Dipoles and Monopoles

Dipoles and Monopoles

The vertical dipole—or its electromagnetic equivalent, the monopole—could be considered one of the best antennas for LMR applications. It is omnidirectional (in azimuth) and, if it is a half-wavelength long, has a gain of 1.64 (or G = 2.15 dBi) in the horizontal plane. A center-fed, vertical dipole is illustrated in figure 5(a). Although this is a simple antenna, it can be difficult to mount on a mast or vehicle. The ideal vertical monopole is illustrated in figure 5(b). It is half a dipole placed in halfspace, with a perfectly conducting, infinite surface at the boundary.

Figure 5. The vertical dipole electromagnetic equivalent, monopole

A monopole over an infinite ground plane is theoretically the same (identical gain, pattern, etc., in the half-space above the ground plane) as the dipole in free space. In practice, a ground plane cannot be infinite, but a ground plane with a radius approximately the same as the length of the active element, is an effective, practical solution. The flat surface of a vehicle’s trunk or roof can act as an adequate ground plane. Figure 6 shows typical monopole antennas for base-station and mobile applications.

Figure 6. Typical monopole antennas for (a) base-station applications and (b) mobile applications