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Often I will see a ferrite bead a couple of inches from one end of a cable. Does the position of a ferrite bead really matter?

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My answer hopefully applies to beads but also ferrite clamps on cables.

If it's preventing susceptibility problems due to interference coming down a cable then its position is not that critical. However, if it's stopping emissions getting out from a device, a better position is as close to the interfering source as possible. There is every chance that interference getting onto a cable (such as from a switched mode power converter) can radiate from the cable so, best to keep the ferrite up as close to the source of noise as much as possible.

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  • \$\begingroup\$ Why don't you see them on both ends then? I'm thinking of a High Speed USB Camera cable. Wouldn't the computer have just as much potential creating noise than the camera? \$\endgroup\$ – Scott May 22 '14 at 21:51
  • \$\begingroup\$ If its at one particular end then that end is the noisier or more susceptible. \$\endgroup\$ – Andy aka May 22 '14 at 22:54
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The position is important. There's two cases I can imagine. In the first case, a device is radiating through the cable, and this is unwanted. In this case, the bead should be as near to the transmitting device as possible. This reduces the length of the radiating element. In the second case, a device is receiving unwanted signals picked up on a cable. In this case, again, the bead should be as close to the device as possible. The reason we don't use two beads on a cable is that usually it's not required. Either the communication link is unidirectional, only one device is susceptible, or one bead is enough to change the electrical length enough to attenuate the unwanted signal.

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  • \$\begingroup\$ So that's is why it's on the end of the cable because it is close to the computer (that causes noise)? \$\endgroup\$ – Codebeat Mar 21 '17 at 3:48
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Beads provide suppression by impeding (developing a voltage across the bead) and absorbing (converting to heat) magnetic fields (resulting from to currents on the wire). Beads provide the most suppression at VHF and UHF current maxima (as-in standing waves). The most likely position for a current maxima for all frequencies is near a chassis, bulkhead or other electromagnetic boundary. Undesired RF energy can form standing waves bilaterally when leaving a chassis or entering it. Placing a bead at a current maxima will shift that maxima to another yet weaker frequency dependent location where a second bead may provide some additional improvement for a particular frequency (test equipment required).

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