Propagation of Radio Signals

The propagation of radio signals refers to the way in which the radio signals are transmitted from the sending station to a receiving station.

Very High Frequency, VHF

The propagation of VHF radio waves is basically by line-of-sight only. Consequently the range of VHF is limited by the height of the transmitting and receiving aerials, the higher the aerials, the longer the range. Merely increasing the power of a VHF transmitter will not affect its range.

Medium Frequency, MF

MF radio waves have a natural tendency to follow the curvature of the earth, a phenomena known as the 'ground wave' effect. Although this results in a much greater range, the radio waves are dissipated by their interaction with the ground, an effect known as 'attenuation'. Consequently, the greater the power of the transmitter, the greater the range.

High Frequency, HF

High frequency radio waves have the greatest range of all of the frequencies used by vessels, in fact an HF transmitter has virtually global coverage. This is achieved by bouncing the radio waves off of the ionised layers of gas that make up part of the earth's atmosphere, producing an effect known as a 'sky wave'. Because these layers are constantly changing, and as this affects the propagation of the signal, it is essential to have some knowledge of how the phenomena works. Lifeboats do not have a HF transmitting capability

The Earth's Atmosphere

The earth's atmosphere is composed of many layers, from the breathable layers of nitrogen and oxygen found at ground level, to the rarefied gases that exist in tiny amounts in the upper Stratosphere. Some of these layers are utilised in radio wave propagation and a basic understanding of the principle is a requirement of the Long Range Certificate.

The Troposphere

The Troposphere is the name given to the dense layer of gases that extend approximately 10 miles above the earth's surface. This is the layer of breathable air and is chiefly composed of nitrogen and oxygen. As the height increases so this air becomes thinner and the temperature and pressure decrease.

The Stratosphere

Above the Troposphere is the Stratosphere, which extends upwards to the limits of the earth's atmosphere. Although there are gases in the Stratosphere, they are rare and remain in the lower levels. However as there is no 'weather' to disturb them, these gases separate out into layers. The layers, which occur between 60 and 240 miles above the earth's surface, are known collectively as the Ionosphere.

Ionosphere

Gases in the Ionosphere are affected by the the emission of ultra-violet rays from the sun which causes them to become electrically charged, or 'ionised'. It is this phenomena that reflects the HF radio waves, an effect known as a 'sky wave'. As the ionisation process is fuelled by ultra-violet emissions from the sun, it increases during the day, reaches a peak at local noon before diminishing as the sun sets. This in turn effects the way that the HF radio waves are refracted, which is why different HF frequencies have to be used at different times of the day. The Ionosphere is also affected by sun spot activity which produces a great deal of ultra violet and tends to occur in ten to eleven year cycles. This sun spot activity also effects HF propagation, but normally the effect is minor and can be safely ignored.