Manufacturing Requirements for Hi-Def Foam
Coaxial Cable

By Gary R. Geppert, Gepco International Inc.

The frequency requirements for High-Definition (Hi-Def) broadcasting far exceed those of any other video format. The good news is that as facilities begin to migrate to the Hi-Def format, they won't have to worry about the transmission of less-demanding formats such as analog or serial digital. In the past couple of years, connector and jack-field manufacturers have been busy making subtle changes and tweaking impedance in order to improve structural return loss, especially at the higher frequencies above 2 GHz. For the most part, broadcast cable manufacturers have been ready for quite some time with products that meet Hi-Def requirements.

The mission for anyone making a Hi-Def video cable is simple: manufacture a product that has no impedance variations over a very broad frequency range.

How is this done?

That's where the simplicity ends. First, you need a center conductor that has no variations. Remember, impedance is determined by the diameter of the insulation over the diameter of the conductor. Conductor uniformity for solid wire is achieved by paying close attention during the drawing process in order to eliminate die chatter or scratching. This spurious phenomenon will cause multiple high-frequency structural return loss spikes. Die chatter can create so many small spikes that, when viewed electrically on a spectrum analyzer, it looks as if it is some type of hash or noise instead of independent spikes. By looking closely under a microscope, one may observe what looks to be randomly dispersed small divots in the wire. Upon even closer inspection with a measuring gradient in the microscope, a specific distance from one divot to the next can be observed. There are usually many groups of these periodically spaced grooves and this is why it creates so many structural return loss spikes.

Another method to ensure center conductor uniformity of the copper is to stop a couple mils short of the final required diameter at the intermediate drawing process, then draw the remaining two mils at the dielectric extrusion process. By using a clean precise diamond die to perform this single drawing operation, you can clean up many imperfections that were generated by previous drawing operations. This holds true as long as the imperfections do not exceed the depth of your final drawing operation. At worst, you will still reduce many anomalies and lower the structural return loss number at the higher frequencies.

Tension control is extremely important when manufacturing coaxial cable. To picture this process I usually like to use the analogy of recording signal onto magnetic tape (or, in the early days of recording, onto wire). In manufacturing coaxial cable, one also uses a head but instead of applying magnetic information, the application is polyethylene. The consistency at which the plastic is applied is as important and comparable to "wow and flutter" in recording. The control of periodicity at application of plastic is extremely important to reducing impedance variation of the final product. Tension control systems usually consist of a motorized let-out system followed by an accumulation method integrated into a regenerative metering and take-up capstan, monitored through strain gages with a dampened method to adjust and hold a constant tension. Ensuring that the wire does not elongate over a long distance takes some very exacting equipment with the correct control algorithms that properly dampen the system so as not to over-correct. Correct tension control cannot only ensure the uniform application of the plastic, it also can maintain the exacting size of the wire, two extremely important components with regard to manufacturing high-quality Hi-Def coaxial cable.

In the manufacturing process, cable must pass over a series of guides in order to keep it properly aligned through its various paths from payoff to take-up. The wire, both un-insulated and insulated, takes a route over guide wheels called sheaves, which have bearings at their center. In order to predict a potential problem with regard to periodicity, it is best to, wherever possible, use the same size sheaves with the same size bearing. This will ensure that when structural return spikes are discovered they may be easily traced back to a specific size sheave or bearing diameter. To determine the diameter of a spurious sheave or bearing, simply convert the speed of the cable in feet per second, divide by the frequency of the spike in megahertz, multiply by 12 to convert to inches, and then divide by pi (3.1416) to get the diameter.

Coaxial cable anomalies can also be further reduced by the use of precision laboratory bearings. They are usually more costly and harder to find, but with a little perseverance the results are worth the efforts.

Creating foams is achieved by blending specific types of polyethylene. The recipe must be repeatable over a long time span for a very broad product range. In other words, consistency of the compound must be maintained over a long duration while the cable is being run, and repeatable for future runs of the same product. The recipe for one product, for example, a 6/U style, can be completely different for another, perhaps a 25-gage miniature.

Development Process of Gepco's Hi-Def Cables

Gepco's development of a precision video cable for High-Def began back in 1986 with the introduction of VPM2000. This cable literally revolutionized the broadcast market with regard to wiring, due to its small size, low attenuation and ease of termination. Until its introduction, all precision cables for broadcast were constructed with solid polyethylene dielectrics. Velocity of propagation was restricted to 66%, shields were constructed of double braids and outer jackets were also solid polyethylene. All of which made for a very user un-friendly product that was heavy, stiff and did not meet any of the new UL requirements. Video broadcast cables up to this point were modeled after the Western Electric 724 construction, the standard since the 1940's.

Up to this point, the use of foam dielectrics' was prohibitive for use as a precision coaxial product because of inconsistency in the blending of chemical foams, which caused variations of diameters, shrinkage and cracking with variations in temperatures.

With the development of nitrogen gas injection systems, creating foam for use as a dielectric with electrical consistency had arrived. It made it possible to manufacture a product that could be regarded as precision. Blends of materials and delivery of the gas at the extrusion process now made foams harder, more consistent, with broader temperature operating range. Velocity of propagation rose to 82%, four percent better than chemical foam products. Attenuations were much lower than any of the other precision cable available at the time.

So, for Gepco, the research for Hi-Def cable started back in 1985 and has continued to the present. As broadcasters continue their migration into Hi-Def, and as market demands continue to change, Gepco will also evolve its products to stay ahead of the market.

Gepco International, Inc., with headquarters in Des Plaines, IL, and a branch location in Burbank, CA, has been a leading provider of studio and broadcast audio and video cables for more than 20 years. The company also specializes in manufacturing custom cable assemblies, panels, patch bay harnesses and breakout boxes. In addition, Gepco stocks and distributes connectors, patchbays and other cable-related products. For more information about Gepco, visit our Company Profile.