ferite ring , rod or ?

[forest]

ok i want to make a filter for my fpv set up from a cap and ferite rod i say rod cos at my local maplins all i could get areis a ring thats is far to big or a two bits in a plastic cover holder that goes around a cable  still a little larger but half of it would be good size wise
i think  ive seen some where you could use a rod can i just cut the ring up? and use a section of it ? i have a small ring on my bec that powers the fc  does that need to be  there or can i use that for the filter? it would be just right
the ring  is big enough that you could wrap tv coax round

thank for taking the time to help
forest

this is for a 250