Ladder line or "window line"
is a variation of twin lead which is constructed similarly except that the
polyethylene webbing between the wires which holds them apart has rectangular
openings ("windows") cut in it. So the line consists of two insulated wires with
bands of plastic holding them together every few inches, giving it the
appearance of a ladder. The advantage of the windows is that they lighten the
line, and also reduce the amount of surface on which dirt and moisture can
accumulate, making ladder line less vulnerable to weather-induced changes in
characteristic impedance. The most common type is 450 ohm ladder line, which has
a conductor spacing of about an inch.
ref. wikipedia.org
Balanced Ladder Transmission Lines
are available in several types, including; 300 ohm, 450 ohm solid conductors,
and 450 ohm flexible, stranded conductors.
