I recall you could make a fm filter trap using a 300 ohm ribbon cable at some
length and put a cap on the end.


An electrical half wavelength with a shorted end or an electrical quarter
wavelength with open end are effective traps. Their bandwidth is rather
narrow.
What bandwidth and db rejection would I get with one L/C network? Would need
like 10MHz of BW.

For that type of bandwidth, I would think a single section L/C would not work.
Probably need multiple sections.

73 Gary N4AST