Q. Mode of propagation of electromagnetic waves?
The mode of propagation of electromagnetic waves in free space and atmosphere may be subdivided into three categories:
• Ground-wave propagation
• Sky-wave propagation
• Line-of-sight (LOS) propagation.
In the frequency bands that are primarily used to provide navigational aids from shore to ships around the world (VLF to LF to MF), the available channel bandwidths are relatively small, and hence the information that is transmitted through these channels is relatively slow speed and generally confined to digital transmission. Noise at these frequencies is caused by thunderstorm activity around the globe, whereas interference is caused by the many users.
For frequencies of 0.3 to 3MHz, in the MF band, ground-wave (or surface-wave) propagation, illustrated in Figure, is the dominant mode used for AM broadcasting and maritime radio broadcasting. Dominant disturbances include atmospheric noise, human-made noise, and thermal noise from electronic components. The range is limited to about 100 miles for even the more powerful radio stations.