There are a number of ferrite rod parameters that are of key interest when considering their use in any radio circuit design.
The two main parameters are the Q of the tuned circuit, and the radiation resistance. These two ferrite rod parameters govern the areas in which they can be used and the design of the circuits in which they are used.
As with any electronic circuit design, the choice of the various parameters for the ferrite rod antenna may require some compromises, but generally a good level of performance can be achieved within the overall limits of this type of antenna.
Ferrite rod antenna Q parameter
The Q or quality factor of the overall circuit in which the ferrite rod antenna coil of maintained is important in any radio design. The circuit design should have a high Q at the frequencies over which the ferrite rod antenna operates if it is to operate efficiently.
At frequencies of a few hundred kilohertz, a medium permeability material would be used within the ferrite rod antenna and this would enable a Q of about 1000 to be obtained. With a Q of this value it will mean that the antenna circuit will need tuning if it is to operate over more than a single channel or frequency.
When used in a portable receiver, the tuning can be linked to the overall receiver tuning and indeed the ferrite rod antenna normally provides the input tuning for the set. Typically two section ganged tuning capacitors are used in traditional radio sets, although modern sets are likely to use two electronically tuned circuits, steered by the same or nearly the same control voltage.
The Q levels of the overall ferrite rod antenna may appear very high, and in fact the ferrite in a rod form has a much higher Q than the basic material as a result of the fact that the rod forms an open magnetic circuit.
Radiation resistance of a ferrite rod antenna
One of the advantages of using a ferrite in the antenna is that it brings the radiation resistance of the overall antenna to a reasonable level. This is of considerable importance in any antenna and hence circuit design.
The ferrite rod antenna can be considered as a very small loop antenna. In view of its size, the loop is very much less than a wavelength in length and without the ferrite it would have a very low radiation resistance. Accordingly the losses due to the resistance of the wire would be exceedingly high. Placing the ferrite core in the coil has the effect of raising the radiation resistance by a factor of µ2. This brings the value into more acceptable limits and reduces the resistive losses caused by the wire.
While the introduction of the ferrite rod raises the radiation resistance of the antenna, it does introduce other losses. The ferrite itself absorbs power because energy is required to change the magnetic alignment of the magnetic domains inside the granular structure of the ferrite. The higher the frequency, the greater the number of changes and hence the higher the loss. It is for this reason that ferrite rod antennas are not normally used above frequencies of a few MHz.
Ferrite rod antenna summary
The ferrite rod antenna is a particularly useful form of RF antenna design despite its limitations and drawbacks in terms of efficiency, top frequency and the need for tuning. Nevertheless ferrite rod antennas are widely used, being used almost universally as the RF antenna in portable radios for long and medium waveband reception as well as being used in a number of RFID applications.
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