WILEY ANTENNAS FOR PORTABLE DEVICES phần 6 doc

134 Laptop Antenna Design and Evaluation

Table 4.4 Integrated and PC Card Wireless Solutions SNR Comparison.

Distance

(m)

0 (deg) 90 (deg) 180 (deg) 270 (deg)

Int Card Int Card Int Card Int Card

0 59 54 52 47 54 45 49 49

5 53 50 49 43 49 49 53 50

10 45 37 47 35 45 38 43 36

15 42 31 51 34 46 35 45 26

20 40 22 52 32 45 35 45 26

25 41 33 49 29 43 30 48 22

30 37 21 46 30 43 30 46 24

35 42 22 43 31 42 29 45 20

40 34 23 46 23 42 23 46 27

45 37 29 46 25 42 25 46 28

(From [6]. Reproduced by permission of IBM.)

was used for this study. Two slot antennas were implemented in the ThinkPad, one on

the upper left side and another on the top right edge of the display. An IBM High Rate

Wireless LAN PC card was used for the comparison study. Table 4.4 lists the SNR values

for distances from 0 to 45 meters with laptop orientation angles 0, 90, 180, and 270.

The SNR values were obtained through the IBM WLAN Client Configuration Utility gain

test program. Distances were measured from AP to laptop. Angle 0 is the laptop rear cover

toward the north, 90 is toward the west (AP direction), 180 is toward the south, and 270 is

toward the east. These actual tests indicate that integrated wireless is 47% better on average

than the PC card version. When the laptop is far from the AP, the integrated antenna has

much higher gain values than the PC card antenna, resulting in much higher SNR. Above

25 meters, the SNR for the integrated wireless system is more than 10 dB larger than that of

the PC card system. The higher SNR values imply longer distance for the same data rate or

higher data rate for the same distance.

As a practical example, an iSeries ThinkPad with the integrated antenna was tested against

a PC card version and shown to have superior performance. The test was conducted on the

fifth floor of an IBM building in Yamato Japan. This floor has three APs. When the RF

signal was weak, the PC card switched to another AP, while the iSeries integrated antenna

performance was still good and maintaining a connection to the same AP.

4.8 Dualband Examples

The 2.4 GHz ISM band has become extremely popular and is now widely used for several

wireless communication standards. As a result, system interference and capacity are of

concern. IEEE 802.11 a devices at the 5 GHz band do not have these concerns. For world-wide

applications, an antenna covering the 5.15–5.85 GHz range is currently needed. Dualband

antennas with one feed point have been proposed by many authors [23–53]. Most antennas

proposed either provide inadequate coverage at the 5 GHz band or are not suitable for

integration in portable devices. In this section we will present three designs that have been

used in laptop computers.

4.8 Dualband Examples 135

4.8.1 An Inverted-F Antenna with Coupled Elements

This antenna structure [47] as shown in Figure 4.16 is a bent version of the closely coupled

triband antenna proposed by Liu [51]. This antenna inherits many properties of the closely

coupled antenna. Therefore, most conclusions drawn in [51] apply to the antenna here. For

the low band (the 2.4 GHz band), the antenna behaves as an INF antenna. Much of the

current flows in the INF section. The current in the L-shaped and tab sections is very weak,

so it has negligible effect on the low band. At the middle and high bands, much of the

current is concentrated either in the L-shaped section or on the tab section. The dominant

effect is on the middle/high band resonance and the radiation pattern. However, since the

INF section is fed directly, it has a relatively strong influence on the middle and high bands.

The antenna behaves in a complicated way at the middle and high bands. Depending on

the applications and available volume for antenna implementations, the middle and high

bands can be exchanged. As referenced in Figure 4.17, R2 provides the middle band, while

R3 provides the high band. Figure 4.17 also shows the evolution from the original triband

antenna to the low profile triband antenna. For the WLAN applications, the middle and the

high bands are combined to cover the 5 GHz band. As a result, the triband antenna is used

as a dualband antenna in this case.

The resonant frequency of the low frequency band is determined primarily by L1+H1−W1

as shown in Figure 4.16. Increasing H1 and the width of the metal strips will widen

the bandwidth of the antenna at the lower band. Moving the feed point FP horizontally

will change the antenna impedance. Moving FP to the left (open) side will increase the

impedance and to the right (grounded) side will reduce the impedance. Changing the feed

point will have some effect on the resonant frequency as well. The middle and high band

elements have negligible effects on the lower band. The middle band frequency is primarily

determined by H2+L2. The impedance in this band is primarily determined by D12 and S2,

Figure 4.16 INF antenna with coupled elements implemented on PCB. (From [47]. Reproduced by

permission of © IEEE.)

136 Laptop Antenna Design and Evaluation

Figure 4.17 Triband antenna evolution.

the coupling distances. Generally speaking, reducing D12 and S2 will increase the coupling

and consequently the impedance at this band. Widening the L2 width will broaden the

impedance bandwidth. Tapering the corner near H2 seems to improve the bandwidth as well.

The high band is primarily determined by H3, S3 and W2. H3 is the major controlling factor

for adjusting the resonant frequency. S3 changes the coupling between this band and the

lower band. The substrate thickness and the substrate dielectric constant will also affect the

2 2.5 3 3.5 4 4.5 5 5.5 6

1

1.5

2

2.5

3

3.5

4

4.5

5

Frequency (GHz)

SWR

Figure 4.18 Measured SWR of the dualband prototype antenna.