Thanks guys

D4D - don't want to drill and loctite would be too permanent.
Bushbandit - I have a home made fold down bracket on the rack that can be swivelled.
BigJules - it's a 320mm ZCG aerial.
I'm thinking that if I go down to the woods, I may be in for a surprise when I come back from exploring or walking and it's gone. I was thinking about some way to tether it to the rack.
Mace - have to agree with the other guys.
Jim - it's a ZCG Scalar but its not tapped for a grub screw - unless some of the larger Scalar aerials are tapped.
Appreciate the ideas.
Tony