__________________________________________________________ Radio Wave Propagation
2-11. With a high-frequency generator (a transmitter) connected to the
However, the generator potential reverses before the electrostatic field has
had time to collapse completely. The reversed generator potential neutralizes
the remaining antenna charges, leaving a resultant E field in space.
2-12. Figure 2-3 is a simple picture of an E field detaching itself from an
antenna. (The H field is not considered in this example, although it is
present.) In view A, the voltage is maximum and the electric field has
maximum intensity. The lines of force begin at the end of the antenna that is
positively charged and extend to the end of the antenna that is negatively
charged. Note that the outer E lines are stretched away from the inner lines.
This is because of the repelling force that takes place between lines of force in
the same direction. As the voltage drops (view B), the separated charges come
together, and the ends of the lines move toward the center of the antenna.
However, because lines of force in the same direction repel each other, the
centers of the lines are still being held out.
Figure 2-3. Radiation from an Antenna
2-13. As the voltage approaches zero (view B), some of the lines collapse back
into the antenna. At the same time, the ends of other lines begin to come
together to form a complete loop. Notice the direction of these lines of force
next to the antenna in view C. At this point the voltage on the antenna is
zero. As the charge starts to build up in the opposite direction (view D),
electric lines of force again begin at the positive end of the antenna and
stretch to the negative end of the antenna. These lines of force, being in the
same direction as the sides of the closed loops next to the antenna, repel the
closed loops and force them out into space at the speed of light. As these loops
travel through space, they generate a magnetic field in phase with them.
2-14. Because each successive E field is generated with a polarity that is
opposite the preceding E field (that is, the lines of force are opposite), an
oscillating electric field is produced along the path of travel. When an electric
field oscillates, a magnetic field having an intensity that varies directly with
2-5
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