Coaxial Cable: Flexible Machining Meets Rising Demand
Coaxial cables have a very broad area of application in networks, test and measurement, radar and navigation, satellite television and the many others based on the transmission of very high frequencies. Use and diversity of cables have strongly increased. Lets have a look at modern automated and semi-automated wire-stripping equipment.
The trend to transmit even larger amounts of data in a shorter time continues. Those technologies that transmit the largest amounts of data or allow a faster access are based on high frequency, which requires high-end copper conductor, fiber-optic cable or a coaxial cable. The large demand is created by the numerous and emerging wireless applications. Wireless data transmission requires antennae in which often one or more coaxial cables are needed.
These trends lead to a higher need for coaxial conductors and higher diversity of cables. Coaxial cables are used on short to medium distances, from a few centimeters to a few meters, to transfer signals. The damping on such distances is neither to big nor does it require an expensive modular technology like optical fibers. Moreover, coaxial cables can transport capacity that is vital for transmitters. Further advantages of the coaxial cables are their flexibility, mechanical stability, large diversity of materials, easy contact ability and signal speed, which are independent of the frequency. The demand for coaxial cables is continuously increasing.
The basic design of a coaxial cable
In principle, a coaxial cable is an electric line consisting of two concentrically arranged conductors. In between these wires a dielectric material is located that insulates both conductors from each other. The surface of these metallic conductors shows an exceedingly good electric conductivity. An electromagnetic field spreads out between the inner and outer conductor in the dielectric material. In the majority of cases, a jacket that performs several functions covers the outer conductor. First, it increases the mechanical stability, and in many wire types it is required for secure plug assembly. Furthermore, it protects the cable from virtually all kinds of environmental influences, especially water penetration, chemical damage and heat that would otherwise diminish its electrical and mechanical properties.
The Main Types Of Coaxial Cables
Depending on the requirements, the cables vary in their build-up. For example, the semi-rigid type can have an outer conductor made of a massive metal tube, and are so stable in their use that they do not require an additional cable jacket. The closed tubes guarantee a very good shielding; however, they can only be bent once. The Heliax type, for instance, is more flexible and can be bent repeatedly thanks to its corrugated structured metal tubes. The Sucoform version has a tin-plated copper meshwork outer conductor as an alternative to stiff or corrugated metal tubes. The tin closes the holes in the meshwork, to provide good shielding. This design allows for repeated cable bending. It has a disadvantage on the damping due to the high specific resistance of tin. Very flexible are cables with an outer conductor made of wire netting. The increased flexibility however is at the cost of a lower fault resistances since the shielding effect of the outer conductor greatly decreases at frequencies above 1GHz due to the openings in the cable netting.
Coaxial cable with tinned shielding, with and without outer conductor (Sucoform) - Typical application area GSM, GPRS and UMTS anennae
A typical coaxial cable with plaited shielding, the most used type of different applications
Coaxial cable with corrugated, massive shielding (Heliax) - Typical application area is connection of antennae with base station
Micro-coaxial cable (< AWG 38) with braided shielding - Typical application area is medical technique
Coaxial cable with stiff, massive shielding including plug (semi-rigid) - Application area: Used for high frequency, large outputs and intensive mechanical strain
Machining Of Coaxial Cables
Coaxial cables can be processed manually, semi-automatically or automatically. The manual processing is only appropriate for simpler cables with low-quality requirements as to stripping and for small quantities. Most often, it is not possible to achieve the required stripping tolerances, and the quality depends upon the slits with hand tools. Besides, the processing time is longer than with other equipment.
The semi-automatic processing is suitable for small to middle sized lots. With this procedure, the cables first get cuts to conform to the required length, and are subsequently fed by hand into the machine. The machine initiates the stripping process, and all other steps are performed automatically as soon as the cable hits the stopper.
With a fully automated machine, cables get processed through the entire operation, for example, they are cut to length, stripped on both ends and discarded. Cutting to length and stripping on an automatic machine are worthwhile for middle to large batch sizes or for special types of stripping. For example, a so-called window stripping can only be done using such equipment.
A coaxial cable will be used for achieving high-quality connections. The most important prerequisite to deliver those cables is an automated, highly precise machine-processed stripping of wide spectra of different types. To achieve and guarantee the required quality, all mechanical parts of the stripping head must be set with extreme precision, and the stripping process itself must be reliably monitored and guided. The following parameters should be adjustable and programmable:
- Cutting position
- Cutting diameter
- Blade-feed speed
- Revolution of the stripping head
- Rotation direction of the stripping head
- Cutting time
- Blade retraction before stripping (wayback)
- Stripping length (part or full strip)
- Stripping speed
- Stripping with or without rotation of the stripping head
- Free selectable stripping cycle
- Retention force of the jaws / pressing force of the tape drive
In addition, it is of utmost importance that the cable is treated gently during processing. Damage to the insulation material reduces mechanical stability and resistance against environmental influences such as water and chemicals. Too large a pressing force on the face during operation can change the geometry of the cable and impair its electrical properties. This is unacceptable in high-frequency technology, and a main reason why belt drives with a relatively large, soft contact surface and programmable jaws have prevailed.
|