Selection of the appropriate coaxial cable

Our last article was about choosing the right connector for your application. But just as important, if not more important, is of course choosing the right type of cable. With this guide, we want to give you some basics to look out for when choosing the right cable for your application.

But let’s start at the beginning, namely with the question of how a coaxial cable is actually constructed:

A coaxial cable consists of an inner conductor. Around it is an insulating layer (dielectric) and a concentric outer conductor. The whole thing is usually surrounded by another insulating layer (sheath). Due to the shielding by the outer conductor, the signal can be transmitted without interference.

Which coaxial cable is right for my application?

When selecting coaxial cables, there are some criteria to consider.

Characteristic impedance

Common characteristic impedances are 50 ohms and 75 ohms. The characteristic impedance of the source (transmitter) and the sink (receiver) must match that of the cable and the connectors. A mismatch will deform the signal or make it unusable.

50 Ohm cables are used in measurement technology, professional antennas and audio technology. 75 Ohm cables are commonly used in video technology, consumer antenna technology and for internal connections (e.g. IF stages).

Attenuation and frequency range

The signal must arrive at the receiver with sufficient level so that the signal can be evaluated. The attenuation of the cable and the inherent capacitance are disturbing factors.

Temperature range

The temperature range of a cable is determined by the materials used, more precisely by the insulating plastics. Common plastics such as the thermoplastics PVC, PUR or PE (polyethylene) can only be used at temperatures up to 70 or 80 °C. The high quality materials such as PTFE are more suitable here. Here, the high-quality materials such as PTFE (Teflon ®), Kapton®, FEP and PFA are superior, as they can withstand temperatures of 165 to 250 °C.

Mechanical properties

In the use of cable connections, specific requirements arise; a solid inner conductor, hard insulating materials and a large cable diameter limit flexibility considerably. A cable with a soft outer sheath (e.g. PUR) and foamed dielectric is sensitive to kicking and bending. Therefore, a choice should be made here as to whether the cables are permanently installed or frequently changed.

Environmental properties

If a fire breaks out, PVC and other halogen-containing insulation materials produce hazardous pollutants. For this reason, the installation of cables with FRNC or LSNH sheathing materials is mandatory in public buildings. In aircraft construction, cables with PTFE, Kapton®, FEP and PFA insulation materials are used because they resist fires longer.

Cables with PE, TPX and PUR jackets are suitable for outdoor use, they are waterproof and UV-stable.

PVC, RADOX® and RESIN® are only suitable for outdoor use to a limited extent because they attract water.

Special properties

Compared to standard cables, low-loss cables have less attenuation with the same thickness. This is achieved by better dielectrics (air, PE foam, PTFE tape). Thus, the inner conductor can be thicker and has a higher conductivity.

Low-noise cables have a conductive layer between the outer conductor and the dielectric, which dissipates the electrons on the surface of the dielectric. This suppresses noise and the microphonic effect when the cable moves.

Cryogenic cables are designed for very low temperatures, such as those found in satellites and special applications.

Triaxial cables are a special case of coaxial cables. Here, a second concentric outer shield is used that does not carry current. This additionally shields the magnetic field of the current-carrying conductors. Triaxial connections are used for highly sensitive measurement technology.

Conductor materials

The outer conductors are usually braided copper strands, which are either bare, tinned or silver-plated.

Inner conductors are solid wire or stranded wire. Copper, steel or aluminium is used here, silver-plated or copper-plated. Bare copper as solid wire is sensitive to tension and crushing. Copper-plated steel stranded wire is very stable, but has a higher electrical resistance.

Standardised cables have the advantage that you can replace them with a cable from another manufacturer without noticeably changing the values. They are specified in the MIL-17F, for example.

Manufacturer-specific cables are usually better than standard cables for the same diameter.

coaxial cables 50Ohm:

coaxial cable with 75Ohm:

Selection of the appropriate coaxial cable

Which coaxial cable is right for my application?

coaxial cables 50Ohm:

coaxial cable with 75Ohm:

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