What is the nominal characteristic impedance of “window line” transmission line?

"Window line" is a highly efficient type of parallel-conductor feed line. It is constructed similarly to traditional ladder line but uses a solid ribbon of polyethylene plastic with rectangular "windows" cut out of it. This design reduces dielectric losses and improves performance, especially in wet weather. The physical dimensions of standard window line—specifically the spacing between the two parallel wires and the gauge of the wire used—are engineered to yield a nominal characteristic impedance of 450 ohms. While other feed lines have different standard impedances, such as 50 ohms or 75 ohms for typical coaxial cables, or 300 ohms for traditional TV twin-lead, 450 ohms is the standard impedance specific to window line.