Ferrite Devices

A FERRITE is a device that is composed of material that causes it to have useful magnetic properties and, at the same time, high resistance to current flow. The primary material used in the construction of ferrites is normally a compound of iron oxide with impurities of other oxides added. The compound of iron oxide retains the properties of the ferromagnetic atoms, and the impurities of the other oxides increase the resistance to current flow. This combination of properties is not found in conventional magnetic materials. Iron, for example, has good magnetic properties but a relatively low resistance to current flow. The low resistance causes eddy currents and significant power losses at high frequencies (You may want to review NEETS, Module 2, Introduction to Alternating Current and Transformers , Chapter 5). Ferrites, on the other hand, have sufficient resistance to be classified as semiconductors.

The compounds used in the composition of ferrites can be compared to the more familiar compounds used in transistors. As in the construction of transistors, a wide range of magnetic and electrical properties can be produced by the proper choice of atoms in the right proportions. An example of a ferrite device is shown in figure 1-72.

Figure 1-72. - Ferrite attenuator.

Ferrites have long been used at conventional frequencies in computers, television, and magnetic recording systems. The use of ferrites at microwave frequencies is a relatively new development and has had considerable influence on the design of microwave systems. In the past, the microwave equipment was made to conform to the frequency of the system and the design possibilities were limited. The unique properties of ferrites provide a variable reactance by which microwave energy can be manipulated to conform to the microwave system. At present, ferrites are used as LOAD ISOLATORS, PHASE SHIFTERS, VARIABLE ATTENUATORS, MODULATORS, and SWITCHES in microwave systems. The operation of ferrites as isolators, attenuators, and phase shifters will be explained in the following paragraphs. The operation of ferrites in other applications will be explained in later NEETS modules. Ferromagnetism is a continuation of the conventional domain theory of magnetism that was explained in NEETS, Module 1, Matter, Energy, and Direct Current. A review of the section on magnetism might be helpful to you at this point.

The magnetic property of any material is a result of electron movement within the atoms of the material. Electrons have two basic types of motion. The most familiar is the ORBITAL movement of the electron about the nucleus of the atom. Less familiar, but even more important, is the movement of the electron about its own axis, called ELECTRON SPIN.

You will recall that magnetic fields are generated by current flow. Since current is the movement of electrons, the movement of the electrons within an atom create magnetic fields. The magnetic fields caused by the movement of the electrons about the nucleus have little effect on the magnetic properties of a material. The magnetic fields caused by electron spin combine to give a material magnetic properties. The different types of electron movement are illustrated in figure 1-73. In most materials the spin axes of the electrons are so randomly arranged that the magnetic fields largely cancel out and the material displays no significant magnetic properties. The electron spin axes within some materials, such as iron and nickel, can be caused to align by applying an external magnetic field. The alignment of the electrons within a material causes the magnetic fields to add, and the material then has magnetic properties.

Figure 1-73. - Two types of electron movement.

In the absence of an external force, the axis of any spinning object tends to remain pointed in one direction. Spinning electrons behave the same way. Therefore, once the electrons are aligned, they tend to remain aligned even when the external field is removed. Electron alignment in a ferrite is caused by the orbital motion of the electrons about the nucleus and the force that holds the atom together. When a static magnetic field is applied, the electrons try to align their spin axes with the new force. The attempt of the electrons to balance between the interaction of the new force and the binding force causes the electrons to wobble on their axes, as shown in figure 1-74. The wobble of the electrons has a natural resonant WOBBLE FREQUENCY that varies with the strength of the applied field. Ferrite action is based on this behavior of the electrons under the influence of an external field and the resulting wobble frequency.

Figure 1-74. - Electron wobble in a magnetic field.

FERRITE ATTENUATORS. - A ferrite attenuator can be constructed that will attenuate a particular microwave frequency and allow all others to pass unaffected. This can be done by placing a ferrite in the center of a waveguide, as shown in figure 1-72. The ferrite must be positioned so that the magnetic fields caused by its electrons are perpendicular to the energy in the waveguide. A steady external field causes the electrons to wobble at the same frequency as the energy that is to be attenuated. Since the wobble frequency is the same as the energy frequency, the energy in the waveguide always adds to the wobble of the electrons. The spin axis of the electron changes direction during the wobble motion and energy is used. The force causing the increase in wobble is the energy in the waveguide. Thus, the energy in the waveguide is attenuated by the ferrite and is given off as heat. Energy in the waveguide that is a different frequency from the wobble frequency of the ferrite is largely unaffected because it does not increase the amount of electron wobble. The resonant frequency of electron wobble can be varied over a limited range by changing the strength of the applied magnetic field.

FERRITE ISOLATORS. - An isolator is a ferrite device that can be constructed so that it allows microwave energy to pass in one direction but blocks energy in the other direction in a waveguide. This isolator is constructed by placing a piece of ferrite off-center in a waveguide, as shown in figure 1-75. A magnetic field is applied by the magnet and adjusted to make the electron wobble frequency of the ferrite equal to the frequency of the energy traveling down the waveguide. Energy traveling down the waveguide from left to right will set up a rotating magnetic field that rotates through the ferrite material in the same direction as the natural wobble of the electrons. The aiding magnetic field increases the wobble of the ferrite electrons so much that almost all of the energy in the waveguide is absorbed and dissipated as heat. The magnetic fields caused by energy traveling from right to left rotate in the opposite direction through the ferrite and have very little effect on the amount of electron wobble. In this case the fields attempt to push the electrons in the direction opposite the natural wobble and no large movements occur. Since no overall energy exchange takes place, energy traveling from right to left is affected very little.

Figure 1-75. - One-way isolator.

FERRITE PHASE SHIFTER. - When microwave energy is passed through a piece of ferrite in a magnetic field, another effect occurs. If the frequency of the microwave energy is much greater than the electron wobble frequency, the plane of polarization of the wavefront is rotated. This is known as the FARADAY ROTATION EFFECT and is illustrated in figure 1-76. A ferrite rod is placed along the axis of the waveguide, and a magnetic field is set up along the axis by a coil. As a wavefront enters the section containing the ferrite, it sets up a limited motion in the electrons. The magnetic fields of the wavefront and the wobbling electrons interact, and the polarization of the wavefront is rotated. The amount of rotation depends upon the length of the ferrite rod. The direction of rotation depends upon the direction of the external magnetic field and can be reversed by reversing the field. The direction of rotation will remain constant, no matter what direction the energy in the waveguide travels, as long as the external field is not changed.

Figure 1-76. - Faraday rotation.

Q.59 Ferrite devices are useful in microwave applications because they possess what properties?
Q.60 Which of the two types of electron motion (orbital movement and electron spin) is more important in the explanation of magnetism?
Q.61 The interaction between an external field and the binding force of an atom causes electrons to do what?
Q.62 The resonant frequency of electron wobble can be changed by variation of what force?
Q.63 Rotating the plane of polarization of a wavefront by passing it through a ferrite device is called what?