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4.3: Antenna Components and Definitions

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  • Antennas used for radio frequency communication are made from conducting wire elements. These elements may be classified as driven or parasitic [50]. All antennas have at least one driven element. In a transmitting antenna, power is supplied to the driven element. Current flowing through the antenna induces an electromagnetic field around the antenna. In a receiving antenna, the driven element is connected to the receiving circuitry. Some antennas also have parasitic elements. These elements affect the antenna's radiation pattern, but they are not connected to the power supply or receiving circuitry [50]. The electric field inside a perfect conductor is zero, so putting a good conductor near an antenna influences the antenna's radiation pattern. Parasitic elements may be included in the antenna to focus the electromagnetic field in a particular direction, alter the bandwidth of the antenna, or for other reasons. Antennas are often mounted on a metal rod for mechanical support, and this rod is called a boom.

    Antennas may be used individually or as part of an array. Arrays may also be driven or parasitic. In a driven array, all elements are connected to the power supply or receiving circuitry [50]. In a parasitic array, one or more of the elements are parasitic and not connected [50]. Arrays are also classified based on the direction of radiation compared to the axis of the array. In a broadside array, radiation is mostly perpendicular to the axis of the array while in an end fire array, radiation is mostly along the direction of the axis of the array [50].

    A transmission line is a pair of conductors which is used to transmit a signal and which is very long compared to the wavelength of the signal being sent. Communications engineers and power systems engineers both use the term transmission line, but they make different assumptions. To a communications engineer, it is a long pair of conductors over which a signal is sent. To a power systems engineer, it is a cable that is part of the power grid. The communications definition will be used in this text. The conductors of a transmission line may be a pair of parallel wires, they may be a waveguide formed by a pair of parallel plates, they may be a coax cable, or they may have another geometry. Coax cable is formed by a wire and cylindrical tube separated by an insulator, both with the same axis, so they are coaxial. For example, a coax cable connecting a transmitter operating at a frequency of \(f = 88 \text{ MHz}\) on the first floor of a building and an antenna on the top of the tenth floor of the building is a transmission line because the length of the cable is long compared to the wavelength of \(\lambda = 3.4 m\). As another example, a pair of wires connecting a transmitting circuit operating at \(f = 4 \text{ GHz}\) on one end of a printed circuit board and an antenna on the other end 25 cm away is also a transmission line because the length of the wires is long compared to the wavelength of \(\lambda = 7.5 cm\).

    Some antennas have a balun. Balun is a contraction for balanced/ unbalanced. It is used between balanced loads and unbalanced transmission lines [15, p. 406] [50]. A typical transmission line, made up of a coax cable, is constructed from an inner conductor and an outer conductor. These conductors have different radii, so they have different impedances. The transmission line is called unbalanced due to this impedance difference. Suppose that this transmission line is connected to a dipole antenna formed from two symmetric conductors. The impedance of the two arms of the dipole are equal, so we say that it is a balanced load. A balun can used in this type of situation when a balanced antenna is connected to an unbalanced transmission line. By properly choosing the impedance of a balun, reflections at the interface between the antenna and transmission line can be reduced so that more energy gets to or from the antenna and less remains stored in the transmission line.