Ferrite cores are used everywhere in radio from RF transformers to filters to chokes. They are often made from different materials that will have different properties at different frequencies and they are often seen as a bit of a Black art.
I came across some recent articles on using ferrite cores as RF chokes and I'm posting the links here so I don't lose them.
1) QRM GURU : The truth about ferrites... This website looks at a selection of ferrite cores and how they can be use to prevent interference at HF & VHF.
2) K9YC ... This website has a detailed PDF on using ferrite cores as RF chokes.
