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CHAPTER 3
MICROWAVE ANTENNAS
LEARNING OBJECTIVES
Upon completion of this section the student will be able to:
1. Explain the basic characteristics of coupling, directivity, reciprocity, and efficiency in microwave
antennas.
2. Describe the construction and basic theory of operation of reflector antennas and horn radiators.
3. Explain construction and operation of microwave lens antennas.
4. Describe the construction and theory of operation of driven and parasitic antenna arrays.
5. Explain the basic operation and applications of frequency-sensitive antennas.
INTRODUCTION
In this chapter you will study the general characteristics of microwave antennas that are widely used
in radar and communications applications. The basic principles of operation of microwave antennas are
similar to those of antennas used at lower frequencies. You might want to review the principles presented
in NEETS, Module 10, Introduction to Wave Propagation, Transmission Lines, and Antennas, at this
time. Pay particular attention to basic antenna principles in chapter 4 for a review of microwave antennas.
Antennas are devices used to radiate electromagnetic energy into space. The characteristics of
transmitting and receiving antennas are similar, so a good transmitting antenna is often a good receiving
antenna. A single antenna performs both functions in many modern applications.
ANTENNA CHARACTERISTICS
Since the operating principles of low-frequency and microwave antennas are essentially the same,
the electrical characteristics are also very similar. You will need a fundamental knowledge of radar and
communications antenna electrical theory in your shipboard antenna maintenance work. Antenna theory
is primarily a design consideration of antenna size and shape requirements that depend on the frequency
used. A brief description of antenna electrical characteristics is sufficient for the needs of most students of
electronics.
Antenna Efficiency
The effectiveness of an antenna depends upon its ability to couple or radiate energy into the air. An
efficient antenna is one which wastes very little energy during the radiation process. The efficiency of an
antenna is usually referred to as the POWER GAIN or POWER RATIO as compared to a standard
reference antenna. The power gain of an antenna is a ratio of the radiated power to that of the reference
antenna, which is usually a basic dipole. Both antennas must be fed rf energy in the same manner and
must be in the same position when the energy is radiated. The power gain of a single dipole without a
reflector is unity (one). An array of several dipoles in the same position as the single dipole, and fed with
the same line, has a power gain of more than one.
