Can a 20 or 40 meter antenna be used to transmit on different bands?
Yes. It depends on how you mean that, though. Either can be 'cut
down' for use on higher frequencies, or 'trapped' and used on lower
ones (so some ridiculous point). If you mean used as a directional
antenna as they work on their design bands, then no, they won't work
that way without that physical modification. There may be some
'directionality' to it, but not like they are supposed to be (that
make sense?). It can work, but not 'well'.
- 'Doc