“Why doesn’t it LOOK like a ladder?”

07Apr16

An RF transmission line short story about open wire feeder, ladder line, twin lead and the window line that we now use.

In years past, before the advent of coaxial cable and the need for SWR meters, uninsulated open wire feed lines were built by hams using non-conductive ceramic insulators. These efficient, low-loss RF transmission lines had a typical impedance between 200 and 600 ohms because they were between three and six inches wide.

Perfectly suited for the vacuum tube era of high impedance link-coupled tuner coils, their evenly-spaced insulators made those feedlines appear somewhat like rope ladders, hence the term “ladder line.” This moniker was indelibly etched into Amateur lore with the introduction of manufactured versions in the 1960’s and ’70’s, often made with white plastic insulators melted into the parallel solid copper wires. Conversely, earlier TV twin lead and transmitting twin lead were much narrower, old plastic-clad parallel transmission lines that had much higher losses at high SWR, like coax.

So let’s fast-forward a couple of decades to our modern, efficient 300 ohm and 450 ohm parallel-wire feedlines which have far lower losses than the old twin lead. Just as with open wire feeders, elevated SWR isn’t a problem for the present day version due to advances in plastics and the use of air. Originally called “window line,” because of the shape of missing sections of dielectric, now these feedlines are generically relabeled as Ladder Line, because hams just do that.

These lines are sometimes colloquially referred to as twin lead or open wire feeder. Our Ladder Lines are now used on DX Engineering Reversible Beverage Antenna Systems or DX Engineering baluns with wide range tuners to create extremely efficient HF transmitting multi-band wire antenna systems.

In contrast to the impedance extremes of true open wire feeder, and avoiding the excessive losses of coax on non-resonant systems, our lowest impedance, 7/16 in. wide, 300 ohm Ladder Line allows tuners to cover more HF frequencies. DX Engineering “ready-to-use” Ladder Lines eliminate the tedious construction process of true open wire feeders. Comparatively, our Ladder Lines exhibit a virtual constant impedance and equivalent low-loss with the absolute practicalities of lower weight, less wind load, full power handling and, most importantly, durability.

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Tags: coaxial cable, Ladder Line, Window Line