Surface and Atmospheric Attenuation

In this topic, let us understand about Surface and Atmospheric Attenuation. Surface attenuation of radio waves refers to the weakening or loss of signal strength that occurs when radio waves travel close to or along the Earth’s surface. Factors such as ground conductivity, terrain, and obstacles can cause absorption, reflection, and scattering of the waves, reducing their power and range in surface based communication systems.

Variation of Surface Attenuation with Polarisation

Surface attenuation occurs due absorption of energy by the earth’s surface. Vertically polarised waves have comparatively lesser contact with earth hence suffer from lesser surface attenuation. On the other hand, horizontally polarised waves have greater surface attenuation since they are constantly in contact with ground.

Variation of Surface Attenuation with Frequency

Surface attenuation increases with frequency of the radio wave. Submarine Communication follows the surface of sea from one submarine to another. This is the reason that very low frequency bands are used for submarine communications. On the other hand, satellite communications are carried out using Extremely High Frequency bands.

Variation of Surface Attenuation with Earth’s Surface

Surface attenuation is greater over land than over sea. Therefore, the range over land is proportional to twice the square root of power whereas sea range would be proportional to thrice the square root of power transmitted. Reflection attenuation is caused due reflections from buildings and hills. In order to reduce reflection attenuation, antennas are mounted on tall masts.

Fading of Radio Waves

Fading is caused by signals received through different paths which are out of phase with each other. For example, a radio transmission may be intended to travel directly from an ATC to aircraft. However, some parts of the wave may be received by the aircraft not directly from the ATC, but on being reflected from objects like hills on the surface of earth. Fading causes increase and decrease voice signals which also referred to as waxing and waning of signals.

Atmospheric Attenuation

Atmospheric attenuation causes weakening of radio waves due to atmospheric absorption and static interference. Atmospheric absorption or scatter is caused by small particles in atmosphere. Obviously, Higher frequency radio waves are affected more by atmospheric absorption. Frequencies of approximately 5 Gigahertz are affected badly by absorption since the wave length at these frequencies is almost same as size of water droplets.

Static Interference

Static interference is caused due to negative charges in atmosphere which build up due to rain, thunderstorm or solar activity. Static interference is higher in lower frequencies affecting voice clarity. At times, equipment’s electronic circuits also cause static interference.

Signal to Noise Ratio

Signal to noise ratio is the ratio of strength of signal to static noise in decibels. Better signal to noise ratio is achieved by either increasing transmission power as per inverse square law or by improving sensitivity of receiver. This is done by reducing its internal noise by using better electronics. Doubling the range however requires four times increase in power.