What is a ladder line and how does it function?

Ladder line is one of the three types of twin-lead parallel wire balanced transmission lines used for radiofrequency applications. It is a balanced transmission line unlike the popular coaxial cable which is an unbalanced transmission line. Unbalanced transmission lines have an unbalanced current while balanced transmission lines have two balanced currents in opposite directions on the two wires.

In case of coaxial cable, in addition to two currents in opposite directions on the inner conductor and inner aspect of the outer conductor, there is a third unbalanced current on the outer aspect of the coaxial shield. That is also known as a common mode current.

Other two types of twin lead parallel wire feeders in use are the twin lead ribbon cable and the window line. Ribbon cables were commonly used for TV installations a few decades back and had a characteristic impedance of 300 ohms.

A balun was used to match impedance at the TV receiver for transforming impedance from 300 ohms to 75 ohms. Similar impedance transformers or a balanced antenna tuner is needed while connecting a high impedance ladder line to a low impedance radio.

While the twin lead ribbon cable had continuous dielectric sheet between the two leads, the window line had rectangular openings cut at fixed intervals as ‘windows’. Nominal impedance of window line was 450 ohms. Windows helped to reduce the weight of the the feed line. The surface available for accumulation of dirt and moisture was also lower for the window line. The size of the rectangular window helped the manufacturer to adjust the characteristic impedance of the transmission line.

Coming back to the ladder line, it is called so because of resemblance to a rope ladder. Impedance of the ladder line may be in the range of 500 – 600 ohms, depending on the construction. The rungs of the ladder hold the parallel conductor lines apart. In the ladder line which I homebrewed, the spacers were made of two inch long drip irrigation pipes of 6 mm diameter. They were held in place by nylon ties. Spacing between the rungs was about 20 cm each. The ratio between the wire space and the wire diameter determines the characteristic impedance partly. I had used 2.5 sq mm insulated copper wire. Dielectric properties of the insulators will also affect the characteristic impedance.

Twin lead lines have much lower transmission line losses when compared to coaxial cable. One figure mentioned is that RG-58 coaxial cable has a loss of 6.6 dB per 100 meters. 300 ohm twin lead losses are in the range of only 0.55 dB for the same length of feedline. But twin leads are more vulnerable to interference by proximity to metal objects like metal fences and metal roofing. They need standoff insulators when run up metal antenna masts.

Interference is likely because nearby metal objects, including the iron in concrete, inject currents induced in them into the feedline, thereby causing it to lose its balanced nature. That is also the reason why the ladder line has to take off from the antenna feed point at 90 degrees angle for at least one quarter wavelength.

This public domain animation from Wikipedia shows a wave travelling along a lossless transmission line. Black dots represent electrons and the arrows show the electric field. Transmission line losses are higher as the frequency goes up and needs special methods of construction to reduce losses.

Purpose of a transmission line is to take the signals from the radio to the antenna without radiation along its path. Radiation along its path can cause radio frequency interference to other equipment in the shack and reduce the signal available for proper directional radiation from the antenna. Of course, there are special situations like in tunnels and mines where leaky cables are used when communications from outside is not possible otherwise. A leaky feeder, otherwise known as a radiating cable, is used in underground mining and other tunnel environments to provide a communication system. There are gaps in outer conductors to allow radio signals to leak out and into the cable in leaky feeders.

As frequency increases, losses from ordinary wires by radiation increases. That is why ordinary wires are useful for low frequency alternating currents of mains supply, but not for radiofrequency currents. In a parallel wire transmission line, the two currents in opposite directions produce electromagnetic fields in opposite directions. The net effect is that there is no significant radiation of electromagnetic waves from parallel wire transmission lines.

In a coaxial cable, the common mode current can radiate from the feedline as it is an unbalanced current. Special arrangements like common mode chokes are used to prevent the flow of unbalanced current in a coaxial cable.