Driven element

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
A Yagi antenna with one driven element (A) called a folded dipole, and 5 parasitic elements (B and C). A feed line leading to the receiver (not shown)) is attached to the driven element at D.

In a multielement antenna array (such as a Yagi-Uda antenna), the driven element or active element is the element in the antenna (typically a metal rod) which is electrically connected to the receiver or transmitter. In a transmitting antenna it is driven or excited by the RF current from the transmitter, and is the source of the radio waves. In a receiving antenna it collects the incoming radio waves for reception, and converts them to tiny oscillating electric currents, which are applied to the receiver. Multielement antennas like the Yagi typically consist of a driven element, connected to the receiver or transmitter through a feed line, and a number of other elements which are not driven, called parasitic elements. The driven element is often a dipole. The parasitic elements act as resonators and couple electromagnetically with the driven element, and serve to modify the radiation pattern of the antenna, directing the radio waves in one direction, increasing the gain of the antenna.

An antenna may have more than one driven element, although the most common multielement antenna, the Yagi, usually has only one. For example, transmitting antennas for AM radio stations often consist of several mast radiators, each of which functions as a half-wave monopole driven element, to create a particular radiation pattern. A two-element array with the elements spaced a quarter wavelength apart has a distinct cardioid radiation pattern when the second element is driven with a source -90° out of phase relative to the first element. A log-periodic antenna (LPDA) consists of many dipole elements of decreasing length, all of which are driven. However because they are different lengths, only one of the many dipoles is resonant at a given frequency, so only one is driven at a time. The dipole that is driven depends on the frequency of the signal. Phased arrays may have hundreds of driven elements. Household multiband television antennas generally consist of a hybrid between a UHF Yagi with one driven dipole and a log-periodic for VHF behind that with alternating active elements. The driven elements between the UHF and VHF are then coupled and often matched for a 75 ohm coaxial downlead to the receiver.

When a "driven element" is referred to in an antenna array, it is often assumed that other elements are not driven (i.e. parasitic, passive) and that the array is tightly coupled (spacing far below a wavelength).