Antenna customization is an expensive process and can take place throughout the design phase of a phone. Virtually all modern handset antennas, including dielectric variants, are unbalanced meaning what we think of as the antenna is only half of the radiation mechanism and the ground plane is the other half. If the ground plane size is changed, or other components are moved around on the ground plane, the antenna detunes and it has to be redesigned. If antennas were to be integrated with other RF components into an RF module, the cost of customization would be even greater. Unbalanced antenna architecture has thus been a major stumbling block to the development of full radio and antenna integration within handsets.

The alternative strategy is to use a balanced antenna, as it has recently been shown that a balanced handset antenna is effective for reducing the currents on the ground plane. This can be thought of as one in which the reference is not the ground plane but an inverted version of the antenna itself. A dipole is probably the simplest and best known example of a balanced antenna but like most balanced antennas it works best in free space away from other conductors. The problem with using them for mobile communications is that modern handset PCBs has a full ground plane and the antenna usually sits a tiny fraction of a wavelength above it. Dipoles just don’t work in this sort of environment. A solution to this predicament has been developed recently and involves using two unbalanced antennas arranged back-to-back and fed as a balanced or complementary pair. By back-to-back we mean a pair disposed with mirror symmetry along the axis of charge acceleration, as with a dipole. Using two unbalanced antennas together has the advantage that each antenna expects to have the ground plane present, but the combination is free from ground plane effects. This phenomenon arises because, although each antenna does generate currents in the ground plane directly beneath the antenna, the current pair cancels leaving negligible currents fl owing on the rest of the ground plane. This effect is shown in Figure 3.

Figure 3. Surface RF currents at 1900 MHz on

a PCB with a radiating balanced antenna pair.

The plot shows good cancellation everywhere

except under the antenna. A 0-30 A/m linear

scale has been used.