The ferrite rod aerial or ferrite bar antenna is a form of RF antenna 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 popularity and listeners of long, medium and short wave broadcasting decreases, as there are higher quality services available via VHF FM and digital services, this sector is decreasing.
However ferrite rod antennas are being used increasingly in wireless applications in areas such as RFID. Here the volumes of antennas required is huge. The antennas for RFID need to be compact and effective, making the ferrite rod antenna or ferrite bar aerial an ideal solution.
Ferrite rod antenna basics
As the name suggests the antenna consists of a rod or bar made of ferrite which is an iron based magnetic material.
A coil is would around the ferrite rod or bar and this is brought to resonance using a variable tuning capacitor contained within the radio circuitry – typically within the RF stages of the radio receiver with which it will be used. In this way the antenna can be tuned to resonance.
In this diagram the ferrite rod antenna is seen as the inductor and core at the input (left hand side) of the circuit. The signal is passed from this into the transistor which acts as an RF amplifier, mixer and also an oscillator – the inductor assembly linking the emitter and collector forms this. The tuning capacitor is mechanically linked or ganged with the RF antenna ferrite rod tuning. Finally the signal reaches the IF transformer and the signal enters the intermediate frequency or IF chain.
As the antenna is tuned it usually used to provide RF tuning circuit for the receiver, enabling both functions to be combined and use the same components. This reduces the number of components needed and hence this provides a cost saving within the radio set.
Typically ferrite rod antennas often have two coils – dependent upon the location, these might be for the long and medium wave broadcast bands.
The ferrite rod antenna operates using the ferrite material to "concentrate" the magnetic component of the radio waves through the pick-up coil. This is brought about by the high permeability µ of the ferrite. It means that the ferrite rod antenna has a much greater level of sensitivity than if the coil had been used on its own.
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 the ferrite rod antenna is at right angles to the direction of the transmitter. This also 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.
Ferrite rod antenna performance
This form of RF antenna design is very convenient for portable applications and this is its great benefit. However convenience is traded for efficiency – the efficiency of the ferrite rod antenna 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 the operation of ferrite rod antennas to frequencies of 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. 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. If power levels rise too high then heat dissipation in the ferrite becomes an issue.
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.