Ferrite rod antenna

- an overview, summary, tutorial about the ferrite rod antenna or aerial, a form of RF antenna that is widely used in RFID and 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.

Ferrite rod antennas are also being used increasingly in wireless applications in areas such as RFID. Here the volumes of antennas required can be huge. The antennas also need to be compact and effective, making ferrite rod antennas an ideal solution.

Ferrite rod antenna basics

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.

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 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.

Diagram showing the operation of a ferrite rod antenna and how the lines of magnetic force are concentrated therough the ferrite rod.
Operation of a ferrite rod antenna

Ferrite rod antenna performance

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 up to 2 or 3 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.

By Ian Poole

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