Mobile Applications

Mobile Applications

Nearly all vehicular antennas are monopoles mounted over a (relatively) flat body surface (as described above). In this application, the monopole is often called a “whip” antenna. At VHF low-band, a quarter-wave monopole can be 2.5 m (approximately 8 ft) long. However, an inductor (coil) at the base of a monopole adds electrical length, so the physical length of the antenna can be shorter. Although this kind of “loaded” antenna will appear to be a quarter-wave antenna, it will have a gain value somewhat less than a true quarter-wave monopole. This disadvantage can be somewhat offset, however, by the ability to mount the (shorter) antenna in the center of a surface that will act as an acceptable ground plane (e.g., the roof or trunk of the vehicle). Figure 10(a) shows an illustration of this kind of antenna.

Figure 10. Typical mobile antennas

Many of the vehicular antennas at VHF high-band are quarter-wave monopoles. At 150 MHz, this would mean that a whip antenna, approximately 0.5 m (1.5 ft) long, is needed. Half-wave and 5/8 wave monopoles also are used, but they require some sort of matching network (i.e., inductors and/or capacitors) in order to match the antenna impedance to that of the transmission line. These longer antennas have a gain of approximately 3 dBi.

At UHF, a quarter-wave whip is approximately 15 cm (6 in) long. Since this length is physically small, some design considerations can be used to increase the gain. For example, as shown in figure 10(b), two 5/8 wave monopoles can be “stacked” with a phasing coil between them. This is, effectively, an antenna array (see sec. 5.5) that provides a gain of approximately 5 dBi.

At 800 MHz, a quarter-wave monopole does not perform well, so the approach of stacking two monopoles, with a phasing coil between, is used. Such an antenna, illustrated in figure 10(c), looks much like a mobile cellular phone antenna and has a gain of approximately 3 dBi.

The azimuthal pattern of all monopoles is ideally a circle. In other words, the gain versus azimuth angle in the horizontal plane is constant. In practice, the pattern in the horizontal plane generally is not omnidirectional, since the portion of the vehicle used as a ground plane is not symmetric, and usually there are other obstructions. Figure 11 shows the horizontal plane pattern for an 840 MHz whip located in the center of the roof of a vehicle [13]. The dotted line in the figure shows the effects, on the pattern, of a law-enforcement light bar mounted on the roof ahead of the antenna.

Figure 11. A mobile antenna horizontalplane pattern [13]