Is antenna length for a baseband signal infinity?
Sort of. Certainly antenna length is proportional to wavelength, so if you take the limit going to DC then you'll need an infinitely long antenna. However, even if you make an antenna infinitely long, it won't work very well. Antennas have limited bandwidth. Say you design a 7.5 cm long antenna for a 1 GHz signal. This antenna will work very well at 1 GHz, but at 2 GHz it's too long and the radiation pattern will change. At 500 MHz, it will be too short and won't radiate efficiently. One of the features of RF transmissions is they usually have a small bandwidth relative to the transmit frequency. Say, a 10 MHz wide band sitting at 1 GHz. The frequency only varies by 1% in this case, so you can pretty much ignore that variation when designing the antenna and design it for a single frequency. So, if you wanted to design a 10 MHz baseband antenna that works from DC up to 10 MHz, you need to design an antenna that works properly not over a 1% bandwidth but over 7 orders of magnitude, which is pretty much impossible.  Actually, it's not even 7 orders of magnitude-that would be 1 Hz to 10 MHz-it's infinite orders of magnitude. The infinitely long antenna that you would need for DC will not work well at all at 10 MHz.