Directional antenna

A directional antenna is an antenna in which special constructional measures have been taken in order to enhance a design-related effect of a single directional radiator addition. Thus, the directional antenna as a transmitting antenna focuses the transmitted energy, or as a receiving antenna for maximum sensitivity in a desired direction, for example to establish a radio link connection between two points. Due to the directivity of reception of interfering signals whose sources are outside the main lobe attenuated.

A directional antenna is better than a round spotlights a highly anisotropic radiation pattern with high directivity, narrow half-width of the opening angle and a high back ratio. Depending on the antenna design can be up to less than one degree, the opening angle of more than 30 °. The strong focusing the energy in a narrow beam gives a high antenna gain, whereby the transmit power for the same field strength can be kept low. As EIRP ( equivalent isotropic radiated power ), the transmission power designated, one would perform a isotropic radiator, in order to achieve the same range as the directional antenna.

Directional antennas are used in all frequency bands. Your design and feasibility depends on the wavelength range, since the directivity depends on the geometrical dimensions of the antenna in relation to the wavelength. Most directional antennas are used only with a strong preferred orientation. In certain radio networks with linear supply areas (so-called " line radio " ) can also use directional antennas that have multiple main lobes which are oriented in opposite directions, for example, diametrically. For this dipole groups are often used ( without reflectors ) or a dipole with two diametrically arranged Wellenleitstrukturen ( as the Directors at a Yagi antenna ). This will show a so-called "Janus - directional characteristics ".

Examples

Arrangements of two or more radiating transmitting masts for long and medium waves as a directional antenna used for transmission. Here, at least one mast to the transmit power is always fed. The other masts ( phase ) or fed (if over Abstimmglieder ) be grounded, then serve as a passive reflector or director masts. Furthermore, in the center shaft, a cage antenna in which one or more elements are fed out of phase may be used. A direction-sensitive receive antenna ferrite or loop antennas are used in these frequency bands.

For short waves Dipolwände or log-periodic antennas ( LPDA ) are often used as directional antennas. Also dipole antennas are used.

For frequencies in the VHF range, the Yagi antenna is the most widely used directional antenna for receiving signals. It is also used as a transmitting antenna of radio amateurs and radio services. When directional antennas for greater transmission power Dipolzeilen be used in this frequency range often. By vertical stacking ( " bush hammered dipoles " ) one reaches a horizontal compartments, which covers a large area. Frequently dipoles also be configured as a vertically polarized omnidirectional antenna. The term " directional antenna " but may still apply if the special design measures to improve the directivity related to the vertical antenna diagram. The radiation is then carried out as flat as possible, the side lobes are often distributed asymmetrically reinforced and directed downward.

Helical antennas already be for frequencies above 150 MHz used ( weather image reception ), from about 1 GHz parabolic antennas are used as direction-sensitive antenna for both transmission and reception. In high high demands but rather shell antennas are used, since they have better radiation characteristics compared to satellite dishes. Horn radiators are rarely used today because the shell antennas exhibit at roughly the same transmission characteristics of a smaller size. Furthermore, in this frequency range are array antennas with high directivity, such as the patch antenna and the phased array antenna is used.

Frequencies from 10 GHz allow small compact antennas with very good directivity. Examples are the parabolic antennas of satellite reception systems. For frequencies 92-98 GHz, it is possible to build highly directional antennas for radar devices with high resolution in a radome on the order of a flashing beacon. Such radars ( Surface Movement Radar ) monitor the traffic in advance at airports.