What is “twin-lead” transmission line

Twin-lead line was once the dominant transmission medium for consumer products such as TVs; it has been largely, but not entirely, made obsolete by coaxial cable.

When engineers need to make a wired RF connection to create a transmission path or line between two subsystems such as a transmitter/receiver and its antenna, their first and perhaps only thought is to use coaxial cable (coax). That makes sense, as coax has a modest loss, is fully shielded, offers a variety of terminations and connectors, and has some environmental ruggedness as well.

Alternatives to coaxial cables exist, but they don’t offer the flexibility. Waveguides offer better loss and interference performance but are costly, complicated, and difficult to terminate. They are still used in specialty situations, such as when thousands of watts are being transferred, but these are only a small fraction of the transmission-line scenarios. Another option for very low power is to use PC-board stripline or microstrip topologies, but these are limited to low power and for use within a single circuit board.

However, there’s another transmission-line option that was once widely used, especially in consumer and mass-market situations, but has fallen out of favor: the open-wire or twin-lead transmission line. This article explores its construction and technical characteristics, why it was once so popular, and why it has largely but not entirely been superseded.

Twin-lead basics

Open-wire is a balanced ungrounded) transmission line which is conceptually and mechanically simple. The electromagnetic (EM) principles of this approach and the mathematics that support it (think Maxwell’s equations) were analyzed in the early days of EM field theory and understood fairly well.

The arrangement has two parallel conductors that do not need to be insulated (usually made of stranded copper or copper-clad steel wires) (Figure 1). This separation is maintained by using insulating spacers every few inches (often polyethylene or Teflon for better performance). The separation distance between wires is critical and is determined by the desired line impedance.

Figure 1. The open-wire transmission line is balanced with respect to the ground; its electromagnetic field performance was fully analyzed using Maxwell’s equations. (Image: Slideplayer)

Figure 2. (top) Open-wire line is also available as twin-lead with a continuous separation using a low-loss dielectric while also providing physical protection; (bottom) an actual piece of twin-lead with ends stripped for connecting. (Image: Slideplayer; MFJ Enterprises)

Off-the-shelf versions are available with a low-loss dielectric between the wires as a solid body which protects and insulates the entire arrangement (Figure 2). Note that “open-wire” is the general name for this class of transmission line with parallel-wire construction; when it is commercially fabricated with protective insulation and continuous separation along its body, it is usually referred to as “twin-lead.”

As with any transmission line, the characteristic impedance is a key parameter. For open wire, the impedance is primarily a function of the wire gauge, spacing, and material between the two wires. The equations that define the relationship between the dimensions and impedance Z_o are complicated. However, they can be reduced to create a fairly accurate approximation.

where d is the wire diameter, D is the separation of the wires measured between their centerlines, and ε_r is the absolute permittivity between the wires (for air, it is very near unity at 1.00054). The actual impedance value will also be affected by the spacers which are used between the wires, or the solid material used for separating and encapsulating the two leads. The most common twin-lead impedance in use is 300 Ω, but other higher and lower values can be constructed.

Where twin-lead was used

Open-wire lines in various versions have been used by wireless experimenters, researchers, and radio amateurs for transmission lines since the earliest days of RF in the 1910s. They were inexpensive and could be made up “in the shop” as needed, with the desired impedance to minimize standing waves (VSWR). Those were true DIY days, as you could not purchase components but had to make them.

However, the mass-market, consumer-driven use of twin-lead really took off with the development and adoption of consumer TV in the 1950s with the use of roof/chimney-mounted antennas before cable TV as we know it came into being. There are several strong reasons for the use of twin-lead in this situation:

It is expensive per foot, and the antenna-to-TV distance could be 50, 100, or more feet, so cabling costs would be an issue in many consumer applications.
With small dimensions of about one-half inch across and the right wire size, its impedance was 300 Ω. This matched the natural impedance of the widely used folded dipole antenna. If a larger non-folder dipole was used with its 75-Ω characteristic impedance, a simple inexpensive 4:1 impedance transformer could be added in line.
Twin-lead was also convenient for connecting dipole FM-radio antennas to receivers.
Finally, the twin-lead has very low RF loss at the lower RF frequencies. This figure of merit is critical for getting enough signal strength from the antenna to the receiver front end. Although numbers vary with fabrication details, twin-lead has a loss of about 0.55 dB/100 meters at 30 MHz, while common RG-58 coaxial cable has a much-higher loss of 6.6 dB/100 meters.

