Directional antenna
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A directional antenna is an antenna in which special constructive measures have been taken in order to additionally strengthen the directional effect of a single radiator due to the design.
A directional antenna can be both a transmitting antenna and a receiving antenna. As a transmitting antenna, it concentrates the transmitted energy in a desired direction and thereby generates directional radiation . As a receiving antenna, its maximum sensitivity is in a certain direction, for example to establish a radio link between two points. The directional effect attenuates the reception of interference signals whose sources are outside the main lobe .
Compared to an omnidirectional antenna, a directional antenna has a strongly anisotropic radiation pattern with a high directivity factor , with a narrow half- width of the opening angle and a high front / back ratio . Depending on the type of antenna, the opening angle can range from more than 30 ° to less than one degree. The strong bundling of the energy in a narrow beam causes a high antenna gain , which means that the transmission power can be kept low with the same field strength . As EIRP (equivalent isotropic radiated power) , the transmission power is called, would make you an isotropic radiator to the same range to achieve with the directional antenna as shown.
Directional antennas are used in all frequency bands. Their design and feasibility depends on the wave range , since the directional effect is dependent on the geometric dimensions of the antenna in relation to the wavelength . Directional antennas with only one pronounced preferred direction are usually used. In certain radio networks with linear coverage areas (so-called “line radio”), directional antennas are also used which have several main lobes which, for example, are oriented diametrically opposite. Dipole groups (without reflectors) or a dipole with two diametrically arranged waveguide structures (like the directors in a Yagi antenna ) are often used for this. These then show a so-called "Janus directional characteristic".
Examples
Below 30 MHz
For long and medium waves , arrangements of two or more self-radiating transmitter masts are used as directional antennas for transmitting. At least one mast is always fed with the transmission energy. The other masts can be fed (out of phase) or earthed (if necessary via tuning elements); these then serve as passive reflector or director masts. Furthermore, a trap antenna can be used for medium waves , in which one or more elements are fed out of phase. Ferrite or loop antennas are used as directionally sensitive receiving antennas in these frequency bands .
Dipole walls or log-periodic antennas ( LPDA ) are often used as direction-dependent antennas for short waves . Dipole antennas are also used.
Below 300 MHz
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 by radio amateurs and radio services. Dipole lines are often used as directional antennas for higher transmission power in this frequency range. By stacking them vertically ("bushed dipoles", dipole wall ), you get a horizontal fan that covers a large area. Dipoles are often configured as vertically polarized omnidirectional radiators. The term "directional antenna" can still apply, however, if the special constructive measures for improving the directional effect concern the vertical antenna diagram. The radiation is then as flat as possible, the side lobes are often asymmetrically distributed and increasingly directed downwards.
Over 1 GHz
For frequencies from approx. 150 MHz , helical antennas are already used; from approx. 1 GHz, parabolic antennas are used as direction-sensitive antennas for both transmission and reception. In the case of high quality requirements, however, shell antennas are more likely to be used, as they have better radiation characteristics compared to parabolic antennas. Horn antennas are rarely used today, as the shell antennas are smaller in size with approximately the same transmission properties. Group antennas with a strong directional effect, such as the patch antenna and the phased array antenna , are also used in this frequency range . Furthermore, the group of slot antennas used in ship radar systems , mostly rotatable and in a horizontal position, are directional antennas.
Frequencies from 10 GHz enable small, compact antennas with very good directivity. Examples of this are the parabolic antennas of satellite receiving systems. For frequencies from 92 to 98 GHz, it is possible to build highly bundling antennas for radar devices with high resolution in a radome the size of a rotating beacon. The so-called surface movement radars monitor traffic in advance at airports.
literature
- Alois Krischke: Rothammels Antennenbuch. 11th edition, Franckh-Kosmos-Verlags-GmbH, Stuttgart 1995, ISBN 3-440-07018-2
- Communication technology part 1 Basics of radio frequency. 1st edition, Institute for the Development of Modern Teaching Methods e. V., Bremen 1980
- Eberhard Spindlert: The great antenna book. 11th edition, Franzis-Verlag GmbH, Munich 1987, ISBN 3-7723-8761-6