Which of the following types of feed line has the lowest loss?

Feed line loss (attenuation) reduces the amount of power that successfully reaches your antenna from the transmitter. Different types of transmission lines use different dielectric (insulating) materials, which greatly impact how much RF energy is absorbed as heat. Air-insulated hardline uses air or inert gas as the primary dielectric between the center conductor and the outer shield, rather than solid plastic or foam. Because air is an almost perfect insulator with minimal RF absorption, it provides the lowest signal loss among the listed options. Standard flexible coaxial cables, whether 50-ohm or 75-ohm, use solid or foam polyethylene dielectrics. These solid materials inherently absorb more radio frequency energy, resulting in higher losses than air-insulated transmission lines.