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post #6 of Old 03-10-2006
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Radiated vs Conducted Noise

I’m not expert in RFI Shielding, nor EMI Suppression, so take the following /w a grain of salt.

Motors can cause one or both of two types of noise: Radiated RFI& Conductive E.M.I.
Except for the refrigeration motors, which can definitely produce EMI, most of the motors aboard (ie: Bilge Pump) cause RFI, sometimes known as spark-gap noise. Tackling EMC is daunting, whether suppressing or shielding (or both).

A Faraday Cage will only shield against radiated RFI, not conducted EMI noise. An effective shielding cage is very difficult to build, and I wouldn’t recommend them in this application.

Conducted interference travels from the source (ie: Refer’ Motor) to your radio or tuner over the DC wiring. The wiring acts as an antenna, actually funneling the radiated signal to the location where it can cause the greatest interference.

When you encounter a really stubborn interference signal, you might use a toroid coil, or Ferrite Beads, wrapping BOTH the positive and negative wires as close to the motor, or other device, as possible. Use a (40 - 60 micro farad) capacitor across the positive and negative motor leads. You must contain the emission as near the source as you possibly can. Common-mode chokes are wound in a manner that produces a high impedance to signals appearing in-phase on both windings while having little effect on differential- or normal-mode signals.
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