The lowest loss transmission line on HF is:

Open-wire line, often referred to as ladder line, consists of two parallel conductors separated predominantly by air. Because air is an outstanding dielectric with virtually zero RF absorption, dielectric losses are kept to an absolute minimum. In contrast, coaxial cables rely on a solid or foam dielectric material between the center conductor and the outer shield, which inherently absorbs some radio frequency energy and dissipates it as heat. Because it avoids these solid dielectric losses, open-wire line provides the lowest loss transmission of all the options on HF frequencies.