Diffraction and refraction are phenomena that occur when radio waves encounter obstacles or pass through different mediums, affecting their propagation. Diffraction refers to the bending or spreading of waves around obstacles, while refraction involves the change in direction and speed of waves as they pass through materials with varying densities, such as the Earth’s atmosphere. These phenomena play a significant role in the behaviour and propagation of radio waves in different environments.
Diffraction of Radio Waves
Diffraction refers to the bending of radio waves around any obstacle. In the context of radio navigation it refers to bending of radio waves around the surface of earth. Higher diffraction is seen in lower frequencies like VLF, LF and HF. That means these waves bend around the earth’s surface and could travel longer distances. Long range of surface communication is possible in these bands.
Ionospheric Refraction of Radio Waves
Refraction is bending of radio waves while transiting from one medium to another with different densities. Refraction occurs due to change in speed of radio waves while crossing over from one medium to another. 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. Ionospheric refraction decreases with increase in frequency. In short, higher frequencies do not bend as much and exit the atmosphere. Therefore, we use extremely high frequencies for satellite communications.
Super and Sub Refraction
Normally only lower frequencies are affected by ionospheric 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, subrefraction is caused due to low pressure systems and cold air flowing over warm surface. The effect of super and subrefraction is to unexpectedly increase the range of VHF Radio Telephony.
Coastal refraction causes an error in radio aids located close to coast line due to bending of radio waves while crossing coastline. We are aware that the speed of radio waves is faster over sea than over land due to change in the medium. Radio waves therefore bend towards land which is the slower speed medium. Coastal refraction is lesser in higher frequencies. Therefore, NDBs located near coastlines use higher frequencies as compared to NDBs located inland.