WHY IS SEMI RIGID THE RIGHT RF CABLE

Mechanical circumstances

The Semi Rigid coaxial cable is easy to bend, wrap, strip, mechanically heard and solder. The cable ends can be seen quickly and quickly. The conductors can be gelled directly onto the printed circuit boards. Miniaturization, components can be quickly managed in the smallest of spaces.

Semi-Rigid coaxial cables are mainly used for applications in high frequency bands up to 110GHz. The cable is unique and it is easily bent to finished shape and still maintains its set after bending. This property makes it ideal for the use with automated bending equipment as well as hand forming. Our vendors are qualified to MIL-DTL-17 and is listed on the Qualified Products List of the U.S Defense Logistics Agency. A very special advantage of it is the 100% shielding with low VSWR, available cable diameters range from .020inch to .500 inch and available impedance range from 10 Ohms to 100 Ohms.

Our diverse range of 27 QPL cables includes copper and aluminum jacketed cables in sizes of .034, .047, .086, .141 and .250 diameters. These are available in copper, tinned copper or silver-plated copper.

Low Loss 50 Ohm Cable: Low Damping Semi Rigid coaxial cables transmit higher performance with low line losses.These cables reduce attenuation by a further 20% and extend the operating temperature to up to 250°C. Please request detailed test reports.

Stainless steel cable 50 Ohm: Stainless steel cables meet cryogenic or medical applications where low thermal conductivity is required.

The center conductor is either a solid or stranded metal wire which acts as the primary electrical signal carrier for any coaxial cable. Most attenuation occurs at the surface of the center conductor due to the „skin effect“ of microwave signals making the finish or plating a very important element. Stranded center conductors are generally only used in flexible cable constructions for added flexibility and longer flex life. In comparison, solid center conductors have lower attenuation and tend to be more amplitude stable with flexure while stranded center conductors tend to be more phase stable with flexure. For larger Semi-Rigid cables, a tubular center conductor can be substituted. The tubular center conductor reduces weight and thermal conductivity without any impact to the electrical performance.

The outer conductor serves many purposes. It is the electrical shield which contributes to cable attenuation and controls RF leakage. Through precision mechanical tolerances, the outer conductor minimizes return loss (VSWR) by maintaining a constant characteristic impedance. The outer conductor is the primary strength member that keeps connectors firmly attached to the cable. It often provides environmental protection and determines the flexibility or how easy the cable can be formed or bent. 

The electro-mechanical performance specified for Semi-Rigid Cables is achieved by a compression fit between the outer conductor and the dielectric core which, in turn, necessitates manufacturing processes that cause deformation of the core by compression and elongation. The resulting stress that is initially non-uniform tends to equalize by cold flow within a few weeks after manufacturing, and will cause withdrawal of the core into the cable. If this occurs in cable that has become part of a cable assembly, the resultant development of an air-void of the cable-connector interface may cause VSWR increases. It is therefore advantageous to achieve core stress relief by preconditioning cable before it becomes a cable assembly.

Preconditioning is not effective on long lengths of cable. Bending of cable, which is usually involved with the manufacture of cable assemblies, tends to introduce non-uniform core stresses; therefore, CarlisleIT recommends preconditioning after bending and before attaching the connectors. Since preconditioning will result in the withdrawal of the dielectric into the cable, preparation of the cable assembly should allow for a ¼ inch length on each cable end beyond the design dimension. The outer conductor and the core should not be cut to the final dimensions until preconditioning has been completed.

A recommended preconditioning procedure consists of three cycles of the following routing:

After the last temperature cycle, maintain the specimen at room temperature for 24 hours minimum before proceeding with further processing. Important is to test the cable in your assembly condition before you produce the cable assemblies.

Stripping outer conductor :

Sections of solid tubular outer conductors can be re­ moved by making a circumferential score or scribe mark and breaking the conductor at this mark. (This is similar to the cutting of glass, scribing a line and breaking it on that line.) The scribe line should be of proper depth, singular and the ends should meet together. This prevents a "step" on the broken edge. The edge must then be rolled between two hard surfaces to minimize the burr on the O.D. of the conductor edge. A lathe or a modified rotary wire stripper can be used for scoring the conductor. Do not penetrate into the Teflon while scoring since this would cause the outer conductor edge to be rolled into the dielectric, resulting in an electrical discontinuity at that point.  If done properly, the edge will have only slight irregularities and will provide satisfactory electrical performance for non-critical ap­ plications.

Cutting the outer conductor with a rotating blade saw or jewelers saw will require subsequent facing operations and careful removal of any chips imbedded in the dielectric. This method is not recommended for ap­ plications which require superior electrical performance. Removal of the section of outer conductor which has been scribed off is accomplished by quickly breaking at the line (similar to snapping a piece of glass) by flexing in two opposite directions.

Do not flex too far since this wil l result in distortion of the conductor edge. The piece can then be pulled off with pliers. Care should be taken not to deform the dielectric by apply ing too much pressure. lf a flush strip to the center conductor is re­ quired here, a razor blade can be used to cut the dielec­ tric. A twist wi l l remove both conductor and dielectric simult aneously (refer to " Trimming dielectric" ). When cutting the dielectric it is important to have a guide to limit penetration, thus avoiding damage to the center conductor. Such damage would not only affect electrical performance but any mechanical Stresses during  subsequent fabrication or operating conditions could cause the conductor to break.

Forming or bending :

A suitable fixture should be used for forming, one having a clamping arrangement for the cable and a mandrel of appropriate dimensions. An ideal mandrel should be grooved to allow the cable to nest thereby avoiding flattening. The tool used to bend the cable should also be grooved so that the cable is completely captivated at the bend tangent point. Large diameter bends may be made on flat mandrels with little or no deformation occuring. Dedicated tooling may be required because of form complexity but automated or semi-automated universally programmed machines are available which will allow versatility and high production capabiIities.

To avoid wrinkling the cable in the bend area, pressure must be applied against the cable at the bend tangent point , forcing the cable against the mandrel. Too much pressure will cause a bulge in the outer conductor at the end of the bend area on the inside. The cable will have a slight amount of "spring-back;' requiring the mandrel to be slightly smaller than the specified bend. This is not a predictable behavior so some experimentation is necessary (usually the larger the bend, the greater the spring-back). These cables should not be formed smaller than the " Safe bend radius" specified in this catalog (see tables). Otherwise, severe electrical and mechanical degradation will occur.