I have one doubt according to modulation of a real signal.
there are so many examples where the real signal was modulated with frequency bigger then 0.
Why the signal could not be putted in f=0?
is there connection with a sending power? ( Antenna send bandpass signal $\rightarrow$ signal is matched $\rightarrow$antenna send low P $\rightarrow$power will be reflected, Tx get P again)