**6.4 Keyboard grounding requirements**

208 Wireless Communications and Networks – Recent Advances

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

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

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

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

Fig. 43. Isolation and S11 of antennas for various orientation with antennas separation 20mm

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

Fig. 42. Isolation and S11 of antennas for various orientation with antennas separation 20mm

orientations.

and EBG-antennas spacing 7.5mm.

and EBG-antennas spacing 15mm.

between antennas.

The large metal plate of a keyboard can act as an integrated shield to prevent noise radiating from the base chassis. However, the mechanical grounding structure needs to be taken serious care of its EMI effect on radio performance, because it is not uncommon that EMI noise radiated from the ground-base to antennas mounted around LCD panel when the keyboard is removed as shown in Figure 44. This means that we are not getting any shield effect from the keyboard, and in fact the keyboard helps EMI ground noise radiating from the base like a large antenna. To make the keyboard a useful shield for all WWAN frequencies, it is necessary to conductively contact ground structure every 1/20th wavelength or so as shown in Figure 45.

Fig. 44. Field distribution on metal shield of LCD panel and nearby antenna.

Fig. 45. Metal plate for LCD panel shielding purpose.

Grounding the motherboard to the chassis wherever possible (screw holes, connectors, etc.) will help reduce EMI radiation from the motherboard. The grounding contacts should follow EMC design guidelines for length to width ratio requirement to prevent from adding a radiation structure rather than providing a noise-reduction ground point. Every effort should be made for each screw and contact point of the motherboard to the chassis for a good chassis ground point.

The grounding of the heat sink also raises a problem for ESD, EMI, and platform noise of wireless devices, therefore improvement should be made in the grounding of the heat sink (cooler). The improvement should include: enabling better DC grounding at "spring contact" points, and no non-grounded arms longer than 7mm are allowed.
