The electromagnetic spectrum
is the complete range of electromagnetic frequencies from 3 kHz to beyond 1024 Hz.
Diffraction
can account for the ability of radio waves with lower frequency to bend around an obstacle.
Refraction
occurs when a wave traveling through two different mediums passes through the boundary of the mediums
and bends toward or away from the normal.
Reflection
occurs when a wave strikes an object and bounces back (toward the source).
The wave that moves from the source to the object is called the incident wave,
and the wave that moves away from the object is called the reflected wave or
backscatter.
The Earth's atmosphere
is divided into three separate layers: The troposphere, stratosphere, and ionosphere.
The troposphere
is the region of the atmosphere where virtually all weather phenomena take place.
In this region, rf energy is greatly affected.
The stratosphere
has a constant temperature and has little effect on radio waves.
The ionosphere
contains four cloud-like layers of electrically charged ions which aid in
long distance communications.
The Radar Horizon
is one-third farther than the line-of-sight horizon.
A temperature inversion causes channels, or
ducts,
of cool air to form between layers of warm air, which can cause radio waves to travel
far beyond the normal line-of-sight distances and witch causes ambiguous returns.
