Introduction to the Production of Coaxial Cable
Coaxial Cable production is a very complex process. Coax cable is an electronic component that has to meet strict manufacturing specifications. These specs include chemical, electrical, and mechanical characteristics. The process of bringing all of these factors to production of coaxial cable is a result of many years research in several fields of science.
Coaxial cable is called unbalanced cable since it contains a inner conductor inside the outer conductor. A balanced cable contains two wires the same, side by side, like the type used for TV antennas that is 300 ohms. Coax is used for TV antennas also but since it’s impedance is so low, usually 75 ohms, it requires an amp and a balun (balanced to unbalanced) transformer adapter to provide enough signal.
As a young, recently graduated, electronic technician, I had the privilege of working for the world’s largest manufacturer of coaxial cable designed for cable TV. It was an interesting job for an electronics technician working for a company making a basic electronics component. I also became involved later with the company, at the ground floor, developing fiber optic cables for telecommunications. Some of the equipment and production methods carried over to the new fiber optic cables but will be the topic of another article.
Physical Characteristics of Coaxial Cable
Physically, coaxial cable is made starting with a center conductor of wire. By the use of rotating reels the wire is pulled through an extruder that applies the plastic to the wire. Uniformity and size is determined by precise speed, extruder pressure , temperature, and proper addition of a catalyst to the polyethylene. Nitrogen is the most common catalyst although there are several others. When nitrogen is added to the polyethylene the plastic forms a foam like texture. The initial polyethylene is more like the plastic used for plastic milk cartons. The resulting plastic is lighter and is much less dense.
Once the plastic is extruded onto the center conductor a metallic layer is added. If coax is the type to be used for home connections, called drop cable, it is sent to the braiding department. Trunk distribution cable has a more rigid, solid, metallic layer that is less flexible. The braided layer is applied by a machine that uses many reels of very small wire and forms a braid on the plastic coated center conductor as it passes through the braiding machine. The solid outer conductor is typically applied by passing the foamed center conductor through a long tube and swagging it. The swagging process uses a dye that presses the tube to the desired size.
The next production step is to apply an outer layer of protective plastic. Polyethylene is used for the outer layer and applied much the same as the first layer minus the foaming process. The outer layer must be stronger to protect the internal materials and maintain the structural integrity. Carbon black is added to the plastic to give it resistance to ultraviolet radiation from the sun. The suns rays causes polyethylene plastic to slowly break down and become less flexible over time and crack. Coax installed inside buildings can be any color, but outside cable will always be black because of the carbon black addition.
Electrical Characteristics of Coaxial Cable
The electrical characteristics are complex, yet built on simple principles. The main thing is to maintain consistency of specific material measurement that result in certain electrical specifications. These measurements vary according to what nominal impedance you need (usually 75 or 50 ohm), electrical power and band width of frequencies. Bandwidth is very important. The more bandwidth the cable can pass without causing undo resistance of any of the frequencies, the more channels that can be passed through the cable.
The most important electrical spec results from the measurement of the ratio of the inner conductor and the outer conductor. The inner conductor’s plastic later insulates the inner and outer conductor and determines the spacing size. Dielectric constant of dry air is close to one and makes the best dielectric but is impractical. The separation of materials is achieved close enough with the dielectric constant of foamed polyethylene since the foam has pockets of air, has low density, and remains flexible. The ability of the finished product to be bent and maintain it’s electrical integrity is one of many quality control tests. Another quality control is the ability of the inner conductor to maintain it’s position while being pulled on. A regiment of other tests are performed to produce a quality product.
Conclusion of Coaxial Cable Manufacturing
A large number of things go into setting up a coaxial cable production facility. Some of this knowledge is proprietary to the industry and it would be unethical for me to reveal it. The electrical characteristics of coax is well known and documented in many electronic textbooks. The production process and quality control procedures are less known. This article provides some unique insight into the manufacturing and production of coaxial cable.
Sources: Personal/professional experience