All over the world, the predominant impedance for RF/microwave coaxial transmission lines and connectors is 50 Ω. Here is why….
The theoretical impedance for minimum attenuation is 77.5 Ω while the maximum power transfer is 30 Ω. The average of these two impedances is 53.75 Ω (rounded off to 50 Ω). Therefore, 50 Ω is a compromise between the minimum attenuation and maximum power transfer in a coaxial transmission line, and that is why it was selected. There are connectors available with other impedances, the next most popular impedance being 75 Ω (approximate minimum attenuation performance) that is in fairly wide use internationally and in long line communication systems.