Antennas embedded within compact devices, such as handheld computers and cellular telephones, must be optimized to limit interactions with surroundings. Such antenna isolation permits good efficiency within different enclosures and reduces the engineering time needed to incorporate antennas within new enclosures. Evaluation of antenna performance can be performed on an antenna within an anechoic chamber or measurements can be made on the complete wireless system of which the antenna is a part. The isolation can then be characterized by measuring the resonance frequency shift when parasitic elements are near the antenna. Such measurements will show that antenna isolation is a critical parameter when evaluating wireless designs as part of multiple systems within the same enclosure.

For practical reasons, multiple or multiband antennas are often implemented within the same enclosure. For efficiency and ease in integrating multi-band antennas, it is essential for their radiating elements to be highly isolated.

Antennas have been studied for over 50 years.¹ Antennas radiate electromagnetic (EM) waves that interact with resonant and absorbing materials nearby, including the enclosure or package in which they are mounted.² This interaction can reduce the overall efficiency of an antenna, with real-world antenna patterns representing some reflections due to the interference between directly radiated antenna waves and waves reradiated by different parts of an enclosure (Fig. 1). The radiation pattern of an isolated antenna can be smoothed and its efficiency improved by shaping its near-field pattern away from perturbators and absorbers. Moreover, the coupling between different antennas mounted inside the same enclosure is reduced.

An example will help to illustrate the importance of antenna isolation, based on experimental results obtained with a wireless communications system.³ Intrinsically, antennas interact with the surroundings as they radiate the EM energy carrying the information. Figure 2 presents the radiation patterns from two different antennas mounted on the side of a laptop computer (as in Fig. 1). The radiation pattern of antenna 1 (a printed stub) has many ripples whereas the pattern of antenna 2 [an Isolated Magnetic Dipole (IMD) from Ethertronics] is very smooth. The ripples are due to resonances created by the near-field energy of the antenna interacting with the enclosure.⁴ Part of this energy is reradiated and interferes with the direct emission in the far field. By measuring the full three-dimensional radiation patterns of both antennas and integrating the gain, it is possible to determine the efficiency difference between them.⁵ Such measurements reveal 4-dB better efficiency for antenna 2 compared to antenna 1, due to a dramatic amount of EM energy from antenna 1 absorbed by the enclosure. These results indicate that antenna 2 is better isolated than antenna 1.

Anechoic-chamber measurements are sometimes critical, as the antenna mounted onto the enclosure may not be exactly as it would be mounted onto a system.⁶ In the case of antennas 1 and 2, both antennas were tested with a wireless network system based on the Home-RF protocol, connected to laptop computers by means of PCMCIA cards. The measurements consisted of timing the transfer of a calibrated file (for example, a 1-MB file) for different antennas. This measurement can be expressed as an "active-bit-rate" function of the transmitted distance. Antenna 1, which is sold with the Home-RF system, and antenna 2 were measured inside a parking lot in order to reduce multipath effects. Figure 3 shows the evolution of the active-bit-rate measurement as a function of the distance. Whereas the bit rate obtained with antenna 1 drops very fast, either when the laptop computers are facing each other or when they are away from each other, the bit rate obtained with antenna 2 remains constant to a distance of 60 feet. At that point, it drops slowly, finally vanishing for a transmit range twice that of antenna 1. This measurement also shows that even in a reduced multipath configuration, the directionality of the antenna is not critical, at least for a Home-RF system. Unfortunately, this measurement does not give any figure of merit concerning the efficiency difference between antennas.