Yagi-Uda antenna
A Yagi -Uda antenna ( common name and Yagi antenna ) is a directional antenna for receiving or transmitting electromagnetic waves in the range of about 10 MHz to about 2500 MHz. It consists of a front -fed dipole, a number of directors and usually a reflector behind the dipole.
History
The Yagi -Uda antenna has been developed since 1924 by the Japanese Hidetsugu Yagi and Shintaro Uda. In 1926 she published the first description in a Japanese magazine. In June 1928 an English article by Yagi was released in the U.S., so that the antenna in our language area usually wore only Hidetsugu Yagi's name. The basic patents mention just a Yagi as an inventor. The correct name Yagi -Uda antenna is rarely used.
Construction
Characteristic of the Yagi -Uda antenna is a dipole directivity receives through at least one or a number of correspondingly arranged directors and possibly reflectors.
The dipole ( 1), carried out in the picture on the right as folded dipole, via the feed line ( 4) is fed. The dipole has a length of a half wavelength ( λ / 2), based on the propagation speed. It is slightly less than λ / 2 in vacuo. Directors (3) are slightly shorter, the reflectors ( 2 ) is slightly longer than the dipole. Dipole, reflector and directors are not electrically coupled. They can be mounted on a common conductive base bar, because in the middle of the elements are voltage node. The distance from the reflector ( s) and dipole is typically about 0.15 λ, dipole and first director about 0.1 λ; this can be improved at the cost of bandwidth or vice-versa through a suitable choice of the length and spacing of the element gain.
Properties
The total length of the antenna determines the gain and directivity. The appropriate choice of the parameters length, spacing, diameter and number of directors optimizes the antenna characteristics.
A three-element Yagi -Uda antenna of length 0.3λ provides an antenna gain of 4-8 dBd. A length of 4λ (15-30 directors) amplifies with 15 dBi, and reaches an opening angle less than 40 °. The practical upper limit is 20 dBi.
Operation
The active dipole excites the parasitic elements, that is, the reflector and directors. The parasitic elements also act as a spotlight, the phase with the active dipole send however. By deviating from the dipole (resonant ) Length arise inductive or capacitive behavior, with appropriate phase shifts of the currents in the elements. The phase shift is not only influenced by the length of the member to position on the side member is determined ( the so-called boom ), but also by the length of the element. The vibration of the pathogen is passed from director to director. The delay from director to director presents itself as a phase velocity that about 0.7 to 0.98 times the speed of light reached depends on the ratio of the geometrical antenna length to the wavelength.
The far field of the Yagi arises ultimately from the direction-dependent and phase-dependent superposition of the radiation components of all elements of the Yagi -Uda antenna. In the direction of the boom (forward) superimposed on the field shares constructively (in addition ), right and left of the overlay is, however, with increasing angle rapidly destructive ( destructively ) which results in the desired preferred direction.
In the relevant literature, there are several indications for an "optimal design ", suggest which different gradations of element lengths and distances. Experiments have demonstrated here that small continuous lengths and spacing variations lead to higher profits than with simple addition " short " antennas with other elements of the same length and the same distance. This was traceable spreads to the consideration of the Yagi - Uda as " waveguide structure " on which (starting from the fed dipole ) a traveling wave in the direction of the antenna tip. The element variation is to make a smoother transition to the open space here, according to theory.
Area of application
Long Yagi -Uda antenna for the 70 cm amateur radio band.
Two Yagi -Uda antennas for television reception house (center). The upper reflector of the Yagi antenna is designed as a grid.
Antenna array of six Yagi -Uda antenna with cross dipole (2 m/137 MHz) for communicating with satellites.
Yagi antenna array for Earth-Moon - Earth connections
Yagi -Uda antennas can also be used at very high frequencies, such as for receiving Meteosat ( 1690 MHz). There antennas come with up to 30 elements and antenna gains of up to 16 dBd for use. However, above about 2 GHz, the desired radiation characteristics can be better achieved with other types of antennas, such as horns.
Amateur radio operators often use a rotatable Yagi - Uda of three or more elements in the wave ranges from 0.1 m to 20 m. Yagi -Uda antennas for longer wavelengths are rare, because the elements then become very large and heavy. In addition, such antennas require because of the ground effect a construction height of at least one-half wavelength. Furthermore, alternative designs such as the Hexbeam used.