Ferrite rod antenna
A ferrite rod antenna or ferrite antenna ( English loop stick antenna ) refers to a magnetic antenna in which a coil made of insulated jumper wire or high-frequency braid is attached to a ferrite rod . The antenna is suitable for the reception of long , long ; Medium waves or, if necessary, also short waves and, for special applications in telecommunications, occasionally also with ultra-short waves .
Structure and wiring
With a parallel- connected variable capacitor , the coil forms an oscillating circuit which is brought into resonance when it is tuned to a transmission signal . Capacitance diodes can also be used for coordination . For the basic adjustment at the factory, the coil is moved on the ferrite core and then fixed with adhesive. The high-frequency amplifier input of the radio device can be connected directly to this resonant circuit or it is inductively coupled to the ferrite rod via another coil. The latter solution is advantageous for low-impedance amplifier inputs. In the case of a feedback audion (reception principle of simple tube radios), a feedback coil can also be attached to the ferrite rod. Depending on the length, a ferrite rod can also carry several voice coils for different reception bands (long, medium and short wave).
As an alternative to ferrite as the material for the antenna core, other soft magnetic materials are also suitable. Depending on the operating frequency and the required quality , laminated rod cores made from magnetically soft metal strips can be used. These antenna cores have the advantage of high mechanical strength without the tendency to break known from ferrite. Amorphous metal strips are particularly suitable, with a thickness of typically 0.020 mm and special alloys with the lowest magnetostriction . Such antennas can be "bent" without losing the antenna properties. However, the use of these antennas is restricted to special areas because of the higher costs for the core.
properties
Ferrite antennas have a polarization and directional effect:
- If the end of a ferrite rod points towards the transmitter, the reception field strength is minimal.
- If it forms an angle of 90 degrees with the direction towards the transmitter and the usually vertical transmitter mast, the field strength is maximum. The reason lies in the course of the magnetic field lines of a vertical transmission mast: They leave it as horizontal rings. If the transmitting antenna is suspended horizontally, the ferrite rod must point in a vertical direction.
This effect is also used for direction finding with Peilempfängern used ( minimum Sight ), but these are two bearings from different places needed to determine which end is the ferrite antenna to the transmitter.
Ferrite antennas are often built into the receiver so that you sometimes have to rotate the entire device for optimal reception. Tube radios, however, often had a manual rotating device for the built-in ferrite antenna.
Ferrite rod antennas largely replaced the previously common loop antennas . The advantage is that ferrite rod antennas only take up about 1/20 of the space required by a loop antenna with comparable reception powers.
application
Ferrite antennas are a very good solution for the reception of radio waves with frequencies below 2 MHz, because their directional sensitivity, the exclusive absorption of magnetic fields and the frequency selectivity make them less sensitive to interference than rod or wire antennas. They can also be accommodated within the device housing without any problems - this must, however, be made of insulation material. Today, ferrite antennas are used in most radios - except car radios - to receive long and medium waves. They can also be found in radio clocks and RFID systems. Another area of application are professional pager systems (radio pagers, e.g. fire brigade ) that work on VHF frequencies and require the use of particularly small antennas. Because of their small dimensions and wire cross-sections and because of the hysteresis losses of the ferrite material, ferrite antennas are less efficient than loop antennas and are therefore only suitable to a limited extent as transmission antennas and only for low transmission powers. Ferrite antennas are very well suited for applications in the long and long wave range, such as for receiving weather and geophysical sferics , Schumann resonances and atmospheric impulse radiation . The advantage of a ferrite antenna lies in the possibility of interference radiation suppression.
Web links
literature
- Karl Rothammel u. Alois Krischke, Rothammels Antennenbuch , Franckh-Kosmos Verlag Stuttgart, 11th edition 1995, ISBN 3-440-07018-2
- Wolfgang Friese, Extraordinary receiving antennas and their adaptation for the long-wave to short-wave range