Well a square wave has a lot of harmonics, and these will get radiated, but not very effectively. Using a Fourier series for the square wave, the third harmonic will be one third the amplitude (in voltage terms) compared to the fundamental. That’s pretty high. But then again, a narrow-band tuned antenna like a dipole (or monopole) will not radiate the third harmonic as effectively as the fundamental. The exact extent of the radiation obviously depends on the matching and radiating efficiency at the third harmonic. Ordinarily I would expect to filter the signal to keep the unintentional radiation very low, depending on the power level being transmitted of course. If you are transmitting tens of watts at the fundamental, then the harmonics will probable be large enough to be a nuisance, if not illegal. If you are in the low power region, say less than 1mW, then the harmonics will almost certainly not be a problem.

For your software, just change the operating frequency to the third harmonic and re-simulate. This should give you the radiation efficiency and matching characteristics at the harmonic. In principle you would need to do this with all the odd harmonics until the power level was low enough to ignore. In practice, if the third harmonic is ok then the rest will probably be ok as well.

The only possible exception is that some harmonics may fit nicely into the bandwidth of the antenna and therefore get radiated quite effectively. Consider this; a particular half wave dipole is resonant at 100MHz. It is therefore also full-wave resonant at 200MHz. Come to think of it, all harmonics may therefore be resonant as well, although I have never seen anything like that in print.

It seems to me that unless you have a low power transmitter, filtering is essential.

Since you're just starting, find a way to set the excitation to a simple sin wave (single frequency). There is plenty of learning to accomplish before you start designing Ultra Wide Bandwidth antennas.

Thanks for the help. The information was very interesting, it clarify me a bit. I am planning to design a folded dipole or spiral antenna but as VEIBLL said, I need to learn more on it. Where I can find some stuffs which can help A to Z, example, I have a dipole with a sin wave, etc... for starting. Then, it will explain all the caracterics on it, and so on. I need some simple and complet examples (if possible with modellingcodes). Maybe a book?

Thanks again,

Mica.

For a good basic grounding on antennas buy the ARRL Antenna Handbook.  It should be at the right level for you and cover the antennas you want.