Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 19
Direction of travel
A
Direction of travel
Figure 2.6 Wave polarization is determined by the direction of the electric field lines of force. (A) Vertically
polarized electromagnetic wave. (B) Horizontally polarized wave.
B
the case of small mobile antennas, the reference is usually the closest large body of conducting
material—such as the roof of a car. Figure 2.6A is an example of vertical polarization
because the E-field is vertical with respect to the earth, the assumed reference
plane. If the fields are interchanged (as in Fig. 2.6B), the EM wave is said to be horizontally
polarized.
As mentioned earlier in this chapter, an EM wave travels at the speed of light, designated
by the letter c, which is about 300,000,000 m/s (or 186,000 mi/s). To put this
velocity in perspective, a radio signal originating on the sun’s surface will reach earth
in about 8 minutes. The velocity of the wave slows in dense media, but in air the speed
is so close to the “free-space” value of c that the same velocity is used for both air and
the near-perfect vacuum of outer space when solving most practical problems. In pure