With the antennas we need a 50 Ohm cable, right ? And how if I use a 75 Ohm cable ? or 300 Ohm ? What can it causes ?
If you use a different impedance cable and / or antenna, you have what is officially called an impedance mismatch.
When you have an impedance mismatch, you create a point of reflection in the system; part of the energy is sent back to the origin.
Depending on the strength and nature of the mismatch, and the design and nature of the source, you would have, at the very least, reduced performance due to the reflection. You'd also likely have significant reduced performance because the cable has too much loss (2.4GHz and 5GHz lose a lot of energy, even in good cable).
Severe mismatches can result in burning out the transmitter and literally heating up the cable (all that energy has to go somewhere!).
SO, the short story: at the least, bad performance to virtually no performance (variable with the situation), worse case, you kill the transmitter / kill the AP, and have to buy a new radio or AP (depending on the type).