Why use open wire feed line for a dipole instead of coax?
My good friend Mike sent me this information when I asked him about open wire fed dipoles. He knows a thing or two about antennas!
In my opinion, there are 2 reasons to use ladder line over coax.
Loss and Tunability.
There are three kinds of loss in RF transmission lines. Resistive losses from wire resistance, Reflective losses from mismatched impedance, and Dielectric losses. Coax uses a dielectric, making contact with the center conductor the whole length of the feedline. Dielectrics have a property called the loss tangent, which is a major contributor to the loss per unit length of coax. The dielectric insulation material absorbs RF power to a very measurable extent. With Ladder Line there is very little contact area between the spreaders and the conductors, and when the other two type of loss are addressed, can be an extremely low loss feed system.
Coax feedlines are fine for single frequency use at low frequencies where length can be short and losses are insignificant however, impedance matching options for multiband use are more limited with coax. Re-tuning the feedpoint of coax for a match does not guarantee that the system will radiate efficiently, it only makes the transmitter see a resistive 50 ohm load. Also the losses of coax are applied to reflected power as well and can make a mismatch appear better than it actually is.