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Introduction to Antennas — purpose, basic operation, and design goals

An antenna is an electromagnetic transducer: it converts electrical signals on transmission lines or circuits into free-space electromagnetic (EM) waves (transmit mode) and converts incident EM waves back into electrical signals (receive mode). In engineering terms an antenna provides control over where EM energy is sent or received (the radiation pattern), how efficiently power is transferred between a circuit and free space (impedance match and loss), and how the electromagnetic field is polarized and spectrally distributed (bandwidth and polarization). Key design goals include maximizing useful radiated power (efficiency), meeting a desired directional pattern or coverage (gain/directivity and beamwidth), operating across required frequencies (bandwidth), ensuring proper polarization, maintaining a match to the feeding network (low VSWR / good S11), and meeting size, cost and environmental constraints. Practical examples to illustrate these ideas: 1) A 2.4 GHz IoT sensor often uses a quarter-wave monopole (length ~31.25 mm since wavelength at 2.4 GHz is 125 mm) to get an omnidirectional pattern so devices nearby can communicate regardless of orientation. 2) A parabolic dish used for satellite TV concentrates energy into aAntenna introduction and functions very narrow beam (high gain) so it can communicate with a geostationary satellite tens of thousands of kilometers away; circular polarization is commonly used to avoid signal fade caused by orientation. 3) A smartphone uses compact patch or PIFA elements arranged for multi-band coverage and MIMO (multiple antennas) so it can handle cellular, Wi-Fi and GPS simultaneously and exploit spatial multiplexing. 4) A phased array radar uses many small radiating elements and digital/analog phase control to steer a high-gain beam electronically without moving parts. Some useful relations (concise): Friis transmission equation for far-field line-of-sight links: Pr = Pt·Gt·Gr·(λ/(4πR))^2, where Pt is transmit power, Gt/Gr are linear gains, λ is wavelength, and R is range. Effective aperture of a receiving antenna: Ae = (G·λ^2)/(4π). A classic practical constant: a half-wave dipole has a radiation resistance of about 73 Ω and a broadside gain about 2.15 dBi. Reciprocity theorem: the transmit and receive patterns of a passive antenna are identical in the same environment. These relations guide design tradeoffs between size, frequency, gain and coverage.