Magnetic antenna
The magnetic antenna , also magnetic loop or Magloop used other than as a dipole , primarily the magnetic component of the electromagnetic field for generating the electromagnetic waves. In the far field , the fields of both antenna forms cannot be distinguished. The magnetic antennas include, for. B. Ferrite antenna and loop antenna .
A magnetic antenna in the narrower sense - a loop antenna - consists of one, less often a few turns with the largest possible coil area, and u. U. a capacitor. Without a capacitor, such an antenna is very broadband; with a capacitor it becomes an oscillating circuit and is very narrow-band.
construction
The magnetic antenna consists of a loop that forms a coil .
Magnetic antennas are preferably used in the frequency range below 30 MHz because their significantly smaller dimensions compared to a dipole make them appear attractive despite their limited efficiency. On the receiving side, the efficiency in this frequency range does not play a major role anyway, because the noise temperature of the atmosphere leads to output voltages that are far above the receiver noise .
A magnetic antenna is well suited as a receiving antenna inside rooms because the magnetic fields are usually significantly less disturbed than the electric ones. This applies both to radiation from power supply cables and to attenuation by building materials.
The circumference of a loop antenna must be less than 1/4 of the minimum wavelength so that the antenna can even be tuned with a capacitor (λ / 4 resonance). The limited size of the antenna leads to a very low radiation resistance, which has to be compensated for with a very high quality factor of the resonant circuit generated in this way. The resonance increase associated with the quality factor leads to very high currents and voltages in a magnetic antenna, which enforces the construction of a tube and a plate capacitor with a large plate spacing. Even at low transmission powers (e.g. 10 W), high currents and very high voltages occur in the capacitor.
The best results are obtained when the conductor is as short as possible and encloses the largest possible area. Therefore the loop should be as circular as possible. For structural reasons, however, rectangular and square shapes as well as five, six and other polygons are used.
In commercially manufactured transmitting antennas, aluminum tubes are preferably used as the material of the loop. Of course, other good electrical conductors are also suitable for self-construction, e.g. B. copper pipes from the installation area, common. It is important that the surface of the conductor is as large as possible, because the skin effect means that only the outermost layer of the conductor makes a significant contribution to conductivity. In some cases, however, square and flat profiles as well as the outer conductors of coaxial cables are also used . The capacitors usually used are plate or tube capacitors , which should ideally be welded or soldered directly to the loop. The high frequency (HF) is coupled in and out by means of a coupling loop or gamma match. The gamma match is a tap on the loop or coil. This must be set so that an input or output impedance of, for example, 50 ohms is achieved.
Mode of action and properties
The magnetic component outweighs the electrical component, the more the smaller the circumference of the antenna is compared to the wavelength (λ). For example, magnetic antennas of 0.3 to 0.1 λ are also referred to as electromagnetic antennas, and magnetic antennas for sizes <0.1 λ. Magnetic antennas enable a very compact and space-saving design, but the smaller the antennas compared to λ, their efficiency also decreases. As a result, they are usually not suitable for broadcast purposes. This type of antenna is very selective and works like a preselector , with the advantage that the signal-to-noise ratio is more favorable and the receivers are less overwhelmed by strong neighboring signals. Even when transmitting, it significantly reduces the interference from electronic devices in the neighborhood, which are often susceptible to this due to technical and structural inadequacies. Because of the stronger magnetic component, the magnetic antenna is less sensitive to environmental influences from walls (with the exception of reinforced concrete), trees and the like. A disadvantage is usually mentioned that with every frequency change the antenna has to be readjusted and that, especially with small designs, the efficiency becomes noticeably worse.
When mounted vertically, magnetic antennas have a vertical plane of polarization and also a clear bidirectional directivity. On the one hand, this can be used to determine the position and mask out interfering signals; on the other hand, the problem arises that the antenna must always be correctly aligned (by rotation) to ensure sufficient reception field strength. It is also disadvantageous here that, in the case of transmission, part of the transmission energy is radiated into the ground and vertically upwards. When mounted horizontally, it represents an omnidirectional radiator with, depending on the mounting height, ground conductivity and the environment, a tendency towards flat radiation, which, assuming low-loss construction, also enables good long-distance connections (DX).
Selective magnetic antennas are quite easy to adapt to coaxial cables . Usually a coupling coil is used for this. Broadband magnetic antennas should be terminated with as low an impedance as possible in order to obtain as flat a frequency response as possible. In measuring antennas, transformers are used for this. If good reception performance is important, amplifiers are often used directly on the antenna.
application
Magnetic antennas have long been known in radio technology and were used at least earlier for short-wave radio on ships and aircraft in direction finding systems and for air traffic control . Today they are used for special cases in amateur radio , as well as for communication with submerged submarines and for RFID applications, whereby their directional effect is also partially used by a rotatable assembly. They are also used in speleology for communication and for sensor data, such as in the Cave-Link system .
literature
- Alois Krischke and Karl Rothammel †: Rothammels Antennenbuch. 13th updated and expanded edition 2013. DARC- Verlag Baunatal, ISBN 978-3-88692-065-5
Web links
Individual evidence
- ↑ Description of the DL8NDG loop. Retrieved August 3, 2019 .
- ↑ Archived copy ( Memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
