Passive radiator
Encyclopedia
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 or transmitter
through a feed line
, and parasitic elements, which are not. The purpose of the parasitic elements is to modify the radiation pattern
of the radio waves emitted by the driven element, directing them in a beam in one direction, increasing the antenna's directivity
(gain
). A parasitic element does this by acting as a passive resonator
, something like a guitar's sound box, absorbing the radio waves from the nearby driven element and re-radiating them again with a different phase
. The waves from the different antenna elements interfere, strengthening the antenna's radiation in the desired direction, and cancelling out the waves in undesired directions.
The parasitic elements in a Yagi antenna are mounted parallel to the driven element, with all the elements usually in a line perpendicular to the direction of radiation of the antenna. What effect a parasitic element has on the radiation pattern depends both on its separation from the next element, and on its length. The driven element of the antenna is usually a half-wave dipole
, its length half a wavelength
of the radio waves used. The parasitic elements are of two types. A "reflector" is slightly longer (around 5%) than a half-wavelength. It serves to reflect the radio waves in the opposite direction. A "director" is slightly shorter than a half-wavelength; it serves to increase the radiation in a given direction. A Yagi antenna may have a reflector on one side of the driven element, and one or more directors on the other side. If all the elements are in a plane, usually only one reflector is used, because additional ones give little improvement in gain, but sometimes additional reflectors are mounted above and below the plane of the antenna on a vertical bracket at the end.
All the elements are usually mounted on a metal beam or bracket along the antenna's central axis. Although sometimes the parasitic elements are insulated from the supporting beam, often they are clamped or welded directly to it, electrically connected to it. This doesn't affect their functioning, because the RF voltage distribution along the element is maximum at the ends and goes to zero (has a node
) at the midpoint where the grounded beam is attached.
The addition of parasitic elements gives a diminishing improvement in the antenna's gain. Adding a reflector to a dipole, to make a 2-element Yagi, increases the gain by about 5 dB
over the dipole. Adding a director to this, to give a 3-element Yagi, gives a gain of about 7 dB over a dipole. As a rule of thumb, each additional parasitic element beyond this adds about 1 dB of gain.
In an example of a parasitic element that is not rod-shaped, a parasitic microstrip patch antenna
is sometimes mounted above another driven patch antenna. This antenna combination resonates at a slightly lower frequency than the original element. However, the main effect is to greatly increase the impedance bandwidth of the antenna. In some cases the bandwidth can be increased by a factor of 10.
Not all types of thin conductor multielement antennas have parasitic elements. The log periodic antenna is similar in appearance to a Yagi, but all of its elements are driven elements, connected to the transmitter or receiver.
Driven element
In a multielement antenna array , the driven element or active element is the element in the antenna 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...
" which is connected to the radio 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...
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 parasitic elements, which are not. The purpose of the parasitic elements is 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 radio waves emitted by the driven element, directing them in a beam in one direction, increasing the antenna's directivity
Directivity
In electromagnetics, directivity is a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator radiating the same total power.An antenna's directivity is a...
(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...
). A parasitic element does this by acting as a passive 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...
, something like a guitar's sound box, absorbing the radio waves from the nearby driven element and re-radiating them again with a different 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:...
. The waves from the different antenna elements interfere, strengthening the antenna's radiation in the desired direction, and cancelling out the waves in undesired directions.
The parasitic elements in a Yagi antenna are mounted parallel to the driven element, with all the elements usually in a line perpendicular to the direction of radiation of the antenna. What effect a parasitic element has on the radiation pattern depends both on its separation from the next element, and on its length. The driven element of the antenna is usually a half-wave 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...
, its length half 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...
of the radio waves used. The parasitic elements are of two types. A "reflector" is slightly longer (around 5%) than a half-wavelength. It serves to reflect the radio waves in the opposite direction. A "director" is slightly shorter than a half-wavelength; it serves to increase the radiation in a given direction. A Yagi antenna may have a reflector on one side of the driven element, and one or more directors on the other side. If all the elements are in a plane, usually only one reflector is used, because additional ones give little improvement in gain, but sometimes additional reflectors are mounted above and below the plane of the antenna on a vertical bracket at the end.
All the elements are usually mounted on a metal beam or bracket along the antenna's central axis. Although sometimes the parasitic elements are insulated from the supporting beam, often they are clamped or welded directly to it, electrically connected to it. This doesn't affect their functioning, because the RF voltage distribution along the element is maximum at the ends and goes to zero (has a node
Node (physics)
A node is a point along a standing wave where the wave has minimal amplitude. For instance, in a vibrating guitar string, the ends of the string are nodes. By changing the position of the end node through frets, the guitarist changes the effective length of the vibrating string and thereby the...
) at the midpoint where the grounded beam is attached.
The addition of parasitic elements gives a diminishing improvement in the antenna's gain. Adding a reflector to a dipole, to make a 2-element Yagi, increases the gain by about 5 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
over the dipole. Adding a director to this, to give a 3-element Yagi, gives a gain of about 7 dB over a dipole. As a rule of thumb, each additional parasitic element beyond this adds about 1 dB of gain.
In an example of a parasitic element that is not rod-shaped, a parasitic microstrip patch antenna
Patch antenna
A patch antenna is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat rectangular sheet or "patch" of metal, mounted over a larger sheet of metal called a ground plane. The assembly is usually contained inside a plastic radome, which...
is sometimes mounted above another driven patch antenna. This antenna combination resonates at a slightly lower frequency than the original element. However, the main effect is to greatly increase the impedance bandwidth of the antenna. In some cases the bandwidth can be increased by a factor of 10.