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Old 07-11-2017, 1:43 AM
steve-kc7byp steve-kc7byp is offline
Join Date: Dec 2012
Location: kennewick, wa
Posts: 118
Default I have a problem of RFI effecting my laptop

All the info bellow is nice. But what you are saying contradicts the instructions that came with the snap on ferrites. In that the ferrites are not supposed to be free floating up and down the cable. The tech at Palomar Engineers told me that if the ferrite is to big for the cable then put electric tape on the cable to hold the ferrite in place. And that's what I did.

Originally Posted by prcguy View Post
You don't need to have the cable fill all the space inside a snap on ferrite bead. Usually one bead of any mix will have little to no effect on HF but will start to reduce RFI in the VHF/UHF range. You have to run multiple turns of your suspected wire through the snap on ferrite or have multiple beads in series to get the most benefit and the correct number of turns depends on the frequency range, the ferrite mix, and how large the ferrite is. Bob at Palomar has further information that is helpful like if you have a switching supply in the tens of KHz range, a higher permeability mix like 77 might work better at suppressing the fundamental low frequency RFI thereby killing off its harmonics at a higher frequency which is causing your problems.

I find the best results in the VHF/UHF range are passing two turns of your suspected wire through a #43 mix snap on ferrite before snapping it closed. Or use three beads in series with a single pass through them all. For HF range, passing about 5 turns through a #31 mix ferrite snap on bead is a good starting point. Passing several turns also locks the bead on the wire so it doesn't slide up and down.

Whatever the rated resistance to RF is stated for a single bead over a wire at a particular frequency, that number will be squared every time you double the amount of turns. If a bead has say 200 ohms of resistance with one pass of wire through it, two passes will be 800 ohms. At some point too many passes will be detrimental to the frequency range you want to surpress.
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