CHAPTER TWOAntenna Fundamentals
A radio antenna, transmitting or receiving, is an independent and yet integral component of any wireless communication system. An antenna acts as a transducer that converts the current or voltage generated by the feeding-based circuit, such as a transmission line, a waveguide, or a coaxial cable, into electromagnetic field energy propagating through space and vice versa. In free space, the fields propagate in the form of spherical waves, whose amplitudes are inversely proportional to their distance from the antenna. Each radio signal can be represented as an electromagnetic wave [1] that propagates along a given direction. The wave field strength, its polarization, and the direction of propagation determine the main characteristics of an antenna operation.
Antennas can be divided in different categories, such as wire antennas, aperture antennas, reflector antennas, frequency-independent antennas, horn antennas, printed and conformal antennas, and so forth [2–10]. When applications require radiation characteristics that cannot be met by a single radiating antenna, multiple elements are employed, forming “array antennas.” Arrays can produce the desired radiation characteristics by appropriately exciting each individual element with certain amplitudes and phases. The very same antenna array configurations, when combined with signal processing, lead to multiple-beam (switched beam) or adaptive antennas that offer many more degrees of freedom in a wireless ...
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