Characteristics of Cylindrical Dipoles and Monopoles

Abstract

The cylindrical dipole consists of a highly-conducting tube or rod with radius a and half-length h. In practice, it is center-driven from a balanced open-wire transmission line with a distance b between the axes of the identical conductors of the line as shown in Fig. 2.1a. The cylindrical monopole is essentially half a dipole. It consists of a highly-conducting tube or rod with radius a and length h, erected perpendicular to a sufficiently large (ideally infinite), highly-conducting ground plane in either of the arrangements shown in Figs. 2.1b and 2.1c. The axis of the single wire with radius a in Fig. 2.1b is at a distance b/2 from the ground plane; with its image in the highly-conducting plane it is equivalent to the open line in Fig. 2.1a. The inner radius of the outer conductor of the coaxial line in Fig. 2.1c is b; the radius of the inner conductor is a and its extension of length h above the ground plane is the monopole antenna. In order that radiation from a balanced open-wire line be negligible, the condition

$$k_0b = \frac{2\pi b}{\lambda}<<1$$

must be satisfied.