Equivalent circuit of a two-element spherical small antenna

For convenient portability mobile devices need to be small. To reduce their size, a small antenna is strongly required because the antenna is one of a mobile device´s largest components. In addition, the antenna´s bandwidth needs to be wide because the recent development of many kinds of multimedia services requires a large bandwidth. Designing a small antenna with a wide bandwidth is not easy, however, because an antenna´s bandwidth is inversely proportional to the antenna´s radiation Q-factor, the lower bound of which is determined by the radius of the virtual sphere, which includes the antenna´s whole structure.