Baluns, Chokes, Isolators, UnUns and Ferrite Toroids for HF+VHF. We try to offer the best prices for FT140-43, FT140-61, FT240-31, FT240-43 and FT240-61 toroids in the UK. The following chart shows how different ferrite materials perform across HF. The green band indicates "resistive", which is desired for best performance. As you can see, 9 turns of coax on an FT240-31 will suffice. Coax can be RG316, RG58, RG8 "mini" or RG142 depending upon your power requirements.