AWR eBooks

MICROSTRIP ANTENNA ARRAY Although there are a significant number of applications for the single element microstrip antenna, in many cases, the performance enhancements and features of multiple microstrip element arrays add to an expanding list of new opportunities. A microstrip antenna array is formed by the arrangement, or grouping, of multiple, single-element microstrip antennas. The respective geometric positioning of the individual elements, as well as the element amplitude and phase excitation, determines the characteristics of the antenna array. Microstrip antenna arrays are typically designed to enhance the antenna performance beyond that available from a single element; for example, arrays of single-element microstrip antennas offer increased gain and narrower beamwidth at the cost of larger aperture area. In addition, array antennas also offer the ability to steer the principal radiation intensity beam via differential phase excitation and to reduce sidelobe levels by variable power excitation to the individual elements of the array – properties which compel emphasis in many applications. The individual dimensional elements of a microstrip antenna array may vary and may be spatially configured in a linear, planar or volumetric arrangement. The radiation pattern of an array is determined by the dimensions, spatial distribution and electrical excitation, i.e., amplitude and phase, of the individual elements. Given the number of variables, a general approach to the synthesis and design of antenna arrays is clearly required. To that end, antenna specialists have been successful in formulating a general methodology using the following definitions: ANTENNA ARRAY TERM ANTENNA ARRAY DEFINITION Array Factor (AF) The array factor defines the radiation pattern of spatially distributed isotropic radiating elements in accordance with the amplitude and phase of element excitation. The array factor is a function of the number, dimensional spacing, and the amplitude and phase of the excitation signal of the elements. Element Factor (EF) The element factor is the radiation pattern of the individual elements of an array. The product of the array factor and the element factor is referred to as the pattern multiplication theorem. An example will illustrate the convenience and efficiency of the theorem. Consider the linear distribution of equally spaced, isotropic radiating elements along the z-axis as shown in Figure 10. The E- field radiation pattern of the i th element may be written: 18 The following definitions are applicable to this equation: – represents the radiation pattern of the element, and , and are the amplitude and phase excitation. For n identical elements, the radiation pattern is written: 18 Bahl, I. J. and Bhartia, P., Microstrip Antennas, Chapter 7, Artech House, Dedham, MA, 1980 ( ) ( ) ( ) [ ] i i i i cos z k j exp I , F , E b q f q f q + × = 0 ( ) f q, F 0 0 2 l p / k = i I i b ( ) ( ) ( ) [ ] å - = + × = 1 0 0 n i i i i cos z k j exp I , F , E b q f q f q Figure 10: Linear Array of Isotropic Radiating Elements Microstrip Antenna Design 14 www.cadence.com/go/awr

• Cover