The wavelength of frequencies within the VHF band is approximately 2 m. Practically, this means that the length of a VHF antenna (dipole antenna) for a VHF radio need not be longer than about half this length, about 1 m., which makes VHF radio communications ideally suitable for all vessels: from large passenger ships down to the smallest fishing boat or yacht.

Electro-magnetic waves of such a relatively small wavelength tend to travel or propagate in an almost straight line between Tx and Rx and extend only as far as the visible horizon (line of sight). These space waves, as they are referred to, will not pass through objects such as hills or large buildings, but may be reflected by such structures. For these reasons, it is best to site the VHF antenna at the highest point of the vessel in order to obtain the greatest communication range (in Tx and Rx).



Place the mouse on either a particular vessel or the CRS Tx/Rx tower on the land and you will realize that the red coloured boat is out of radio communication range of the CRS Tx/Rx tower. All other vessels are able to communicate by radio with the CRS Tx/Rx tower, either because they are physically closer to the CRS than the red coloured boat, or their antennas are sufficiently high enough to obtain a good line of sight communication with it. If the red coloured boat were also out of VHF radio communication range of each of the other vessels, then any Distress, Urgency, Safety or Routine alerts or calls from it could not be heard.

The range of VHF radio communications (distance from antenna to the visible horizon) is calculated by the following formula (the symbol √ stands for square root):

Range [NM] = 2.25 x √height of antenna [m]

When calculating the range between two antennas, the range of both antennas have to be taken into consideration. The formula is:

Range between two antennas [NM] = 2.25 x (√height of antenna I [m] + √height of antenna II [m])

Example: the range between vessel antenna (30 m above sea level) and CRS antenna (62 m above sea level) is 2.25 x (√30 + √62) = 30.04 NM

VHF transmission range

The range of VHF radio communications at sea may vary greatly between different types of vessel and between different types of vessel and CRS. This is due to the varying heights of antennas ( h ) in use, and is dependent on the size of vessels and locations of CRS. Under normal radio conditions the expected radio communication range at VHF will normally be expected to be between 20 to 30 NM.

The marine frequency band for VHF radio communication, extending between 156 MHz and 174 MHz, contains 57 individual VHF CH (channels) numbered consecutively from VHF CH 1 to VHF CH 28 and from VHF CH 60 to VHF CH 88.