What is the maximum theoretical range at which an aircraft flying at 5000 ft AMSL can receive a VHF radio transmission from a station at 1525 ft AMSL?
Refer to figure.
The space wave is the direct, line-of-sight transmission of a radio wave through space. Because the Earth is round, VHF communications via space waves are limited by the curvature of the Earth. As such, aircraft at higher altitude and transmission stations at higher altitude have a higher maximum theoretical communication range.
The equation to calculate maximum theoretical range is:
- Maximum theoretical range (in nm) = 1.23 x (√H1 + √H2)
Where:
- H1 is the receiver height in feet
- H2 is the transmitter height in feet
Calculating the range for the problem in the question:
- Range = 1.23 x (√5000 + √1525) = 134.07 NM
Note: Depending on the calculator or method used this answer may also come out to 135 NM. As this is not the answer reported in the exams we are keeping the answer as 134 NM until it is reported as having changed. This question is regularly seen with many different heights included. It is vital that you learn the formula NOT the numbers!
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