How does a Yagi-Uda antenna work

Yagi antenna

A yagi antenna is an antenna for terrestrially broadcast VHF 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 influence the radiation characteristics of the antenna, which is given a more club-shaped expression by the directors. They lie 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 is immediately in front of the dipole. Its width and that of the other directors depend on the reception frequency, but they are always narrower than the dipole. The resulting opening angle is between 20 ° and 25 °.

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 on the dipole. For Yagi antennas, this is 5 dBi to 15 dBi, the latter value being achieved with over ten directors. In addition, the antenna gain can be increased by further constructive measures. At the same time, the rear reflectors increase the back attenuation and prevent radiation from the rear, which can arise, for example, from reflections on mountains or buildings and which 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. The reception 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 receiving frequency of 200 MHz, the wavelength is 1.5 m and thus a dipole width of 75 cm.

The bandwidth of Yagi antennas is relatively narrow and is around 10% of the reception frequency. With a reception frequency of 200 MHz, the reception range is between 190 MHz and 210 MHz.

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 with one another in a cross shape, in such a way that the two systems are at right angles to one another. This construction is a crossed Yagi antenna that is switched via a coaxial relay and can thus receive vertically or horizontally polarized radio waves. For the reception of circularly polarized radio waves, there is also the X-Yagi, which also combines two planes of polarization. Since the antenna elements of the two antennas are at right angles to each other, they are also referred to as orthogonal antennas.