Driven element
Encyclopedia
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 current
s, 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 resonator
s 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 station
s often consist of several mast radiator
s, each of which functions as a half-wave monopole
driven element. They are driven out of phase to create a desired radiation pattern. 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. The advantage of this over an antenna such as a Yagi, which is only designed for one frequency, is that the antenna can be made relatively broadband
(nearly 2:1), by exciting differently sized elements that are resonant at different frequencies. 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. Phased array
s may have hundreds of driven elements.
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
).
Antenna array
Antenna array may refer to:*Antenna array a group of isotropic radiators such that the currents running through them are of different amplitudes and phases*Interferometric array of radio telescopes used in radio astronomy...
(such as a Yagi-Uda antenna)
Yagi antenna
A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element and additional parasitic elements...
, 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
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
. In a transmitting antenna it is driven or excited by the RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
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 current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
s, 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
Feed line
In a radio antenna, the feed line is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitter, it feeds the radio frequency current from the transmitter to the antenna, where it is radiated as radio waves. In a receiver it transfers...
, and a number of other elements which are not driven, called parasitic elements. The driven element is often a dipole
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
. The parasitic elements act as resonator
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
s and couple electromagnetically with the driven element, and serve to modify the radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
of the antenna, directing the radio waves in one direction, increasing the gain
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
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 station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
s often consist of several mast radiator
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...
s, each of which functions as a half-wave monopole
Monopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
driven element. They are driven out of phase to create a desired radiation pattern. A log-periodic antenna
Log-periodic antenna
In telecommunication, a log-periodic antenna is a broadband, multi-element, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency...
(LPDA) consists of many dipole
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
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
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the signal. The advantage of this over an antenna such as a Yagi, which is only designed for one frequency, is that the antenna can be made relatively broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
(nearly 2:1), by exciting differently sized elements that are resonant at different frequencies. A two-element array with the elements spaced a quarter wavelength apart has a distinct cardioid
Cardioid
A cardioid is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It is therefore a type of limaçon and can also be defined as an epicycloid having a single cusp...
radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
when the second element is driven with a source -90° out of phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
relative to the first element. Phased array
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
s may have hundreds of driven elements.
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
Passive radiator
In a radio antenna, a passive radiator or parasitic element is a conductive element, typically a metal rod, which is not electrically connected to anything else. Multielement antennas such as the Yagi-Uda antenna typically consist of a "driven element" which is connected to the radio receiver...
) and that the array is tightly coupled (spacing far below a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
).