Tips Ease Antenna Design For ISM-Band And Short-Range Devices

VARIOUS TYPES OF ANTENNAS are used for short-range and Industrial-Scientific-Medical (ISM) -band devices. For example, the antennas that excite an electrical field are called electrical antennas. In contrast, those that excite a magnetic field are known as magnetic antennas. In the 38-page application note, "ISM-Band and Short-Range Device Antennas," Texas Instruments, Inc. (Dallas, TX) explores the fundamentals of such antennas along with practical design principals.

The paper begins by examining the basics of antennas. Equations are provided for the speed of light, the wavelength in free space, and more. The note also explains how to distinguish between the three field regions where the electromagnetic (EM) wave develops. Those regions are the reactive near field, radiating near field, and far field.

Section 2 focuses on antenna types and their features. The half-wave dipole antenna, which is the basis for many other antennas, also is used as a reference antenna for the measurement of antenna gain and radiated power density. In many cases, however, that antenna is simply too large. In addition, the differential feed is a drawback. For these reasons, the quarter-wave monopole antenna replaces one branch of the dipole antenna with an infinitely large ground-plane. Other types of antennas, such as helix and various loop antennas, also are examined.

To conclude this section, a useful list of the "Rules of Thumb for the Antenna Design" is provided. For example, the groundplane of quarter-wave monopoles is often too small because of space limitations. To create as much groundplane around the feed point as possible, the engineer should measure the resulting antenna impedance and match it to the needed characteristic impedance.

The section on RF propagation explores path loss and multipath propagation effects. It delves into details like the link budget, which forms the basis for an estimation of the achievable distance in a communication link. The note also discusses fading and intersymbol interferences. Suggestions to avoid these problems, which are caused by multipath propagation, also are provided.

The application note ends with examples and measurements. It explores test-module schematics, antenna layouts, radiation patterns, and more. Overall, the note succeeds in examining the basics of antennas for short-range and ISM-band applications. It also provides useful information on the problems they face, tips for resolving those issues, and general antenna design and implementation.

Texas Instruments, Inc., 12500 TI Blvd., Dallas, TX 75243-4136; (800) 336-5236, Internet:

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