7.0 Introduction

In Chapters 2 and 3 we examined the single-turn small loop, one of the two fundamental antenna types. It is a kind of dual to the small dipole. The loop has a conducting path for current back to the source, whereas the dipole s current is reflected back from an open end. When both antennas are vertical they have vertical polarization, but the loop has a figure-8 azimuth pattern and a constant-level elevation pattern, while the dipole has a constant-level azimuth pattern and a figure-8 elevation pattern, all considered in free space. The loop is inductive rather than capacitive, and its radiation resistance depends on its area in square wave-lengths, rather than its length. All of these differences make the loop more useful than the dipole in some applications, mostly receiving.

In this chapter we consider the single-turn loop, the solenoid or uniform helix (which may be thought of as several loops in series or an N-turn loop), the contrawound toroidal helix (CTHA), and an example of a folded antenna. We study them mainly in transmit mode, and defer coils wound on ferrite cores to the chapter on receiving antennas.

Like the dipole, the small loop has a high reactance that can be reduced by designing it to occupy more of the available space. We present modeling methods and results for a doughnut, a sheet barrel, and a thick-walled barrel. Going in the opposite direction, the solenoid clearly has more inductance and should have more radiation resistance than the...