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19 | RF systems, which often hinge on antenna performance. This chapter surveys RF antennas, including how they work, key performance criteria, and common types. It also explores antenna design, selection, testing, and verification in new RF systems. Understanding RF Antennas An antenna is a fundamental transducer that converts electrical signals carried through conductors, transmission lines, or waveguides into electromagnetic waves that travel through space. RF antennas operate reciprocally, meaning they convert electrical signals into electromagnetic waves and vice versa, with the same efficiency in both directions. Transmission occurs when a transmitter directs an electrical signal to the antenna terminals, causing magnetic and electric fields to develop along the antenna structure. These fields eventually transform into electromagnetic waves traveling away from the antenna in a pattern dictated by the antenna geometry. Reception occurs when electromagnetic waves in space collide with an antenna structure. The waves then induce electrical signals carried along a conductor or transmission line or are captured by an aperture and converted to guided waves within a waveguide. Antenna Feeds and Signal Transmission Antennas are often driven via feed lines or feeds, which are typically transmission lines or waveguides designed to efficiently direct the electrical signals to and from the RF system to the antenna terminals. Nearby dielectric or conductive objects can affect the electric, magnetic, and electromagnetic fields around an antenna. These interactions impact performance, as field size, shape, and behavior depend on the antenna's structure and the frequency of the signal. The distance at which electric, magnetic, or electromagnetic coupling can occur depends on the wavelength of the electrical signals, with lower-frequency signals coupling more efficiently at greater distances. In antenna structures, three different field regions exist depending on frequency: near field, radiative near field, and far field. The near field includes the reactive or inductive region, where electric and magnetic field coupling occurs. The radiative near field (known as the Fresnel region) is where electromagnetic fields are not strictly perpendicular and have not yet formed a plane wave. In the far field, electromagnetic waves establish a true plane wave with perpendicular electric and magnetic field vectors. The electric and magnetic fields of a uniform plane wave are truly perpendicular, with a clear direction of propagation orthogonal to the electric and magnetic field directions. The direction of propagation of a uniform plane wave is known as the Poynting vector. Objects within the near-field range can affect antennas via loading or other complex coupling dynamics. Antenna loading occurs when an inductive or capacitive element induces a change of behavior in an antenna, by design or otherwise. This phenomenon is used in antenna design to influence the antenna pattern, gain, directivity, efficiency, bandwidth, and other specifications. Far-Field vs. Near-Field Considerations Most antennas are specified for their far-field performance, though specialized probes and certain types of near-field antennas are often used to test antenna systems. There are no clear boundaries between the antenna field regions, and a variety of different models are used to estimate or predict the transitions between these regions. However, beyond the far-field boundary, electromagnetic energy can be considered a pure radiating plane wave, significantly reducing the complexity of mathematically analyzing an antenna. Antenna Performance Criteria The following are key performance criteria used to specify RF antennas: • Polarization • Radiation/antenna pattern • Directivity • Gain • Beamwidth • Effective area/aperture • Efficiency • Impedance • Voltage standing wave ratio (VSWR) • Return loss • Resonance • Bandwidth • Power handling • Environmental ruggedness • Feed interconnect