How to: Ferrite Beads Filters & EMI Suppression frequency range

[Kiriakos GR]

If you are a professional electrician and ever be challenged to answer what those long Ferrite Beads does over cables?

Here is my own simple explanation that worth keeping it in your memory.

Ferrite Beads they work as resistors at radio frequencies.
Their peak performance as RFI eliminators this is at the range of 80 to 180MHz.

Ferrite Beads over cables their are 100% related to FM Radio transmitting frequency.
And they will proven as useful items if there is a local FM radio broadcasting station next to your home.

Usual wavelength at FM band this is about 1.5 meters long.
About 1.5 meters long it is the total length of a transmitting FM dipole  (antenna).   
Now imagine that most cables that we use in all our devices, them are about 1.5 meters long too.

Therefore any cable that we use no matter the application,  this at the length of  1.5 meters long this is matching very close to FM Band wavelengths and therefore it transforms itself  to RF FM reception Antenna.     

Speaking of Beads placement over cables,  they are placed at almost beginning and the end of cable.
This is not a random selection.

According to FM Radio theory, at transmitting, antennas of 1/4 wavelength translates to max electromagnetic difference between 75cm part of dipole antenna this pointing at the sky,  and it negative pole this pointing at the ground.
An perfect antenna this is considered the one which it wavelength this matching 100% the frequency of our choice that our FM transmitter this is going to use.

Now imagine the picture of a regular sine wave, this will visualize and explain to you, of why the peak of energy this is there at 1/4 of it total length.
     
Now we getting back again to Beads placement over cables, their purpose is them to operate as  Radio Frequency Cut  (RFC) filters, and they will do that as long their spacing of two beads this matching the specific wavelength that causing trouble to us.

Theoretically wavelength of 1.5 Meters this is tuned for the center of FM band.
At 88MHz this is a bit longer, and at 108MHz this is a bit sorter.
 
But the bottom line,  this is that no matter if FM Radio wavelength this is 1.2 meters long or at 1.8 meters long, all of our cables which they do device to device connectivity, them are also in similar length's.



http://docs-asia.electrocomponents.com/webdocs/0596/0900766b805962ed.pdf