High Frequency (HF)

The radio frequency range between 3 and 30 megahertz (MHz), commonly used for long-range marine communications via skywave propagation. 

High Frequency (HF) radio is an essential communication tool in the maritime industry, particularly for vessels operating beyond the range of Very High Frequency (VHF) and Ultra High Frequency (UHF) systems. HF radio waves can travel long distances by reflecting off the ionosphere, a phenomenon known as skywave propagation. This allows ships to maintain contact with coastal stations, other vessels, and maritime authorities even when they are far from shore. HF radio is widely used for ship-to-shore communication, weather updates, distress calls, and coordination between vessels. 

Unlike VHF, which relies on line-of-sight transmission, HF signals can reach thousands of miles, making them indispensable for ocean-going vessels. However, HF communication is subject to atmospheric conditions, solar activity, and interference, which can affect signal clarity. To improve reliability, marine HF radios often use Single Sideband (SSB) modulation, which reduces bandwidth usage and enhances signal strength. 

International maritime regulations, such as those outlined by the Global Maritime Distress and Safety System (GMDSS), require certain vessels to be equipped with HF radio for emergency communication. Despite the rise of satellite communications, HF radio remains a crucial backup system for long-distance maritime communication.