Ferrite rod antenna

- an overview the basics of the ferrite rod antenna, a form of RF antenna that is widely used in transistor radio applications.

The ferrite rod antenna is a form of RF antenna design that is almost universally used in portable transistor broadcast receivers as well as many hi-fi tuners where reception on the long, medium and possibly the short wave bands is required. As the name suggests the antenna consists of a rod made of ferrite, an iron based magnetic material. A coil is would around the ferrite rod and this is brought to resonance using a variable tuning capacitor contained within the radio circuitry itself and in this way the antenna can be tuned to resonance. As the antenna is tuned it usually forms the RF tuning circuit for the receiver, enabling both functions to be combined within the same components, thereby reducing the number of components and hence the cost of the set.

Typical ferrite rod antenna assembly used in a portable radio

The ferrite rod antenna operates using the high permeability of the ferrite material and in its basic form this may be thought of as "concentrating" the magnetic component of the radio waves. This is brought about by the high permeability μ of the ferrite.

The fact that this RF antenna uses the magnetic component of the radio signals in this way means that the antenna is directive. It operates best only when the magnetic lines of force fall in line with the antenna. This occurs when it is at right angles to the direction of the transmitter. This means that the antenna has a null position where the signal level is at a minimum when the antenna is in line with the direction of the transmitter.

Operation of a ferrite rod antenna

This form of RF antenna design is very convenient for portable applications, but its efficiency is much less than that of a larger RF antenna. The performance of the ferrite also limits the frequency response. Normally this type of RF antenna design is only effective on the long and medium wave bands, but it is sometimes used for lower frequencies in the short wave bands although the performance is significantly degraded, mainly arising from the losses in the ferrite. This limits their operation normally to frequencies of only a MHz.

Ferrite rod antennas are normally only used for receiving. They are rarely used for transmitting anything above low levels of power in view of their poor efficiency. It any reasonable levels of power were fed into them they would soon become very hot and there would be a high likelihood that they would be destroyed. Nevertheless they can be used as a very compact form of transmitting antenna for applications where efficiency is not an issue and where power levels are very low. As they are very much more compact than other forms of low or medium frequency RF antenna, this can be an advantage, and as a result they are being used in applications such as RFID.

The need for 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.

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

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 particualrly 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.