Loop antenna

The magnetic antenna, and magnetic loop, uses other than as a dipole primarily the magnetic component of the electromagnetic field. In the far field the fields of both antenna types are indistinguishable. The magnetic antennas include, for example ferrite antenna and loop antenna.

A magnetic antenna in the narrow sense - a loop antenna - consists of one, rarely a few turns with the greatest possible surface coils, and possibly a capacitor. Without capacitor such an antenna is very broad band; with capacitor it becomes a resonant circuit and a very narrow band.

Construction

The magnetic antenna comprises a loop which forms a coil.

Magnetic antennas are preferably used in the frequency range below 30 MHz because compared to a dipole they leave their much smaller dimensions seem attractive, despite the limited efficiency. At the receiving end plays the efficiency in this frequency range anyway not a big role, because the noise temperature of the atmosphere leads to output voltages that are well above the receiver noise.

A magnetic antenna is well suited as a receiving antenna indoors, because the magnetic fields are usually significantly less disturbed than the electrostatic. This applies both to the radiation from power cables as well as for the attenuation by building materials.

The extent of the loop antenna must be less than 1/4 to be the minimum wavelength so that the antenna at all with a tunable capacitor ( λ/4-Resonanz ). As the limited size of the antenna leads to very low radiation resistance, which must be compensated with a very high quality factor of the resonant circuit thus formed. The resonant peak associated with the quality factor leads to very high currents and voltages in a magnetic antenna, which forces the design of a tube and a plate capacitor with a large plate spacing. Even with small transmission powers (eg 10 W ) occur in high currents and capacitor very high voltages.

The best results provides the shortest possible conductor includes a wide area as possible. Therefore, the loop should be as circular. For structural reasons, but also rectangular and square shape, as well as five-, six -, and other polygons are used.

As the material of the loop preferably aluminum tubes are used in commercially prepared transmit antennas. Of course, for DIY and other good electrical conductors, such as copper pipes from the installation area, as usual. It is important to maximize the surface area of the conductor, because significantly contributes only the outermost layer of the conductor to conductivity by the skin effect. Find use but also partly square and flat profiles, as well as the outer conductor of coaxial cables. When capacitors are usually plate or tubular capacitors for use, which are welded most directly with the loop or should be soldered. The input and output coupling of radio frequency (RF ) by means of a coupling loop or gamma match. The gamma match is a tap on the loop or coil. This is set so that an input or output impedance of 50 ohms, for example, is achieved.

Performance and Characteristics

The magnetic component predominates over the electrical component the more, the smaller the size of the antenna compared to the wavelength ( λ ) is. Thus one speaks in magnetic antenna 0.3 to 0.1 λ by an electromagnetic antenna and circumferences < 0.1 λ of magnetic antennas. Magnetic antennas allow a very compact and space-saving design, however, increases with the reduction of the antennas with respect to λ from their efficiency. Thus they are usually not suitable for broadcast purposes. This type of antenna is very selective and acts as a preselector, with the advantage that the signal -to-noise ratio is favorable and the receiver will be less overwhelmed with strong adjacent signals. Also in the case of transmission, it significantly reduced or interference with electronic devices in the neighborhood, which often show up because of technical and constructive shortcomings of this vulnerable. Because of the stronger magnetic component, the magnetic antenna is less sensitive to environmental influences through walls (except concrete ), trees and the like. As a drawback is usually called, that the antenna must be tuned to each frequency change, and that, especially for small designs, the efficiency is noticeably worse.

If mounted vertically, have magnetic antenna on a vertical plane of polarization and also a significant two-way directivity. This can on the one hand make it usable for position determination and suppression of interfering signals, on the other hand, the problem arises that one must always ensure the correct orientation of the antenna for receiving a sufficient field strength ( by rotation). The disadvantage here is that in the case of transmission, a part of the transmitted energy is radiated into the ground and straight up. In horizontal installations, it represents an omnidirectional antenna with, depending on installation height, ground conductivity and the surrounding area, tend to flatter radiation group, provided the low-loss design, also good long distance connections (DX) allows.

Selective magnetic antennas are quite easy to adapt to coaxial cable. Most for a coupling coil is used. Broadband magnetic antennas should be completed as a low impedance to obtain the flattest possible frequency response. When measuring antennas are used for transformers. Does it depend on good reception services, are often used amplifier directly to the antenna.

Application

Magnetic antennas have long been known in the radio art and have been used previously for at least the short-wave radio to ships and aircraft in Peilreinrichtungen and air traffic control. Today they are used for special cases in amateur radio, and to communicate with submerged submarines and for RFID applications, partially with the indicative effect was further exploited by a rotatable assembly and will.