Radio communication known as RT (Radio Telephony) uses a Tx (radio transmitter) to send out radio waves of a certain frequency and an Rx (radio receiver) to receive the radio waves at the same frequency. Radio waves consist of electro-magnetic energy in specific frequency bands within the radio frequency spectrum. Frequency of waves means the number of occurrences of a wave per second. For example, marine VHF RT works in the VHF part of the marine radio frequency spectrum, which extends between 156 MHz to 174 MHz.
The wavelength of frequencies within the radio frequency spectrum is obtained by the following relationship:
Wavelength [m] = Speed of an electro-magnetic wave in vacuum (300 x 106 m/s) / Frequency [Hz=1/s]
Example: Wavelength of 160 MHz wave frequency is (300 x 106) / (160 x 106 ) = 1,875 m
GMDSS terrestrial communications provide services through VHF (Very High Frequency - 30 MHz to 300 MHz), HF (High Frequency - 3 MHz to 30 MHz) and MF (Medium Frequency - 300 kHz to 3 MHz).
The full radio spectrum chart, including VHF, HF, and MF
The term propagation is used to refer to the movement of radio waves from a transmitter to a receiver.
For LRC purposes three types of propagation are of concern:
- Line-of-sight-wave propagation (Direct wave)
- Ground-wave propagation
- Sky-wave propagation
For most marine communication in the range of 50 MHz and above ( VHF communication) the line-of-sight-wave (direct wave) propagation type is used, which works where no physical obstacles are interposed between Tx and Rx.
Ground-wave propagation is used over longer distances at which the curvature of the Earth comes into play. Ground-wave propagation is common for MF communication. The length of propagation over land depends on obstructions (hills, etc.), while at sea, the 'terrain' being flat, the distance is always longer.
Sky-wave propagation is affected by ionospheric reflection. Variations in propagation length occur according to such factors as time of day, year, sunspots as well as other ionospheric conditions. Sky-wave propagation is common for HF communication.
Types of propagation among the radio spectrum