The amount of the variation in the current that directly affects variation in the magnetic field surrounding the conductor depends upon the nature of the current. When the conductor is a power lead to an electric motor, all the frequencies and amplitudes associated with broadband interference are present in the magnetic field. When the lead is an ac power lead, a strong sinusoidal magnetic field is present. When the lead is carrying switched or pulsed currents, extremely complex broadband variations are present. As the magnetic field cuts across a neighboring conductor, a voltage replica of its variation is induced into the neighboring wire. This causes a current to flow in the neighboring wire. When the neighboring wire leads to a sensitive point in a susceptible receiver, serious interference with that receiver’s operation can result. Similarly, a wire carrying a steady, pure dc current of high value sets up a magnetic field capable of affecting the operation of equipment whose operation is based upon the earth’s magnetic field. Shielding a conductor against magnetic induction is both difficult and impractical. Nonferrous shielding materials have little or no effect upon a magnetic field. Magnetic shielding that is effective at low frequencies is prohibitively heavy and bulky. In aircraft wiring, the effect of induction fields should be minimized. This can be done by use of the proper spacing and coupling angle between wires. The degree of magnetic coupling diminishes rapidly with distance. Interference coupling is least when the space between active and passive leads is at a maximum, and when the angle between the leads approaches a right angle. INDUCTIVE-CAPACITIVE COUPLING Capacitive (electric) fields are voltage fields. Their effects depend upon the amount of capacitance existing between exposed portions of the noisy circuit and the noise-free circuit. The power transfer capabilities are directly proportional to frequency. Thus, high-frequency components are more easily coupled to other circuits. Capacitive coupling is relatively easy to shield out by placing a grounded conducting surface between the interfering source and the susceptible conductor. COUPLING BY RADIATION Almost any wire in an aircraft system can, at some particular frequency, begin to act like an antenna through a portion of its length. Inside an airframe, however, this occurs only at very high frequencies. At high frequencies, all internal leads are generally well shielded against pickup of moderate levels of radiated energy. Perhaps the only cases of true inside-the-aircraft radiation at HF and below occur in connection with unshielded or inadequately shielded transmitter antenna leads. COMPLEX COUPLING Some examples of interference coupling involve more than one of the types (conduction, induction, or radiation) just discussed. When more than one coupling occurs simultaneously, corrective actions, such as bonding, shielding, or filtering, used to correct one type of coupling can increase the coupling capabilities of another type of coupling. The result may be an increase in the transfer of interference. For example, an unbended, unfiltered dc motor can transfer interference to a sensitive element by conduction, inductive coupling, capacitive coupling, and by radiation. Some frequencies are transmitted predominately by one form of coupling and some frequencies by others. At still other frequencies, all methods of transmission are equally effective. On the motor used in the example above, bonding almost always eliminates radiation from the motor shell. It also increases the intensity in one of the other methods of transmission, usually by conduction. The external placement of a low-pass filter or a capacitor usually reduces the intensity of conducted interference. At the same time, it may increase the radiation and induction fields. This occurs because the filter appears to interference voltages to be a low-impedance path across the line. Relatively high interference currents then flow in the loop formed between the source and the filter. For complex coupling problems, multiple solutions may be required to prevent the interference. RADIO INTERFERENCE REDUCTION COMPONENTS Learning Objective: Recognize various methods and components used to reduce radio interference caused by electrical noise. Radio interference reduction at the source maybe accomplished to varying degrees by one or more of 10-7