Developing integrated antenna subsystems for laptop computers

IBM Journal of Research and Development, Mar/May 2003 by Liu, Duixian, Gaucher, Brian P, Flint, Ephraim B, Studwell, Thomas W, Et al

Antenna location evaluations

It is very important to have a good understanding of the antenna performance effects due to antenna location. Because of its popularity, an INF antenna (Figure 3) was used to examine performance at different locations on a laptop. Since the antenna characteristics are dependent on its location on the laptop, an antenna tuned for a particular location probably will not work as well for other locations. Therefore, some minor modifications to the antenna are necessary to ensure an acceptable standing wave ratio (SWR) in each case. The SWR is defined as

where [tau] is the reflection coefficient for the outgoing wave from the transmitter. No compensation for the shift of the center frequency is made during these evaluations based on the assumption that adjustments for this shift are straightforward. Figure 4 shows the antenna locations and orientations. Because of laptop symmetry, antenna locations on only the left side of the laptop are considered. The laptop used is an A-Series ThinkPad*, which has acrylonitrile butadiene styrene (ABS) plastic in the display and base covers. Other ThinkPad models use carbon-fiber-reinforced plastic (CFRP, very lossy material at rf frequencies) or metal covers, and the results shown here would not be applicable. Table 1 lists the peak and average gain values at these locations. In the table, 0[degrees] indicates measurements made in the horizontal plane; negative and positive angles indicate measurements made above and below the horizontal plane, respectively. The measurements have an azimuth scan from -180[degrees] to 180[degrees] and an elevation scan from -40[degrees] (above the horizontal plane) to 35[degrees] (below the horizontal plane), both in 5[degrees] increments. The laptop was open, with an angle between the cover and the base of 90[degrees]. Listed in the table are those frequencies corresponding to the maximum average gain value and highest peak gain value. The average gain is defined over an azimuth scan (360[degrees]) for a given elevation angle. The table indicates that, except for the MBaseSideLeftBack location, placing the antenna high (center and top) or in vertical orientation tends to yield maximum radiation on or close to the horizontal plane. This is another indication that one should place the antenna as high as possible. Table 2 lists the center and resonating frequencies and 2:1 SWR of the antenna at different locations. Note that the center frequency, [function of]^sub cen^, is slightly different from the resonating frequency, [function of]^sub min^. The resonating frequency corresponds to the minimum SWR values. Table 2 indicates that the 2:1 SWR bandwidth will be wider if the antenna is placed on a small ground plane (side of display) or the edge of a large ground plane (back side of display). One must remember that even though the laptop uses a plastic cover, metal foil and shields exist inside the cover to reduce emissions from laptops to meet FCC regulations.