"gareth" wrote in message ...

I wonder if there is any experience out there about cutting down mobile
CB antennae (along with their base loading coils) to make a 50 MHz ground
plane?

Sure. Just take the more or less 9 foot long CB whip, remove the mounting
stud on the end and cut the whip down to 50MHz length.
If you are lucky, there won't be much taper in the whip and it will snug
back into the stud. Use the same car mounting bracket.