The following example illustrates the benefits of tuning with respect to antenna volumetric size reduction. Two different antenna configurations were analyzed using a 3D electromagnetic modeling program: one a broadband design, and the other, a narrower band design capable of tuning over the same frequency range but using four tuning states.
Figure 3a illustrates a 50 x 6 x 14 mm seven-band antenna configuration and its associated radiation efficiency over just the lower three-band spectral region from 700-960 MHz.
Figure 3 - Comparison of a) multiband antenna and b) tuned antenna with respect to size and radiation efficiency over the region 700- 960 MHz. (Dimensions in mm.)
A similar, but smaller (50 x 6 x 7mm) antenna configuration is shown in Figure 3b, illustrating that tuning using only four states can produce nearly the same efficiency and total frequency coverage as the larger broadband antenna.
It is clear from the example in Figures 3a and 3b that a physical volume reduction of one half can be achieved by tuning the antenna to one of several states, each supporting a certain set of frequency bands. The antenna during operation would therefore only be required to change state when the operating band is changed. The time required for this change must be compatible with other functions within the radio system. A typical requirement might be 10-20 microseconds or less.
Mutual coupling effects between adjacent antennas operating at the same frequency at the same time can be mitigated using isolation techniques. The most common technique is to physically separate the antennas from each another. The mutual coupling effect drops as the distance increases. However, for handheld devices, it’s not always possible to find locations to give adequate separation to mitigate the effects of mutual coupling. In this case, device designers need a different antenna solution to achieve the performance required by the specification.
One possible solution is to excite two different modes from the same antenna structure using Isolated Mode Antenna Technology (iMAT®) from SkyCross. The iMAT antenna structure is placed on one end of the phone. Each of the two feed points launches a different radiating mode. The feed points are isolated from each other and do not suffer from the losses normally associated with mutual coupling, so the efficiency of each mode is high. In addition, the radiation patterns are different and produce a low correlation coefficient. Figure 4 illustrates implementation of an iMAT antenna showing the isolation between two feed points on the same antenna structure.
Figure 4 – iMAT antenna implementation showing isolation between two feed points on the same radiating structure