Radio waves also called EM waves propagates or travels from a transmitter to a receiver by various means. There are two main types by which radio waves can travel or propagate from one place to another. They are called as Ionospheric Propagation and NonIonospheric Propagation. In this topic, we shall discuss the Non Ionospheric Propagation of Radio waves.
NonIonospheric Propagation of Radio Waves
Radio waves have the capability to travel or propagate with or without the use of ionosphere. NonIonospheric propagation consists of surface and space waves. Ground waves is used to refer to both surface waves and space waves, since they do not pass through ionosphere while travelling from transmitter to receiver.
Low, Medium and High Frequency waves travel as surface waves. Surface wave frequencies range from 20 KHz – 50 MHz. Normally, 20 KHz to 2 MHz frequencies are used in aviation. We are aware that LF, MF and HF waves have less atmospheric and surface attenuation. High diffraction enables surface waves to bend around the earth’s surface. Therefore, these bands of frequencies are used in long range NDB and long distance communication which travel without the use of ionosphere.
Very and Ultra high frequency waves greater than 50 MHz use Space waves which have very less diffraction or bending of waves around the surface of earth. Direct and Ground reflected waves are two types of space waves. Therefore, VHF and UHF frequencies are used in certain equipments like VHF Communication.
Space waves constitute of Direct and Ground Reflected waves
Direct waves travel by Line of sight principle where as ground reflected waves are received after reflection from obstacles like hills or buildings. Although this issue does not affect voice communication, they could cause an error in navigational equipments.
Line of Sight Formula for Maximum Range of Ground Waves
Maximum range of ground waves is given by a line of sight formula. Line of sight formula states that the maximum range in nautical miles is equal to 1.23 times the sum of square root of height of transmitter and receiver in feet. Remember, this formula provides just the theoretical maximum range. The rage would reduce depending on various other factors like transmission power.