RF and Microwave Circuit Design

12RF and Microwave Antennas

12.1 Introduction

This chapter will introduce the fundamental properties of RF antennas through an initial discussion of the power radiated from a simple wire conductor, leading to theoretical expressions for the electric and magnetic fields at a long distance from the conductor. The theory will be extended to consider the half-wave dipole, which is probably the most basic and most useful of all antenna types. The use of wire dipole elements in array configurations will be discussed, and we will consider how arrays can be designed and implemented in planar formats. The chapter will conclude with a discussion of traditional microwave antennas, namely those using aperture techniques and radiation from waveguide slots. In the latter case, it will be shown how modern fabrication techniques allow waveguide slot antennas to be implemented in multilayer planer substrates, using photoimageable thick-film and LTCC technologies.

12.2 Antenna Parameters

1. An antenna is a passive loss-free structure, which converts the field pattern of the transmission line that feeds it to a field pattern in free space, and vice-versa. It can be regarded as a transformer.
2. In principle, the available output power from an antenna equals the available input power, so the available gain is unity, as in a transformer.
3. The radiation pattern of an antenna shows the power or electric field radiated from the antenna as a function of direction. Figure 12.1 illustrates a typical ...