Page 529 - Engineering Electromagnetics, 8th Edition

14

CHAPTER

ELECTROMAGNETIC

RADIATION AND

ANTENNAS

e are used to the idea that loss mechanisms in electrical devices, including
transmission lines and waveguides, are associated with resistive effects
W in which electrical power is transformed into heat. We have also assumed
that time-varying electric and magnetic fields are totally confined to a waveguide
or circuit. In fact, confinement is rarely complete, and electromagnetic power will
radiate away from the device to some degree. Radiation may generally be an unwanted
effect, as it represents an additional power loss mechanism, or a device may receive
unwanted signals from the surrounding region. On the other hand, a well-designed
antenna provides an efficient interface between guided waves and free-space waves
for purposes of intentionally radiating or receiving electromagnetic power. In either
case, it is important to understand the radiation phenomenon so that it can either be
used most effectively or be reduced to a minimum. In this chapter, our goal is to
establish such an understanding and to explore several practical examples of antenna
design.■

14.1 BASIC RADIATION PRINCIPLES: THE
HERTZIAN DIPOLE

The essential point of this chapter is that any time-varying current distribution will
radiate electromagnetic power. So our first task is to find the fields that radiate from a
specifictime-varyingsource.Thisproblemisdifferentfromanythatwehaveexplored.
In our treatment of waves and fields in bulk media and in waveguides, only the
wave motion in the medium was investigated, and the sources of the fields were not
considered. Earlier in Chapter 11, we found the current distribution in a conductor
by relating it to assumed electric and magnetic field intensities at the conductor

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