8.2 Compact elements

In mobile and personal communications, it is normally important to have very compact antennas. Physical compactness means a less intrusive presence at both handsets and base stations. However, there is a tradeoff between antenna performance, in the sense of impedance bandwidth, and its electrical size. For high directivity, an antenna must become electrically large, occupying apertures of several wavelengths. Similarly, both the bandwidth and the directivity of an antenna cannot be large if the antenna is electrically too small.

For personal terminals, it is necessary to have physically small antennas, and with current carrier frequencies of about 1 GHz for cellular systems, for example, this means the handsets must bear electrically small antennas as well. The performance of compact handset antennas is influenced by their immediate surroundings, in particular, the operator's hand and head. In practice, for currently used handset antennas, these effects are almost impossible to isolate from the handset performance. As carrier frequencies increase in future systems, it becomes more feasible to mount large numbers of uncorrelated antennas on a handset because the structure becomes electrically large. However, the performance of these antennas will still be influenced by the presence of the immediate surroundings.

In this section, fundamental limits of the compactness of element antennas are discussed, and some basic compact element designs are compared.