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Air-wound chokes are one of the least effective ways to stop common-mode current. They are also targeted to a narrow band because the way the winding is done dictates what band it will be effective on. They are cheap and easy to implement but have the aforementioned deficiencies. Strings of ferrite beads may also have less effect at eliminating common-mode current. They are easy to employ and do not suffer from the narrow band shortfall. However, it takes a lot of ferrite beads to have much reduction impact and this can become expensive.To have the most effective reduction of common-mode current you must employee ferrite cores of the specific mix for the application and use multiple turns. The best ferrite cores are made in the United States such as those made using Fair-Rite 31 and 43 grade materials. The best performance comes from large cores. The problem with this approach is cost. This is a very expensive way to deal with common-mode current.