Subelement A: VHF-DSC Equipment & Operation— Topic :
Question 10A1
Element 9 (GMDSS Maintainer)It becomes necessary to construct a temporary VHF marine antenna. What should be the approximate length of the antenna?
Explanation
The VHF marine band operates primarily between 156 and 162 MHz. A common and efficient design for a temporary vertical antenna is a quarter-wave monopole.
To calculate the approximate physical length of a quarter-wave antenna in feet, we use the formula:
Length (feet) = 234 / Frequency (MHz)
Using an approximate center frequency of 159 MHz for the marine band:
Length = 234 / 159 MHz ≈ 1.47 feet
To convert this to inches:
1.47 feet * 12 inches/foot ≈ 17.64 inches
Therefore, an 18-inch antenna is the closest approximation to a quarter-wavelength for the VHF marine band. Options A, B, and C are significantly shorter than the resonant quarter-wave length, which would lead to very poor antenna efficiency and high Standing Wave Ratio (SWR).