**6.3 Antennas isolation [8]**

Noise current distribution and antenna surface current are most important message for solving the sensitivity dedradation problem. We can use a network analyzer deliver the energy to TP's LVDS or camera's USB lines and measure the insertion loss at antenna port, and that is the most commonly utilized technique to obtain the isolation situation of platform noise.

With the highly integration of powerful computing and multi-radio communications devices in a single product nowadays, multiple antennas are usually implemented to achieve the seamless and convenient communication services. However, the closely placed antennas have resulted in intra-system coupling interference and therefore severely degraded the performance of various kinds of wireless communications. The isolation technique for antenna systems must be implemented to reduce the mutual coupling between coexistent various RF systems. In this section, we will show the optimal isolation achieved from antennas separation, orientation, and utilization of periodic structure to reduce the mutual coupling interference.

The isolation requirement between coexistent RF systems is shown in Table 4. The placement of two chip antennas under investigation for Bluetooth and 802.11b/g WiFi systems inside the mold notebook computer is shown in Figure 36. The chip antennas are fabricated on FR4 with dimension of 1.6 mm thickness and 35mm × 30mm area, and it is fed by microstrip to achieve 50 Ω impedance-matching. The configurations for different spacing and orientation between coexistent antennas are shown in Figure 37 to analyze the mutual coupling effect.


Table 4. Isolation requirement between coexistent systems.

Fig. 36. Placement of two chip antennas insidemold.

Fig. 37. Three orientation for antennas placement.

The reflection coefficients (S11 and S22 are reflection coefficients for Bluetooth and 802.11b/g respectively) and isolation (S21) between each other were then measured and shown in Figure 38. The results clearly show that the isolation between antennas is much worse when they are both placed in vertical polarization with main lobes coupling. We then oriented the antennas in orthogonal direction to each other and separated antennas by moving 0mm, 10mm, and 20mm respectively. The measured results of reflection coefficient covering 2.4GHz-2.483GHz in Figure 39 show that the isolation is much better due to orthogonal polarization to each other when they are oriented in orthogonal direction. Finally, we placed both antennas in horizontal direction and adjusted the separation between them, the results in Figure 40 also show better isolation than vertical placement due to main lobes decoupling. Table 5 compares the isolation performance between various orientation and separation for both antennas, and it shows that the orthogonal and horizontal orientations gain almost 6-9dB improvement in isolation except 4 dB difference for 0mm separation.

Fig. 38. Measured results for various antenna spacing with both antennas oriented in vertical direction.

The reflection coefficients (S11 and S22 are reflection coefficients for Bluetooth and 802.11b/g respectively) and isolation (S21) between each other were then measured and shown in Figure 38. The results clearly show that the isolation between antennas is much worse when they are both placed in vertical polarization with main lobes coupling. We then oriented the antennas in orthogonal direction to each other and separated antennas by moving 0mm, 10mm, and 20mm respectively. The measured results of reflection coefficient covering 2.4GHz-2.483GHz in Figure 39 show that the isolation is much better due to orthogonal polarization to each other when they are oriented in orthogonal direction. Finally, we placed both antennas in horizontal direction and adjusted the separation between them, the results in Figure 40 also show better isolation than vertical placement due to main lobes decoupling. Table 5 compares the isolation performance between various orientation and separation for both antennas, and it shows that the orthogonal and horizontal orientations gain almost 6-9dB improvement in isolation except 4 dB difference

 (a) 0mm (b) 10mm (c) 20mm Fig. 38. Measured results for various antenna spacing with both antennas oriented in

 (a) Vertical (b) Orthogonal (c) Horizontal

Fig. 36. Placement of two chip antennas insidemold.

Fig. 37. Three orientation for antennas placement.

for 0mm separation.

vertical direction.

Fig. 39. Measured results for various antenna spacing with both antennas oriented in orthogonal direction.

Fig. 40. Measured results for various antenna spacing with both antennas oriented in horizontal direction.

Table 5. Measured results for various antennas placement configuration.

#### **6.3.1 Suppression of mutual coupling interference between coexistent antennas [8]**

The applications of EBG structure in antenna not only could improve gain and radiation efficiency, it could also help suppress side lobes and reduce coupling effect. Since isolation requirement could not be met by orientation and separation arrangement between antennas from the above measurement, we therefore chose the best placement configuration with orthogonal orientation and 20 mm separation for further investigation utilizing EBG structure. We placed the EBG structure beneath the antennas with 7.5mm (less that λ/4) and 15mm (about λ/4) distances as shown in Figure 41 and investigated the mutual coupling characteristics. Figure 42 shows the results with 7.5 mm separation between antennas and EBG structure for various antenna orientations. Because of high impedance surface from EBG structure, the 40 dB isolation requirement between antennas could be achieved and parallel-plate guided wave coupling could also be suppressed. Figure 43 shows the results with 15 mm separation between antennas and EBG structure for various antenna orientations.

Fig. 41. Configuration of antennas and EBG structure placement.

Fig. 42. Isolation and S11 of antennas for various orientation with antennas separation 20mm and EBG-antennas spacing 7.5mm.

Fig. 43. Isolation and S11 of antennas for various orientation with antennas separation 20mm and EBG-antennas spacing 15mm.

Table 6 compares the isolation performance with and without EBG structure for various antenna orientations, and it shows that 37.7 ~ 48dB and 35.7 ~ 40.9dB isolation between antennas can be achieved with EBG structure placed beneath antennas 7.5mm and 15mm respectively. When EBG structure moves closer to antennas, we can obtain better isolation between antennas.


Table 6. Comparison of isolation improvement from EBG structure with antennas separation 20mm.
