[GENERAL] Defining character sets for indicidual fields

Hi,

By default, my postgresql server is set to use UTF8 character set. I was wondering if there is any way to make sure that certain fields like url etc. only makes use of ascii. My main aim is to save space by using only 1 byte / character for urls  (some of the urls are over 200 characters long). Is this possible? Or are all characters eventually converted to UTF8 during storage?

Thanks,

Ram