Ferrite toroids and snap-on chokes for audio and HF purposes - we stock several types of toroid for HF Baluns/Chokes as well as the 31-mix "Problem Solver" which makes a great mains-filter and ad-hoc choke. We also have 5mm+9mm snap-on "domestic" ferrites for taming audio/Hi-Fi/USB issues when near strong RF. 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.