14 - Antennas

Abstract

This chapter focuses on antennas that are designed to accept radio frequency (RF) power from a transmitter and radiate it into its surroundings or alternatively to extract energy from a passing radio wave and deliver it to a receiver. Practical antennas fall into two main groups, those which are self-resonant and those which are not. The simplest resonant antenna is the half-wave dipole (known in the Americas as a doublet). The radiation intensity is a maximum in the plane at right angles to the dipole and is “doughnut” shaped and there is no radiation along the line of the dipole. A vertical dipole is described as “vertically polarized” because the lines of electric field in the direction of maximum radiation are vertical. The bandwidth of a dipole can be increased by making the conductors very fat over most of their length, tapering conically to the feedpoint. In many situations, from a VHF or UHF pocket pager to a military tactical HF communications system, size or weight considerations may enforce the use of an antenna that is much smaller than a half-wave dipole. Such an antenna will not be resonant in its own right, but measures can be taken to bring it to resonance. A patch or “microstrip antenna” consists of a very thin flat metallic region or patch on a dielectric substrate, itself mounted on a ground plane larger than the patch. When fed at two points with signals in quadrature, a patch antenna produces circularly polarized radiation or receives such radiation.