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

Table 3 shows a link budget in spreadsheet form. In the range calculations, 7.5 dB of extra path loss due to the effects of multipath Rayleigh fading was assumed. Since data is typically sent in packets or blocks, a common system metric used to characterize performance is block error rate. This path loss represents an average power loss due to the effect of destructive multipath summation at the antenna while operating at the 10% block-error rate system sensitivity point. The results shown are the ranges for a laptop to an access point (0-dBi transmitting antenna gain) at an 11-Mb/s data rate and for peer-to-peer laptops (-2 dBi transmitting gain) with different data rates at 11, 5.5, 2, and 1 Mb/s, respectively.

Some examples

INF antenna implementation

Figure 7 shows an INF antenna integrated into a laptop prototype for 2.4-GHz applications. The antenna was stamped from a brass sheet and is mounted on a metal support frame of the laptop display. Since the metal support frame is connected to the laptop display and therefore provides a very large ground plane to the antenna, the antenna system has very stable performance. Thus, even when the feeding coaxial cable is moved around, the antenna input impedance changes very little, a problem that plagues the measurement of even freestanding small antennas with long cables and poor grounds.

Figure 8 shows the measured SWR of the antenna. The vertical dashed lines show the recommended SWR mask for the 2.4-GHz band. Note that the 2.4-2.5-GHz frequency range (slightly wider than the strict U.S. band), is used here to cover worldwide applications. The horizontal dashed line indicates 2:1 SWR. It is clear that the antenna has adequate SWR bandwidth and the maximum SWR is less than 1.6 over the whole band, allowing for manufacturing and environmental margin. It should be noted that the effective cable length in this test is zero, minimizing interpretation and use of this data.

Figure 9 shows the measured radiation patterns of the antenna in the horizontal plane when the laptop is open. The solid and dashed lines are for the horizontal (H) and vertical (V) polarizations, respectively. The dash-dot line is for the total (T) radiation pattern. The gain (average/peak) values are shown in the legend of the figure. The vertical polarization has a gain value larger than that of the horizontal polarization. The overall average gain value is about 0 dBi, similar to an isotropic radiator. However, the peak gain value (2.6 dBi) is larger than that of a half-wavelength dipole antenna (2.1 dBi). This is due to the effect of the laptop display surface. In most countries, the peak gain numbers are also important and are tracked by the regulatory bodies creating the need to balance and optimize the average and peak values of each design. For the WLAN case in the U.S., the Federal Communications Commission (FCC) sets a 36-dBm equivalent isotropic radiated power (EIRP) maximum. With a one-watt transmitter output, the maximum peak gain for an antenna is therefore 6 dBi.