Antennas are often overlooked in a system or product design. But they are the beginning of the circuit, for a receiver, and the end of the circuit, for a transmitter, and generally set the guidelines for the performance needed by other components in a system, such as a low-noise amplifier (LNA) for the receiver and a power amplifier (PA) for the transmitter. An antenna's basic functions are simple; implementing those functions in a compact mechanical design can often be challenging, however. An antenna can be thought of as an electronic system's interface to the outside world: it radiates and receives signals generated and processed by the other components in the system. Because of the complex modulation on many modern signals, the antenna should handle those signals with as little distortion as possible, although even the best-designed antennas are subject to variations from the "ideal" design as a result of manufacturing tolerances.
An external electromagnetic (EM) field is capable of inducing current flow in almost any form of metallic or conducting structure. Similarly, that structure can also carry current to radiate an EM field. Many different structures have been investigated as antennas over the years, from simple wires to elaborate meandering patterns on printed-circuit boards (PCBs). The goal for any given antenna design is to make the current/EM-field conversion with as much gain and efficiency as possible, over the required bandwidth, and in the mechanical size and format required by the application. Other key antenna parameters include how well its impedance matches that of its system, measured in terms of its voltage standing wave ratio (VSWR), the type of radiation patterns it produces, its polarization, and its powerhandling capability. Modern antenna designs come in many forms, from chip-sized designs fabricated on low-temperature-co-fired-ceramic (LTCC) substrates to waveguide horns and enormous phased-array systems comprised of thousands of separate antenna elements.
By its electrical length, an antenna is tuned for a specific resonant or center frequency, with some percentage of bandwidth around that resonant frequency. It may also be used at harmonics, or frequencies that are some fraction of the wavelength of the nominal resonant frequency. An antenna may be designed with more than one resonant frequency. With more functions being integrated into single electronic devices, a current trend in antenna designs is to create products that can handle multiple bands and/or polarizations simultaneously (see figure).
The gain of an antenna will vary across its operating bandwidth, usually peaking at each resonant frequency. An antenna's gain is typically referenced to the gain of a known reference antenna, such as an isotropic or dipole antenna. An isotropic radiator, such as might be used in an antenna test chamber, is a theoretical point source that radiates EM energy uniformly in all directions. A dipole, among the simplest of antennas, is basically a straight piece of wire cut in the middle and fed at that point by a transmission line. For example, an antenna having the same gain as that ideal isotropic radiator, is said to have 0 dBi gain. If it has more gain than the reference, the number is positive; less gain than the reference, and the number is negative. Similarly, an antenna having several decibels less gain than a reference dipole antenna is said to have -2 dBd gain.