Marine Radio Equipment Installation on Ships
Having talked with many current, and retired Electro Technical Officers (ETO), it is apparent that radio communications is considred by some, a ‘Black Art’.
The main two frequency bands that you will come across on a Marine vessel are HF & VHF.
HF is short for High Frequency, and are the frequencies in the ‘radio part’ of the electromagnetic spectrum, that are between 0-30 Mhz (Mega Hertz).
VHF is short for Very High Frequency, and are radio frequencies between 30 – 300 Mhz (Mega Hertz).
Different frequencies are used for different purposes, but briefly fall into the following characteristics:-
HF (High Frequency): allows for greater communication distances, which can be up to thousands of miles.
The radio signals do not travel in straight lines, but can ‘bounce’ off various ‘layers’ of the sky and also off the ground.
At different times of the day and night, it is sometimes neccessary to ‘re-tune’ to appropriate HF frequencies to maintain communication.
This is because the various ‘layers’ that the signals bounce off, change during the 24 hour daily cycle.
Some higher HF frequencies are also affected by the 12 year ‘Sun Spot Cycle’, but we will not elaborate here.
VHF (Very High Frequency)
VHF is normally ‘Line Of Sight’ only, with a range of up to approximately 50 Miles.
The reason that VHF is line of sight, is that the radio waves do not ‘bounce’ off the Ionisphere (Sky), or the ground.
Imagine Ship ‘A’ is located east of Bermuda, and Ship ‘B’ is located just west of Madeira, with nothing but sea between them.
It is more than 50 Miles between them, and they cannot communicate on VHF.
This is due to the earths curvature, in other words the fact that it is a round globe.
If two ships were near to each other, and could communicate on VHF, they would lose contact, as one ship sails away from the other, as they would literally ‘ go round the world’.
This is why on land, you see radio transmitters on high towers, and hills. Height gives further range, as it helps overcome the curvature of the earth.
The antenna, is the device responsible for transmitting, and receiving the radio signals.
Therefore the type of antenna, and its positioning on the ship, are important factors in your radio systems performance.
More to follow soon….