Optimal length of an antenna

I understood that the optimal length of an antenna depends on the wavelength of the signal.

But often the antenna is connected via a cable, or wire, to the RF module. Like in this picture here:

Question: Doesn't this wire have a (negative) affect on the antenna, because it changes the length of the antenna?

Yes. It is called "transmission line loss".

edit: But it isn't as much as you might think. The transmission line (cable) has different electrical properties than the antenna does.

SurferTim: The transmission line (cable) has different electrical properties than the antenna does.

Can you please explain this a little bit more?

You would probably be better off doing a Google search than reading the explanation I can offer.