A coaxial cable is normally made of aluminum, copper or other highly conductive material. This is because high frequency signals only travel on the surface of the conductors and not in the entire cross-sectional area of the conductor. In the basic form the coaxial cable is a central conductor completely surrounded by an external conductor where between them there is a separation of air, PVC, Teflon or foam insulating material. The ratio between the diameter of the central conductor and the diameter of the external conductor determines the characteristic impedance that is normally 50 or 75 Ohms.