight, as the waves themselves are also bend a bit.

In maritime usage, a range of 5-10 miles can be depended on, and even 20 miles to a US Coast Guard station. How tall is the antenna and what watts are present in the effective radion power of the antenna will affect the distance.

Last edited by markadlerdallas. Register to edit

Tags: none

The effectiveness of VHF communications is maximized by:
  • The adjustment of squelch for maximum receiver sensitivity, setting transmitter power to 1W & selecting an appropriate channel.
  • Correct Answer
    Appropriate setting of the transmitter power, selecting an appropriate channel & adjustment of squelch for maximum receiver sensitivity.
  • Selecting an appropriate channel, adjustment of squelch for minimum receiver sensitivity & setting transmitter power to 1W.
  • Selecting an appropriate channel, adjustment of squelch for minimum receiver sensitivity, setting transmitter power to 25W.

The effectiveness of VHF communications is maximized by:

(B). Appropriate setting of the transmitter power, selecting an appropriate channel & adjustment of squelch for maximum receiver sensitivity.

Appropriate setting of the transmitter power
FCC asks that you use the minimum of power to achieve connections. Given various obstacles, presence of metal structures, hills, etc., it may be necessary to adjust radiation power to communicate.

selecting an appropriate channel
To match the right frequency (channel) to the nature of the communication would be key to getting the message across and engage other parties. Channels are designated for different types of messages and should be consulted.

adjustment of squelch for maximum receiver sensitivity.
To maximize adjustment, increase squelch but only to a point. If you increase too much, you will block some messages. Too much of a good thing may not help to communicate.

Last edited by markadlerdallas. Register to edit

Tags: none

Go to 48 Go to 51