Figure 1-5. Simple radio communication system.
field decreases as the distance from the antenna increases. Thus, when you are out of effective radio
range, you can barely understand a transmission, or the signal is too weak to activate the receiver circuit.
The term electromagnetic radiation was derived because a radio wave radiated into space by an antenna
contains both electric and magnetic fields.
c. As mentioned earlier, frequencies falling between 3 kHz and 300 GHz are called RF, since
they are commonly used in radio communications. The RF range of 3 kHz to 300 GHz is divided into
bands of frequencies-a part of the RF spectrum. These frequency bands are detailed in Table 1-1, page
1-8. Each frequency band of the RF spectrum is ten times higher in frequency than the one immediately
below it.
d. For a receiving antenna to pick up, or absorb the maximum amount of energy from an
electromagnetic field, it must be located in the plane of polarization. This is simply a matter of
orientation between the Earth and the electric field. If the lines of force in the electric field are
perpendicular to the surface of the Earth, the wave is said to be vertically polarized. If the lines of fore
are parallel with the Earth, the polarization is said to be horizontal Figure 1-6, page 1-9, shows both
vertical and horizontal polarization. Notice that in view A, the electric lines are at right angles to the
Earth's surface. In view B, the electric lines are parallel to the Earth's surface. Polarization affects the
strength and range of a signal and is addressed during the planning process before executing signal
operations. Polarization will be discussed in more detail in Lesson 2.
1-7
SS0130
Previous Page
