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To compare differences by manufacturer consider Palomar's type 43 material is good for attenuation from 2 MHz to 1000 MHz. Fair-Rite's type 43 is good for attenuation from 20 MHz to 300 MHz. As you can see this is significant on the HF band. To compare differences by manufacturer consider Palomar's type 43 material is good for attenuation from 2 MHz to 1000 MHz. Fair-Rite's type 43 is good for attenuation from 20 MHz to 300 MHz. As you can see this is significant on the HF band. Fair-Rite 31 material - the best grade for general purpose HF and low-band chokes. Fair-Rite 31 clamp-on core, Part No 0431177081. An HF ferrite choke will typically need 6-8 turns passing through the center hole. Regardless of the type of core, one pass will probably not be very effective. The same Fair-Rite 31 material is also available in a "FT240" 2.4-inch toroid formatFair-Rite Part No 2631803802. 10-12 turns of coax on a FT240-31 makes a nice low-bands choke or balun. ==Fighting RF Current Flow==Ferrite Chokes and Baluns may be employed to help stop RF currents from flowing where they aren’t wanted. Common-mode RF current on feedline increases the risk of TVI and other types of RF interference (RFI). RF current flows only on the surfaces of conductors. The outside surface of the coax shield is a completely separate conductor from the inside surface so you can think of the outside surface of the shield is part of the outside world. This is where the common-mode currents flow. Common mode current means that the feedline is radiating RF. It is ideal to have only the antenna radiate. Excessive common mode current causes an excess of RF in the shack, which can get into all sorts of things including the building electrical system. To reduce this you can employee chokes and baluns.* common mode choke* feedline choke* mains choke They all do the same job – blocking unwanted common-mode RF current on the feedline cable. Chokes may be needed in several different places: at the antenna feedpoint which is considered a balun, at different points on the coax, on other wiring in the shack including computer cables, and on the mains electrical wiring. 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.