Ferrite Rod Antenna Parameters

- notes and overview about some of the key parameters associated with ferrite rod antennas and their performance.

There are a number of ferrite rod parameters that are of key interest when considering their use in any application.

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. The size of the ferrite rod antenna means that there are some compromises that need to be made in other areas of their performance. Accordingly it is necessary to make the right balance between the important requirements.


Ferrite rod antenna Q

One of the requirements for an efficient ferrite rod antenna is that it should have a high Q at the frequencies over which it operates. At frequencies of a few hundred kilohertz, a medium permeability material would be used and this would enable a Q of about 1000 to be obtained. With a Q of this value it will mean that the antenna 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.

Diagram of a typical ferrite rod antenna showing the ferrite rod with the coupling / tuning coils used to transfer the energy
Typical ferrite rod antenna assembly used in a portable radio

The Qs of the overall 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 more reasonable level. The ferrite rod antenna can be considered as a small loop antenna. In view of its size, the loop is 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 μ^2, and thereby bring the value into more acceptable limits.

While the introduction of the ferrite rod raises the radiation resistance of the antenna, and hence reduce the losses due to the resistance of the wire, it does introduce other losses. The ferrite itself absorbs power. This arises from the energy 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.


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.

By Ian Poole


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