Radio Propagation & VHF Range Flashcards

VHF marine radio waves primarily travel by which type of propagation?

Answer: Line-of-sight propagation limited to the radio horizon

VHF radio waves travel predominantly by line-of-sight propagation and are limited to the radio horizon. Unlike HF (high frequency) radio, VHF waves do not reflect off the ionosphere, which constrains their range to approximately 20–30 nautical miles under normal conditions.

What is the typical maximum VHF communication range between two recreational vessels at sea level?

Answer: 20–30 nautical miles

Due to line-of-sight propagation, two vessels at sea level can typically communicate over approximately 20–30 nautical miles. Range to a coast station with a high-mounted antenna can be considerably greater, which is why HM Coastguard antennas are positioned at height.

How does increasing antenna height affect the range of a VHF marine radio?

Answer: It increases range by extending the radio horizon

Raising the antenna height directly extends the radio horizon and increases VHF communication range. This is why a masthead-mounted antenna gives greater range than a deck-mounted one, and why coast station antennas are sited on high ground or elevated structures.

What is the standard maximum high-power output of a fixed marine VHF radio?

Answer: 25 watts

Fixed marine VHF radios operate at a maximum output of 25 watts on high power. Handheld portable VHF radios are limited to a maximum of 5 watts. The 25-watt maximum is specified by ITU regulations and Ofcom for marine use in the UK.

When communicating with a vessel close by, which power setting should be selected?

Answer: Low power (1W) to avoid causing unnecessary interference to other stations

When communicating with a nearby vessel, low power (1 watt) should be selected. This is sufficient for short-range contact and avoids causing unnecessary interference to other stations that may be using the same channel at greater distances. Good radio discipline requires using only the power needed.

Which factor would most significantly reduce VHF radio range when at sea?

Answer: Physical obstruction by a land mass or the vessel's own superstructure

Physical obstructions — land masses, cliffs, headlands, or the vessel's own superstructure — significantly reduce VHF range by blocking the line-of-sight propagation path. Weather conditions (rain, wind) have minimal effect on VHF signals, and time of day is irrelevant for VHF communications.