Basic Wireless Communication for Microcontrollers

Chapter 1 - Electricity and Magnetism

Interaction with Matter

     At several points so far, we have hinted at what happens when E and B fields encounter matter. Now is the time to discuss it in a little bit more depth.

Conduction and Loss

     Just like static E fields, changing E fields also cause conduction when they encounter a conductive material. People also say that changing B fields induce currents in conductors, and that is true, but it is really due to the E field produced by a changing B field. In other words, if you want to determine the effect of an EM wave on a conductor, it is enough to consider the E field alone or the B field alone, and you typically chose one or the other depending on which is easier to understand and use in calculation(e.g., the E field is usually easier when the conductor does not form a closed loop and the B field easier when it does).
     When currents are induced in a conductor by a passing E field, any resistance in the counductor causes the currents to produce heat. To preserve conservation of energy, some energy must be robbed from the passing wave. This "robbing" is actually a result of the EM wave re-radiated by the currents in the wire, but we will discuss that further in the section on Basic Antennas.

Relative Permitivity

     Materials which are not good conductors are still influenced by electric fields. The groups of electrons (electron clouds) around molecules are distorted by the presence of an external electric field. The distortion causes the molecules to become polarized (that is, have a positive end and a negative end) and create a field in response to the applied field, and which partially cancels the applied field.
     The end result is that many materials, when placed in an electric field, will have a weaker field strength inside than outside, due to the polarized molecules. Materials which have this property are called dielectrics, and most insulating or mildly conductive materials do have this property to some extent. This effect is modeled by considering that the material changes the permitivity of the space inside it. The new permeability is equal to the permitivity of free space (Epsilon_o) times the relative permitivity (Epsilon_r, also called the dielectric constant) of the material.

Relative Permeability

     In a way very similar to the electric field case, magnetic fields can also affect materials and cause the materials to produce magnetic fields. However, there are more varieties of magnetic effects. The strongest effect, called ferromagnetism, is when the material responds by producing a magnetic field much stronger than the applied field, and in the same direction. Only a few materials have this property, mostly iron, nickel, and cobalt, as well as alloys containing them. There are also other much weaker effects, such as paramagnetism and diamagnetism. All of these involve the effects of the magnetic field upon either electron's spin or orbital motion. In cases where the applied field is relatively weak, these effects can be modeled as changes in the permeability within the material. The new permeability is equal to Uo (permeability of free space) times Ur (the relative permeability). Ur >> 1 for ferromagnatic materials and can be either slightly greater or slightly less for the other effects. The ferromagnetic effect (and a slightly weaker cousin called the ferrimagnetic effect) are the only ones of significance for RF electronics.
     When stronger applied fields are involved, a situation can arise in ferro and ferrimagnetic materials where the linear relationship between applied field and internal field breaks down and the internal field levels off and stays constant. This is called saturation. In some materials, if the applied field is shut off at this point, the material will retain its magnetism and still produce a B field. Permanent magnets are made of such materials.

Changes in Propagation Velocity and Impedance

     One of the most striking effects of changes in permeability and permitvity is that it changes the speed of light (and, hence, all EM waves) in such materials. The impedance (ratio of electric to magnetic field strength) is also changed. In most practical cases, the speed of light will be less than the free space value. Some special cases can be contrived where the speed is greater, but in those cases, the speed at which information and energy are transfered are still less than the speed of light in free space. The altered values of velocity and impedance are calculated using the same formulas, but with the altered values, that is: Vphi=1/sqrt(Epsilon_o*Epsilon_r*Uo*Ur) Eta=sqrt((Uo*Ur)/(Epislon_o*Epsilon_r))

Combined Effects

     In some cases, materials will be somewhat conductive, have a relative permitivity greater than one, and a relative permeability other than one. Such materials not only have an altered Vphi and Eta, but they also will exhibit losses due to conduction.

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