Figure 3. The stripped ends of the twin-lead are usually connected to the electronics or antenna using basic spade lugs which go under screw terminals; in contrast, coaxial cables have a variety of more-advanced connectors. (Image: LinkedIn)

In widely available 300-Ω ohm twin-lead, the wire is usually 20 or 22 AWG gauge (0.52 or 0.33 mm²), spaced about 0.30 inches (7.5 mm) apart. Termination of twin lead is generally done with spade lugs that go under a screw head, in contrast to the more-sophisticated coaxial-cable connector (Figure 3).

In addition to long rolls of twin-lead to be used by installers, it was so common and popular into the 1970s and even 1980s that you could buy it almost anywhere including supermarkets, hardware stores, appliance stores, and other non-electronic retail outlets. For mass-market consumers, it was in standard lengths such as 50 or 100 feet and pre-terminated with crimped-on spade lugs (Figure 4).

Figure 4. In addition to long unterminated rolls, flat twin-lead was sold to consumers in standard lengths and pre-terminated to save time and minimize frustration; it is still available that way. (Image: Solid Signal)

What led to the twin lead’s fall from favor

There is no single reason, but there are several very strong ones:

It is unshielded and is very susceptible to electrical interference. While external shielding can be wrapped around it, doing so degrades its performance and raises the cost.
The twin-lead routing path from antenna to TV had to be carefully planned and kept away from nearby metal objects which would affect its impedance and losses. To install it, “standoffs” were used which kept it a few inches away from the objects and structures such as the building’s siding. Installing it properly was a challenge; if not done right, performance would be degraded and inconsistent.
Unavoidable phase shifts due to less-than-perfect twin-lead installation and interference resulted in color shifts in the analog color TV display since that long-lived TV standard used phase detection to establish color rendition.
The RF performance of twin leads, especially with close wire spacing, degrades as operating frequencies reach hundreds of megahertz. At those higher frequencies, the dimensions needed are too small, the tolerances are too tight, and the parasitics are too large.
By far, the largest factor was the widespread adoption of cable TV to replace over-the-air broadcasts (this was before our present “cord cutting”). There was no need for a physical antenna-to-TV transmission line, and twin-lead is not viable for stringing from pole to pole and then down to individual households. Instead, coaxial cable became the far better choice.
It’s difficult to connect an amplifier to a twin-lead transmission line. Yet, for cable TV, amplifiers are needed to boost the signal to each TV and isolate individual “drops” from each other.
For non-cable applications such as small-aperture rooftop dishes where received signal strength is very low and signal-noise ratio (SNR) is marginal, the low loss of twin-lead would seem, at first, to be an advantage. However, the carrier frequencies were too high while the cable-routing issues were challenging. Also, the availability of tiny, transistorized pre-amplifiers that could be mounted at the antenna feed itself (called LNAs), with needed DC power piggybacked on the RF coaxial cable, made these front-end LNAs a very attractive solution to the loss and SNR problem.

Figure 5. Despite the challenges of working with open-wire, it is still used in some specialty situations, such as this ham-radio transmission line, which connects a widely separated antenna and its receiver front end. (Image: Reddit).

Today’s open-wire and twin-lead world

Despite its long fall from a highly favored transmission line to being barely noticed, you can still buy many versions of twin-lead as standard, off-the-shelf items. Today, open-wire and twin-lead are used primarily in specialty situations where some of its positive attributes outweigh the negative ones. These include installations where a unique impedance is needed for the transmission lines and is not available as coaxial cable. It is also used by amateur radio hobbyists who have a location where open wire can be properly installed or where cost and low loss are crucial (Figure 5).

There are times when you do need to go from a balanced transmission line such as open wire to a grounded, single-ended transmission line such as coaxial cable. The solution is to use a balanced/unbalanced passive transformer (usually referred to simply as a “balun”), which provides the transformation in either direction (Figure 6).

Figure 6. The balun transformer is used to interface twin-lead with coaxial cable. (Image: Chegg)

Baluns cost only a few dollars and are still available. The technical benefits and ease of installation of coaxial cable far outweigh its higher loss at lower frequencies — a problem that can be easily overcome with preamplifiers — while the cost of coaxial cables has decreased significantly due to its high volume. Open-wire and especially twin-lead were solutions before coax arrived and even when coax became available, but coax offered a better solution. The market and technical needs make open-wire and twin-lead cables a much less desirable and generally impractical approach to the transmission-line challenge.