In this post, we shall understand about Ionospheric Attenuation and Refraction. We had learnt about Refraction of Radio Waves in our previous post.
Ionospheric layer is found in the upper levels of stratosphere in atmosphere. 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 in the ionospheric layer. Lower frequencies experience higher levels of Ionospheric Attenuation. Hence we use minimum of High Frequency Band if we intend to transmit the signal through the Ionosphere. Ionospheric refraction is caused by slowing down of Radio Waves in ionosphere by the free ions. Radio Waves refract in the ionosphere to such an extent that some of the waves end up returning to earth.
FREQUENCY EFFECT
Ionospheric refraction decreases with increase in frequency. In short, higher frequencies do not bend as much and would end up exiting the atmosphere. Therefore, we use extremely high frequencies for satellite communications. Normally only lower frequencies are affected by ionospheric refraction.
SUPER – SUB REFRACTION
However, special atmospheric conditions may cause refraction in higher frequencies also. These abnormal occurrences are called Super or Sub Refraction. Super refraction is caused due to high pressure systems and warm air flowing over cold surface. On the other hand, sub-refraction is caused due to low pressure systems and cold air flowing over warm surface. The effect of both Super and Sub-Refraction is to unexpectedly increase the range of VHF Radio Telephony.
I am sure that we have understood quite a lot about the properties of EM Waves. Having learnt about the concepts regarding EM Waves, let us understand about Radio Wave Propagation in our next post.