7.4 The Ferrite Rod Antenna
In part 6
we considered the use of a small Magnetic Loop as an antenna.
Although usually better than a Hertzian dipole, the small loop still tends
to have a low efficiency due to its low radiation resistance compared to its
‘real’ – i.e. dissipation – resistance. One way we
can deal with this problem is to try making the loop of a
Superconducting material. Although this can work, and indeed
is one potentially useful application of ‘high temperature’
superconducting materials, it isn’t very convenient in practice due to
the need for cooling to low temperatures.
Fortunately, we can also significantly increase the radiation resistance of
a loop by placing a suitable piece of Ferrite material inside the
loop and modifying it as illustrated in figure 7.7.
The ferrite has the effect of intensifying the magnetic field inside the
loop. The is produced by the high permeability, , of the ferrite
material. Usually, it is convenient to use a rod of ferrite material and
wind a coil around a central part.
This increases the loop’s radiation resistance by a factor of to
Here is the ferrite’s ‘effective’ relative magnetic permeability. This depends upon the choice of material and the size and shape of the rod. (This shape dependence is because some of the magnetic field ‘escapes’ from the rod away from the coil.) For frequencies of a few hundred kilohertz we can obtain ferrites which provide values in the range from around 100 to around 10,000. Taking the example of an we can see that using the ferrite can increase the antenna’s radiation resistance by a factor of a million! Hence the ferrite can have a dramatic effect in improving the antenna’s efficiency.
Sadly, the usual engineering rule of, “You can’t get own for nowt!” applies. In this case we find that the ferrite itself also tends to absorb some of the signal power. This is caused by the requirement that the alternating magnetic field has to ‘flip’ the magnetic alignment of the magnetic domains inside the granular structure of the ferrite. We can’t avoid this as without these domains the material wouldn’t be a ferrite and hence would not have a usefully high value. For a ferrite rod the extra ‘ferrite’ loss has an equivalent resistance,
where is the imaginary (loss) part, and the real part of the ferrite’s permeability, is the cross sectional area of the rod, and is the length of the rod. We must now add this to the wire’s resistive losses to obtain the overall level of loss resistance in the antenna. Fortunately, by choosing a suitable material we can arrange that this increase in loss can be quite small compared to the increase in . Hence, overall, the ferrite significantly improves the antenna’s performance.
When viewed from the connecting wires, we find that – even when using a high ferrite – the antenna’s resistance value is often just a few ohms (or even much less than one Ohm) in series with a significant inductance. This combination of a low resistance with a large inductance can make it awkward to match the antenna as a source or load to the receiver or transmitter electronics. To try and deal with this problem it is usual to connect a capacitor to turn the loop into a resonant circuit/antenna as shown in figure 7.8.
The inductance of this antenna is
and by using a suitable parallel capacitance, , we can convert the antenna’s terminal impedance, at the resonant frequency
into a pure resistance whose magnitude is larger than the actual loop resistance, where is the circuit’s Quality Factor,
where is the half-power half-bandwidth of the resulting resonance.
For signal frequencies up to a few MHz the Tuned Ferrite Rod antenna can provide antenna efficiencies (and hence gains or effective areas) which can be between a thousand and a million times better than a Hertzian dipole of similar size. For this reason they are often preferred and are used a great deal, for example, in portable radios for the medium wave and long wave bands. The tuned nature of the the antenna can sometimes also help filter out unwanted signals at frequencies well away from the required input.
The main disadvantages of the antenna are:
- The may be so high (.i.e. so small) that the antenna filters away some of the required signal modulation.
- The dissipation in the ferrite makes the system unsuitable at a TX antenna
- The value is only larger than unity for small magnetic field levels.
Losses in the ferrite mean that, if we try using the ferrite in a TX antenna the power dissipated may heat up the material until it decomposes or melts. Since the ferrite behaviour tends to ‘vanish’ ( falls to unity) when try to apply a large field we also find that it simply refuses to work as expected when we try to transmit significant power levels. For these reasons the Ferrite Rod makes an excellent RX antenna, but is not used for signal transmission except where the power level to be transmitted is quite low (typically less than a Watt or so).
You should now see how a variety of different sorts of antenna work. That it is possible to choose a specific gain, operating frequency, etc, by assembling suitable arrays of dipoles. That arrays can contain both driven and passive (or parasitic) elements. That the behaviour of a complex antenna system can be worked out using the principle of field superposition to add together the contributions of all its parts. You should also see how the methods used vary with the size of the antenna compared with the free space wavelength. At low frequencies it should be clear why Tuned Ferrite Rod antennas make excellent RX antennas but are not used for TX antennas.
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.