Each case of communicable disease is the result of an orderly progression in. a series of events. This series of events may be described as a three-link chain, each. link representing a factor essential to the transmission of disease. These links (figure 1-1) are: (1) The source of the disease (reservoir) (2) The means by which the disease may be transmitted (mode of.
Radio Waves. Radio waves are a type of electromagnetic (EM) radiation with wavelengths in the electromagnetic spectrum longer than infrared light . They have have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, radio waves travel at the speed of light. Naturally occurring radio waves are made by lightning or by astronomical objects.
1 This type of radio propagation is encountered with signals travelling to and from satellites. 2  Ground wave propagation: When signals travel via the ground wave they are modified by the ground or terrain over which they travel. 3  They also tend to follow the earth's curvature.