How does parallel conductor transmission line compare to coaxial cable with a plastic dielectric?
Why Open-wire or ladder line uses mostly air as its dielectric, with only small spacers touching the conductors, so dielectric heating losses are minimal. The conductors are also spaced widely, which raises the characteristic impedance and lowers the current for a given power, reducing I-squared-R loss. Solid plastic dielectric coax, by contrast, dissipates energy in the polyethylene, especially at VHF and above. That is why open wire is preferred for long runs and for feeding antennas at high SWR.
Watch out The velocity factor choice has it backwards: air-spaced line runs about 0.9 to 0.95, while solid polyethylene coax is near 0.66. SWR and reflection coefficient are set by the load and the line impedance, not by the type of line construction.
Air beats plastic: open wire has less dielectric, so less loss and a higher velocity factor.