Arrays Antenna

An antenna array (or array antenna) is, much like it sounds, several elements
interconnected and arranged in a regular structure to form an individual antenna. The
purpose of an array is to produce radiation patterns that have certain desirable
characteristics that a single element would not. A stacked dipole array, as shown in
figure, is comprised of vertical dipole elements.



This dipole array has an omnidirectional pattern like the element dipole does; but has higher gain and a narrower main lobe beamwidth in the vertical plane. Figure 22
shows how the vertical-plane gain of the dipole element can be “enhanced” by making an array of them. Figure 22(a) represents the radiation pattern of one element. The Figure is kindly the pattern antenna array as well as array schema.




The omnidirectional coaxial collinear antenna (often referred to as an “omni”) is a
very popular array design for base stations. It is comprised of quarter-wave coaxial
sections with inner and outer conductors transposed at each junction. A conceptual illustration is shown in figure. Although more complex than the illustration, this antenna array behaves like a series of vertical dipoles stacked one above the other. The more stacked sections, the greater the gain and the narrower the vertical beamwidth. A vertical-plane pattern for this type of antenna is shown in figure 24. Variations in electrical design can produce a downward tilt of the vertical-plane pattern as shown in figured. This antenna often is enclosed in a fiberglass sheath, called a radome, and appears as a simple pole that can be mounted off the side or on top of a mast or tower.

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