More information needed!
What kind of antenna is it? Is it a base installation, or mobile?
I suppose you could go to Radio Shack and buy a length (or roll) of coax and extend it using the proper coax cable adapters, but more adapters induce more signal loss.
If it's a base installation, I would replace the coax with one single length that's long enough without any splices or add on's. You could do the same thing with a mobile setup, but buying a length of coax to extend the length of existing coax is not the best idea, since splicing coax is a very bad idea, and using coax connectors and adapters will work, but it will introduce more signal loss.