1-2
Microwave frequencies present special problems in transmission, generation, and circuit design that
are not encountered at lower frequencies. Conventional circuit theory is based on voltages and currents
while microwave theory is based on electromagnetic fields. The concept of electromagnetic field
interaction is not entirely new, since electromagnetic fields form the basis of all antenna theory. However,
many students of electronics find electromagnetic field theory very difficult to visualize and understand.
This module will present the principles of microwave theory in the simplest terms possible but many of
the concepts are still somewhat difficult to thoroughly understand. Therefore, you must realize that this
module will require very careful study for you to properly understand microwave theory. Antenna
fundamentals were covered in NEETS, Module 10, Introduction to Wave Propagation, Transmission
Lines, and Antennas.
This module will show you the solutions to problems encountered at microwave frequencies,
beginning with the transmission of microwave energy and continuing through to waveguides in chapter 1.
Later chapters will cover the theory of operation of microwave components, circuits, and antennas. The
application of these concepts will be discussed more thoroughly in later NEETS modules on radar and
communications.
Q-1. What is the region of the frequency spectrum from 1000 MHz to 100,000 MHz called?
Q-2. Microwave theory is based upon what concept
WAVEGUIDE THEORY
The two-wire transmission line used in conventional circuits is inefficient for transferring
electromagnetic energy at microwave frequencies. At these frequencies, energy escapes by radiation
because the fields are not confined in all directions, as illustrated in figure 1-1. Coaxial lines are more
efficient than two-wire lines for transferring electromagnetic energy because the fields are completely
confined by the conductors, as illustrated in figure 1-2.
Figure 1-1.—Fields confined in two directions only.
