c. Ionosphere. The ionosphere is that portion of the earth's atmosphere above the lowest level
at which ionization (splitting of molecules into positive and negative charges, or ions) of low pressure
gasses will affect the transmission of radio waves. It extends from about 30 to 250 miles (48 to 402 km)
above the earth. The ionosphere is composed of several distinct layers in which ionization occurs at
different levels and intensities.
d. In the ultra-high-frequency band (300 to 3000 mc), the direct wave must be used for all radio
transmissions. Communications is limited to a short distance beyond the horizon. Lack of static and
fading in these bands makes line-of-sight reception very satisfactory. Highly directive antennas can be
built into small spaces to concentrate RF energy into a narrow beam, thus increasing the signal intensity.
a. When power is delivered to an antenna, two fields are set up by the fluctuating energy: One
is the induction field, which is associated with the stored energy; the other is the radiation field, which
moves out into space at nearly the speed of light. At the antenna, the intensities of these fields are high
and are proportional to the amount of power delivered to the antenna. At a short distance from the
antenna, and beyond, only the radiation field remains. This radiation field is composed of an electric
component and a magnetic component.
b. The electric and magnetic fields (components) radiated from an antenna form the
electromagnetic field. This field is responsible for the transmission and reception of electromagnetic
energy through free space. Thus, the radio wave may be described as a moving electromagnetic field,
having velocity in the direction of travel, and with components of electric intensity and magnetic
intensity arranged at right angles to each other (figure 11).