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 design, development, testing, and integration methodology for antennas integrated into laptop computers is described. Two key parameters are proposed and discussed for laptop antenna design and evaluation: standing wave ratio (SWR) and average antenna gain. A novel averaging technique was developed and applied to these to yield a measurable, repeatable, and generalized metric. A prototype antenna was built using this methodology, and measurements indicate that the resulting design attains both performance and cost targets. A PC-card-version wireless system is also discussed and compared with the integrated one. The impact of the antenna on the overall wireless system is studied through a link budget model.

Introduction

Wireless use by mobile professionals has increased greatly in the past several years [1-4]. According to Cahners-Instat, the market for wireless LANs is projected to grow from $1.2 billion in 2000 to more than $5.6 billion in 2005. Another estimate from Frost and Sullivan forecasts that manufacturers' revenue in the total worldwide wireless LAN industry will approach $884 million by the year 2005 [5]. As a result, the unlicensed 2.4-GHz industrial, scientific, and medical (ISM) band has become very popular and is now widely used for several wireless communication standards. Examples now include many laptop computers with built-in 11-Mb/s wireless LAN capability (standard 802.11b), and the newly developed Bluetooth technology for cable replacement to connect portable and/or fixed electronic devices. For even higher data rates, standard 802.11a devices in the 5-GHz Unlicensed National Information Infrastructure (U-NII) band are being developed which have data rates up to 54 Mb/s with proposed channel bonding techniques that will extend this to 108 Mb/s [6].

The initial implementations integrated these systems into portable platforms such as laptop computers using

PC cards inserted into the PC card slot. As wireless technology becomes more prevalent and less expensive, manufacturers are moving away from PC cards in favor of integrated implementations. There is an industry-wide effort to avoid the problematic issues of breakage and physical design constraints associated with external antennas, and to completely integrate these communication subsystems directly into the portable platforms such as laptops. Until now, system designers did not take into account the wireless subsystem and the design did not include an antenna, when in reality integrated antennas can provide product differentiation [7]. There are a plethora of articles [8, 9] regarding all of these systems, but few designs fully integrate the antenna as part of the system and platform or achieve the potential performance such integration can offer. The goal of this paper is to highlight the specific design challenges associated with antenna integration into laptops. These challenges are illustrated through practical design examples, including suggested test and integration methodology to solve the problems outlined below.

There are three major challenges for antenna design associated with wireless integration into laptops. First, laptops are very densely packed electronic devices, and there is little room for additional functions. Second, FCC emission requirements have forced laptop manufacturers to make extensive use of conducting materials in the covers of the laptops or conducting shields just inside the laptop covers to minimize radiation from today's very high-speed processors. Thus, it is difficult to place an antenna in an environment free enough of other conductors to create an efficient radiator. Third, the size, shape, and location of the antenna may be affected by other design constraints such as the mechanical and industrial design. It is therefore necessary to make engineering tradeoffs in the design, performance, and placement of the antenna on the one hand (given the industrial and mechanical design) and the size of the laptop on the other. As an example, early results based strictly on analytical modeling, blind cut-and-try, or the use of "integratable" vendor solutions yielded an integrated Bluetooth antenna solution incapable of reliable connectivity much beyond 1-3 meters; this was not even close to the advertised Bluetooth specification of 10 meters. Surprisingly, vendor solutions that touted fully integrated design capability for Bluetooth appeared to be measurements of freestanding antennas. Once integrated with the odd ground planes and cabling of a real system, the antennas fell far short of advertised performance. Selling an integrated system solution that falls short of user expectations creates disappointment and dissatisfaction, and could discourage wide acceptance of wireless technology. Clearly, a better solution to this problem was required.

Laptop-related antenna issues

Possible antennas for laptop applications Figure 1 shows several possible antennas for laptop applications. Dipole and sleeve dipole antennas are basically the same, except that one is center-fed and the other is end-fed. Dipole antennas have a wider bandwidth than sleeve dipoles, but sleeve dipoles are easier to use. These antennas produce their best performance if they are mounted on the top of the display. Helical and monopole antennas should also be placed on the top of the display to achieve their best performance. The helical antenna is physically small, but its bandwidth is narrower than that of the monopole antenna, making it problematic to match transmitter and receiver over the fairly broad ISM bands. Given their large size, traditional slot and patch antennas should be placed on the surface of the display. Ceramic chip antennas are typically helical or inverted-F (INF) antennas or variations of these two types with high dielectric loading to reduce the antenna size. They are small, but their bandwidth is too narrow. Slot and INF antennas belong to the same antenna category and are good candidates for laptop applications because of their broader bandwidth characteristics. These antennas are also very popular for laptop applications because of their overall performance, ease of integration, simple design, and low cost.