MONOPOLE ANTENNAS O. P. Gandhi, G. Lazzi, and C. M. Furse Electrical & Computer Engineering Department University of Utah Salt Lake City, UT 84112-9206 This article describes the concepts underlying monopole antennas and their many applications, such as for broadcasting, car radios, and more recently for cellular telephones. In its simplest form, the monopole antenna above an infinite ground plane shown in Fig. 1(a) can be considered as one-half of a corresponding double-length center-fed linear dipole shown for comparison in Fig. 1(b). The current distribution for a vertical monopole antenna of height h is assumed to be a standing wave of the following form, in terms of the feed point current I (0): (0) ( ) sin ( ) sin( ) I I z kh z kh ′ ′ = − (1) for 0 z h ′ ≤ ≤ . This occurs because the perfect ground plane creates an image of the monopole with a current distribution identical to that for the lower arm of the dipole. Together with this image, the monopole antenna appears to be a center-fed dipole for the upper half-space. There is negligible penetration of fields into a high conductivity ground for a monopole antenna, and all that radiation is directed into the upper half-space creating a power density for any angle θ (see Fig. 1) that is twice as high as that for a dipole radiating the same amount of power. This gives a directivity or gain of the monopole antenna that is twice that for the double-length dipole. Many of the other properties of a monopole antenna above ground are also related to those for the corresponding double-length dipole in a fairly simple manner. We give in Table 1 the relationships between a monopole antenna of length h above ground and the corresponding dipole antenna of length L = 2h. A graph of the variation of the