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yagi antenna

A Yagi antenna is an antenna for terrestrial broadcast FM radio and television. The Yagi antenna is a directional antenna, it has a dipole and one or more directors and one or more reflectors

. Yagi antennas respond to the vertically polarized electric field. The directors affect the radiation pattern of the antenna, which takes on a more lobe-like appearance due to the directors. They are located at a distance of 0.10, 0.15 or 0.20 of the wavelength lambda in front of the dipole and taper in width. The widest director lies immediately in front of the dipole. Its width and that of the other directors depends on the receiving frequency, but they are always narrower than the dipole. The resulting aperture angle

is between 20° and 25°.

Yagi antenna with multiple directors and reflectors

Yagi antenna with multiple directors and reflectors

The reflector or reflectors are located behind the dipole, they are about 5% longer than the dipole and increase the antenna gain by reflecting the incident waves onto the dipole, which is between 5 dBi and 15 dBi for Yagi antennas, the latter value being achieved with more than ten directors. In addition, the antenna gain can certainly be increased by further constructive measures. At the same time, the rear reflectors increase the return loss and prevent rear radiation, which can be caused by reflections from mountains or buildings, for example, and are the cause of ghost images

. The reflectors are wider than the dipole and are located at a distance of 0.20 x lambda behind the dipole.

Dipole with reflector and director

Dipole with reflector and director

The receiving frequency range of Yagi antennas is between about 3 MHz and 3 GHz, with the frequency range

below 1.5 GHz being the most suitable. The dipole itself is tuned to half the wavelength. At a receive frequency of 200 MHz, this results in a wavelength of 1.5 m and thus a dipole width of 75 cm.The bandwidth of

Yagi antennas is relatively narrow and is about 10% of the receive frequency. At a reception frequency of 200 MHz, the reception range is thus between 190 MHz and 210 MHz.

X-Yagi antenna, photo: impextechnology.com

X-Yagi antenna, photo: impextechnology.com

Yagi antennas can receive radio waves with horizontal or vertical polarization, depending on the orientation of the dipole and the directors. Two Yagi antennas can also be combined in a cross shape, in such a way that the two systems are perpendicular to each other. This design is a cross Yagi antenna that can be switched by a coaxial relay to receive vertically or horizontally polarized radio waves. For receiving circularly polarized radio waves, there is also the X-Yagi, which also combines two polarization planes. Since the antenna elements of the two antennas are at right angles to each other, they are also referred to as orthogonal antennas.

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Englisch: yagi antenna
Updated at: 30.12.2014
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