Critical Frequency – is the highest frequency that can be returned by an Ionosphere layer, any frequency higher than Critical Frequency will cause Radio Wave to penetrate more deeply into the Ionosphere layer before returning to the Earth.
Critical Angle – is the lowest angle at which the sky wave returns to the Earth, any angle above this there will be an uninterrupted flow of Sky waves. Any angle below Critical Angle the Radio Wave will penetrate Ionosphere and will lost in the Space.
Skip Zone/Dead Space – The Ground distance between the point at which the maximum Surface wave goes and the point where the first Sky wave returns, No signal coverage is available in this area and it is also termed as Dead Space, The only method to receive signal in the Dead Space is to lower the frequency, or to increase the power of the Transmission, which is not very practical in flight.
Skip Distance – The Ground distance between the Transmitting Antenna and the Point where the first Sky wave is received after returning back to the Earth
Selective Fading – It occurs when the received signal from the Surface Wave and from the Surface Reflected wave meets each other 180* out of phase at the receiver and hence cancel out each other, It can be corrected by Varying the height of Receiver or Transmitter.
Night Effect/Dusk & Dawn Error – At night when Ionosphere layer changes due to the absence of Sun rays that causes a layer consisting of charged Ionized particles, caused by the atoms of high velocity and high energy Ultra violet rays in the Sun’s rays. It is the region of free electrons consisting of positive and negative ionized particles, which surrounds the Earth. The Earth is surrounded by few layers that absorb these ultra violet rays from reaching the Earth. When sun is no longer over a particular area, it is called night, hence the direct ultra violet radiation in the night is absent and the layer that consisted charged ions in the day almost vanishes. For the Sky wave to be deflected from the atmosphere it has to travel much higher than in the day to be reflected back to the Earth. This causes the range of the Radio Aid to be increased in the night time.
Fading – It occurs due to the fluctuations in the signal strength, It mainly occurs due to the continuous fluctuations in the Ionosphere. It also occurs when two Radio waves have traveled different routes and are in a phase difference. If they are In Phase they will strengthen each other giving even stronger reception. But if the waves are Out of Phase, they will eventually cancel out each other
Duct Propagation/Super Refraction – it occurs in Inversion layers, (Inversion is when temperature instead of decreasing with increase in altitude increases with increase in altitude).When temperature and humidity increase with increase in altitude, causing an abnormal distribution of water vapor in the surrounding air mass. It gives rise to an layer very close to the Earth with reflecting properties same as of Ionosphere, causing Radio waves to be deflected again and again in this layer, increasing the range to thousand of miles.
This type of phenomenon takes place usually in summer nights and is mostly found in tropical latitudes. It does not last for a very long time.
Diurnal Changes – It occurs because the solar radiation increases ionized density in the upper layers, that absorb ultra violet rays from reaching the Earth. At night when these ionized intensity decreases, the lowest layer called “D” layer vanishes, and the height of layer that reflects Radio waves rises, causing Skip Distance and Skip Zone/Dead Space to increase.
Seasonal Changes – The intensity of Ionosphere layer depends on the position of sun with reference to particular latitude. The Ionosphere layer is strongest when the Sun is closest (perigee or Perihelion) to the Earth. The max Ions particles occur in the Ionosphere layer in summer.
Sun Spot Activity – It occurs every 11 year and causes considerable changes in the Ionosphere layer. This occurs due to very high ultra violet radiations from the Sun, causing unreliable coverage of Radio Aids. During this period the Ionosphere layer becomes so strong that even the VHF signals may also return to Earth.