iakobos,
>An antenna is always resonant somewhere, the problem is, nowhere else.
There's a nice area for argument! Depends on the antenna, and what you mean by resonance......e.g. a cavity backed spiral antenna used for Electronic Counter Measures is a reasonable match from 2 - 18GHz. Where is it resonant?
Sadly, the older I get, the more I realise that I know less and less about radio engineering, when it really ought to be more and more.....