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

The test setup shown in Figure 5 is for a 2.4-GHz Bluetooth radio subsystem. This experiment is intended to illustrate the effects of antenna placement and laptop materials. One laptop, an IBM 770 ThinkPad, has a popular vendor radio installed in the PC bay using an extender card so that the radio and its antenna are well removed from the conducting surfaces of the laptop. A simulated PC card slot opening fabricated from copper-clad PC board material was placed over the radio. This conducting surface represents shields or conducting plastics used in modern laptops. The card position could be adjusted so that the antenna was outside the slot (positive d displacement), flush with the opening of the slot (d = 0 displacement), or inside the slot (negative d displacement). A second radio, installed in another laptop (not shown), was used to form a link and keep the radio under test transmitting. It was located so that its signal at the probe position was much weaker than the signal from the radio under test. The output of the log amplifier, which is proportional to the log of the power received at the probe antenna, was filtered with a low-pass filter and then displayed on an oscilloscope.

The experiment proceeded by setting the distance, d, and measuring the output power of the radio under test. Since the probe antenna was not calibrated, only relative power levels were determined in the measurement. The position of the slot, d, was varied between -10 mm and 15 mm.

The results are shown in Figure 6 for each of the three possible carrier frequencies used by the radio (2.404 GHz, 2.441 GHz, and 2.459 GHz). The relative output power of the transmitting radio is a function of its antenna position, d, relative to the conducting aperture of the simulated PC card slot. Between -10 mm and 4 mm the sensitivity of the output power to this dimension is almost 0.8 dB/mm. The effect saturates at d = 4-5 mm. That is, once the antenna is located 4 or 5 mm outside the conducting surface of the laptop, there is little additional benefit to increasing the protrusion of the antenna. It should be noted that the transmitting power was measured only in one direction, which is nearly that of peak gain. Increasing the protrusion further might improve the omnidirectionality of the antenna.

The design of the ThinkPad 770 is indicated by the vertical line on the plot at d = -2 mm. The potential for improvement in antenna sensitivity is almost 6 dB if the antenna is moved to a position at which it protrudes 4 mm from the laptop case.

To understand the impact the additional 6 dB has on the link range, consider the following: The radio vendor has reported a range with this radio of 5 m. IBM has developed a link budget model (see the section below) for the Bluetooth radio from which it is possible to determine the range as a function of antenna gain or output. Included in the model is a path loss exponent (1/r^sup n^). For indoor environments and short-range applications such as Bluetooth, n = 2.5 appears to be a reasonable value, while n = 3.5 is used for WLANs. With this model, the 6-dB loss of power when the PC card radio is installed in the 770 reduces the range by more than 40%. If both ends of the link were to have these radios installed in 770s, the range would be expected to be about 1/3 of the case in which the output of the radios is unaffected by the laptops. The reduction in range for the 770, based on the current measurements, is consistent with results reported by the vendor.