The RSGB offers help with EMC problems by supplying filters and ferrite rings. To find more products and information click here .
You can buy here Ferrite rings as recomended by the RSGB EMC Committee these rings are made from Fair-Rite 43 material. Inside diameter 22.85mm and a ferrite width of 12.7mm (total width 35.5mm). This can accommodate 12 turns of 5mm diameter cable.
Please note the price shown is for one ring as these are packed as singles