MARINE RADIO PROPAGATION AND MODULATION
The act of passing a radio wave through the Earth’s atmosphere is known as Propagation. Your radio and its antenna will have been designed to operate across appropriate frequencies and make full use of the best propagation for that frequency range.
Even with the most modern of radio communication systems, three are only three methods of propagation. The basic principles behind this have not changed since the discovery of radio in the late 1800’s. The three methods of propagation (or the three waveforms) are;
- Space/Direct Wave, aka Line of Sight
- Ground Wave, aka Surface Wave
- Sky Wave
The frequency of the wave will influence the types of information that can be passed. It is a common misconception by many users of radio communications that the specific wave being employed is the only waveform being produced, whereas in fact all methods of propagation exist in all frequency ranges.
DIRECT WAVE
As can be seen below, the curvature of the earth dictates the radio horizon. The left hand ship’s antenna is in sight of the shore station so is in range but the right hand ship is over the horizon and out of range. The good news is that due to reflection and refraction radio waves travel further than light, so the radio horizon is a little further than the visual horizon, thus the middle ship is also in range of the coast station.
VHF uses “Direct Wave” It should therefore now be obvious why VHF antennas are positioned as high as possible. On a sailing vessel this is at the top of the mast. On a ship or motor yacht it is at the highest point, often on a mast above the bridge.
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SPACE WAVE
Space wave propagation, like direct wave propagation is “Line of sight,” however due to the altitude of satellites and aircraft we have a much better range. Space wave and Direct Wave are together known as “Line of Sight Propagation.” The transmitting and receiving antennas need to be able to “see” each other, even if this is not possible with the human eye.
The GMDSS systems that use Space/Direct wave propagation are
- VHF (DSC & RT)
- Search and Rescue Transponder (SART)
- Automatic Identification of Shipping (AIS) SART AIS
- Emergency Position Indicating Radio Beacon (EPIRB)
All INMARSAT satellite systems, including the older COSPAS-SARSAT EPIRB systems use Space Wave.
As already mentioned there are three forms of propagation and as we can see above VHF terrestrial radio communications (i.e. comms between 2 separate stations on earth without the aid of a satellite) are limited in range by “line of sight” The is typically regarded as about 35 miles offshore from a coast station to a vessel. As a seafarer you are likely to often be operating beyond this range.
GROUND WAVE
As suggested by the name, a Ground Wave travels along the Earth’s surface. The Ground Wave is not affected directly by changes in atmospheric conditions, and if transmitted with sufficient power in the VLF and LF bands can travel for many hundreds of miles. This makes them reliable, but they are heavily influenced by the terrain they are travelling across. Porous materials, such as sand or water, reduce the effectiveness of the wave because they draw power from the waveform as it penetrates the particles and atoms of the material. This process is known as Attenuation.
Within GMDSS, MF is the most effective user of Ground Wave, but lower frequencies of the HF band make use of this waveform at night. This directly effects range of Navtex reception, but primarily affect MF and HF DSC/RT transmissions.
Although MF (ground wave) gives us a greater range than VHF, it is still limited to a few hundred miles. If we need a longer range (ie we are further offshore) we need to bounce our radio waves off something. This is where we make use of satellites (line of sight) or the earth’s ionosphere using our third method of propagation “Skywave” (used for HF transmissions).
SKY WAVE
The Ionosphere forms the boundary between the earth’s lower atmosphere (the area we live and breathe in) and the vacuum of space. It is between 50 and 400 miles above the earth’s surface. The image below shows the layers of the ionosphere by day and night.
The D layer, is basically radiation, caused by the sun, it is therefore only present by day. The D layer attenuates some of our sky wave transmission, therefore weakening it.
The E layer continues to attenuate our signal but also refracts some of it.
The most important region in the ionosphere for long distance HF radio communications is the F layer. During daytime when radiation is being received from the Sun, it often splits into two: the lower one being the F1 region and the higher one, the F2 region. The F1 region exists in the summer but less so in the winter.
Typically the F1 layer is found at an altitude of around 180 miles with the F2 layer above at around 250 miles. At night the combined F layer centres around 180 miles altitude. The altitude of all the layers in the ionosphere layers varies considerably with time of day and with season, so the figures are a very rough guide.
The F layer acts as a “reflector” of HF signals and is used for long range (sky wave) communications. At night without the D and E layers less signal is lost (attenuated) so range is potentially further.
The distance between the transmitter and the first returning Sky wave is known as a Skip Distance, and the gap between the Ground wave also being produced and the first returning Sky wave is known as the Silent Zone.
The point at which a single Sky wave returns to the Earth is also known as the Lowest Usable Frequency (LUF). When that single wave becomes too heavily attenuated, or sufficiently loses power, and will no longer radiate, that is known as the Maximum Usable Frequency (MUF). When antennas are stationary, the “sweet spot” is known as the Optimum Working Frequency (OWF) and in perfect conditions would be 85 percent of the MUF.
This is of course almost impossible to achieve on a moving vessel. At the receiving station, the radio wave will also reflect off the ship’s superstructure, meaning that the antenna is receiving the same information on the same frequency at the same time! This is known as Interference Fading.
Due to a vessel’s movement in the water, it is rare that a Skywave would strike the Ionosphere at a perfect angle. This may cause a failing of the wave to be refracted. When this happens to a genuine radio wave, it is known as Polarisation Fading.
Due to the changing altitude of the F layer, a simple “Rule of Thumb” is used to choose a radio frequency that is likely to use Sky wave as its propagation. This rule is: “ The higher the sun, the higher the usable frequency”.
In summary each type of GMDSS radio emits “some” radio waves on all three types of propagation, however the primary propagation for each type of radio is shown below.
| HF | MF | VHF | Navtex | EPIRB | SART | AIS | |
| Line of Sight (Direct Wave & Space Wave) | ✅ | ✅ | ✅ | ✅ | |||
| Ground Wave | ✅ | ✅ | |||||
| Sky Wave | ✅ |