Refraction results in bending of radio waves while passing from one medium to another. On the other hand, we had learnt earlier that diffraction refers to the bending of radio waves around obstacles.
The speed of radio waves is 3×108 Meters/Second2 only when it s being transmitted through vacuum. In all other mediums, their speed varies depending on the medium of transmission. This change in velocity of radio waves results in change of their direction when radio waves transit through from one medium to another with varying densities. This process is called Refraction of Radio Waves.
Coastal refraction causes an error in radio aids located close to coast line due to refraction or bending of Radio Waves while crossing coastline. The air over sea is more moist than that of land resulting in change in the air density. The speed of Radio Waves is faster over sea than over land.
While crossing coast lines, Radio Waves therefore bend towards land, which is a slower speed medium. Coastal refraction is lesser in higher frequencies. Therefore, Non Directional Beacons (NDB) located near coastlines use higher frequencies as compared to NDB located inland.
Now that we are aware of refraction of radio waves in Non-Ionospheric transmission, let us understand about Attenuation and Refraction due to the Ionosphere in our next lesson.