I would amend Cecil's statement to:
'We hope antennas are linear systems.'

A nonlinear antenna is a terrible beast indeed. It will mix every
signal it receives with every other signal, creating a nasty mush of
signals. On transmit it is not so bad, maybe generating harmonics and
a wider signal than we'd like.

We've all heard stories of a poor antenna connection causing problems,
or a nearby raingutter joint causing TV inteference. These are
nonlinear antennas.

73,
Glenn AC7ZN