What is the advantage of using a ladder line in amateur radio?

 As you can see in the picture, open parallel wire transmission line is often called a ladder line. It has two bare wires held at exactly the same distance apart by insulating spacers which could be made of plastic. One of my local friends used pieces of drip irrigation tubings as spacers as they were UV protected! Advantage of ladder line is that by using a minimum amount of dielectric insulating material, attenuation or power loss per meter is less less than the usual coaxial cable. As a comparison, if the loss in coax is 2 dB, corresponding loss in a ladder line may be just 0.2 dB. If insulated wire is used, relative permeability of the insulation will alter the characteristic impedance of the ladder line. Other factors which affect the characteristic impedance are the ratio of the wire spacing to wire diameter and conductive loss in the rung insulators. Typical impedance though taken as 450 Ohms, could vary and may even be 600 Ohms, requiring an external antenna tuner for matching.

If I want to use a dipole on non-resonant bands or a long wire antenna, an external antenna tuner will be needed for matching the impedance on non-resonant bands. It is in that situation that the open wire or ladder line has advantage as it has lower losses. Ladder line does not obviate the need for a tuner, but reduces the total losses during multiple back and forth reflections from the antenna to the radio in the transmission line when the SWR in the feedline is high.

Whenever there is a mismatch in the feedline, part of the power is reflected back from the antenna, as measured by the standing wave ratio or SWR. Reflected power when it reaches the tuner, is sent back to the antenna once again. As the process repeats, transmission losses are converted to heat energy and could cause burning off the coax at higher transmitted powers. This possibility is also minimised by a low loss ladder line. In short, advantage of ladder line is when you are trying to match non-resonant antennas to your radio with an external antenna tuner which can cater to higher SWR like 10:1. There is less loss of RF energy as heat and improved efficiency of transmission.