Long distance communication between two points on the earth is achieved through reflection of electromagnetic waves by ionosphere. Such waves are called sky waves. Sky wave propagation takes place up to frequency of about 40 MHz. A space wave travels in a straight line from transmitting antenna to the receiving antenna. It is used for line of sight (LOS) communication as well as satellite communication.
(a) The ionospheric layer acts as a reflector for a certain range of frequendes (3 to 30 MHz). Electro-magnetic waves of frequencies higher than 30 MHz (upto 40 MHz) penetrate the ionosphere and escape.
(b) In television broadcast, microwave links and
satellite communication, space wave mode of propagation is used