We had seen that there are two main types by which radio waves propagate from one place to another. Let us now concentrate on the Ionospheric means of propagation. Atmosphere consists of a layer called ionospheric layer in the upper levels of stratosphere. This layer consists of high levels of free ions therefore called as Ionosphere.
Ionospheric Attenuation of Radio Waves
Ionosphere consists of negatively charged free electrons due to ionized gasses. Ionisation is the process of ejecting electrons from an atom. Radio waves which travel to ionosphere are refracted as well as attenuated in the Ionosphere. Ionospheric attenuation causes loss of energy in the radio wave due to collisions between wave particles and free electrons.
Ionospheric Refraction of Radio Waves
Refraction of waves cause bending of waves towards earth and some of the waves return to ground as sky waves. Ionospheric attenuation is higher in lower frequencies. Solar radiation increases ionisation levels in the atmosphere. If the ionization intensity is constant, refraction decreases with increase in frequency. Energy contained in the radio wave increases with frequency which reduce both attenuation as well as refraction.
D, E and F Layers or Bands of Ionosphere
Ionosphere is seen as three distinct bands called the D, E and F layers. These are also called as Kennely, Appleton and Heavyside layers named after their discoverers. Kennely or D layer is the lowest layer, which is found at an average height of 75 km over earth. D layer has limited refraction capability due to low ionization levels. Heavyside or E layer is found at an average height of 125 km over earth. Refracts up to 2 MHz due to medium ionization levels. Appleton or F layer is the highest layer, which is found at an average height of 225 km over earth. This layer is capable of refracting 2 to 50 MHz due to high ionization levels
Diurnal and Seasonal Variation in Ionosphere
Intense ionisation is found at the centre of D, E and F layers. D layer forms at sunrise and disappears at sunset. E layer reduces in altitude at sunrise and increases in altitude after sunset. F layer splits into two at sunrise and rejoins at sunset. Altitude of F1 reduce below F layer at sunrise. Altitude of F2 layer is dependent on time of year also. In summers, F2 increases in altitude up to 400 Km. However, in winters, F2 remains at approximately same height of approximately 225 Km.
Ionospheric propagation consists of sky waves since they pass through the ionospheric layer. Radio waves entering ionospheric layer refract and may return as Sky waves. Sky waves are possible in the frequency range of 2 to 50 MHz. LF, MF ad HF bands propagate as sky waves under normal conditions.
Critical Angle of Sky Waves
Critical angle is the minimum transmission angle for return of sky wave. That is to say that if the transmission angle is less than the critical angle, these rays would escape the atmosphere and would not return to earth. Escape rays are the name given to such rays caused by transmission at less than the critical angle. Critical angle is dependent on Ionisation levels in the atmosphere. Higher ionisation levels reduces critical angle and lower ionisation levels increase the critical angle.
Variation of Critical Angle with Frequency
Critical angle is also dependent on Frequency of wave. Critical angle increases with increase in frequency. If you maintain the same critical angle and reduce the frequency of transmission, the waves would escape the atmosphere at some frequency. This is called as the Critical Frequency. Critical frequency is the lowest usable frequency for a given critical angle. Remember, higher ionisation levels also would increase the critical frequency.