Antennas are transmitting or receiving devices for electromagnetic waves. As transmitting antennas, they convert the electric current into electromagnetic wave, thereby the RF power of the RF transmitter is converted into a free space wave, as receiving antennas they convert the free space wave into a line wave which is fed to the RF receiving section.
The radiation of an electromagnetic oscillation from an antenna is related to the field characteristic impedance of the free space, which is calculated from the permeability and permittivity of the free space and is 376 ohms at the radiation frequency. The value for the field characteristic impedance of 376 ohms can also be calculated using the impedance of 120 ohms multiplied by 'pi'. It is thus lower than the characteristic impedance of the antenna.
Optimum matching is achieved by the size and shape of the antenna. One condition for the detachment of the electromagnetic waves from the antenna is that the antenna length has at least one tenth of the size of the wavelength to be radiated, another that the characteristic impedance of the antenna is greater than the field characteristic impedance.
The antenna principle
Antennas are based on the resonance principle and are formed by an open resonant circuit. This means that the antenna must be matched to the receiving frequency and the size of the resonant part therefore depends directly on the wavelength or a fixed fraction of the wavelength (`lambda`). Thus antennas can be tuned to lambda, lambda-half, lambda-quarter or lambda-eighth. Using the example of a GSM cell phone, the center frequency of the GSM band of 925 MHz results in a wavelength of 35 cm. This results in antenna lengths of 35 cm, 17.5 cm, 8.75 cm or, in the case of lambda eighths, 4.37 cm.
Important antenna characteristics are the antenna gain, the antenna bandwidth and antenna impedance, the aperture angle, the return loss and the radiation pattern, which has a direct influence on the design. According to their directivity, antennas can be classified as directional and omnidirectional. Omnidirectional antennas include omnidirectional antennas, discone antennas and rod antennas, while directional antennas include dipoles, log-periodic antennas, Yagi antennas and parabolic antennas.
Omnidirectional anten nas transmit circularly, they are usually designed for indoor or outdoor use and radiate the building in all directions. The range can be up to 600 m. They are usually used for multipoint connections and in mobile devices. They include the rod antennas, WLAN antennas, ground plane anten nas and omnidirectional antennas.
Directional antennas, such as the Yagi antenna or the parabolic antenna, are used for point-to-point outdoor connections and can span several kilometers, depending on the power class. They are used for cross-campus connections and also in directional radio.
In terms of design, antennas are divided into rod antennas, flat antennas, loop antennas, ferrite antennas, omnidirectional antennas, parabolic antennas, dipole antennas, panel antennas, Yagi antennas, 3D antennas and horn antennas. There are also shape-flexible antennas such as the spray-on antenna and combination antennas such as the foil antenna. The combi antenna combines different types of antennas so that, for example, one does not need several antennas in the car for broadcasting, digital radio, the GPS system and mobile radio. For mobile devices, there are special antennas such as the patch antenna or the PIFA antenna, which also cover several frequency bands, duplex antennas for radio and mobile radio, and triplex antennas that additionally include the GPS system.
In addition to the passive antennas mentioned, there are also intelligent antennas, smart antennas, and antenna arrays such as the phased array antennas. Smart antenna concepts are being developed in order to make better use of scarce frequency resources and thereby increase the radio capacity of a radio cell. The basic principle of smart antennas is based on controlled radiation of electromagnetic waves. This ensures that the radio reception area to be covered is irradiated with optimum field strength. A distinction is made between various techniques with fixed radiation characteristics, switched fixed radiation and dynamic radiation.