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Coaxial cables - An introduction
Typical coaxial cable constructions are classified as braided, semi-rigid or flat ribbon and described briefly below.
Flexible (Braided) Coaxial Cable is the most common form of coax cable mainly used for its flexibility. It consists of entwined or 'braided' layer of fine
wire acting as the outer conductor; hence the name "braided coaxial cable".
Fig Braided Cable
The braided outer conductor acts a good shield to the center conductor; its effectiveness being measured by the number of strands weaved in the braid and the number of braid layers used in the construction. The shielding offered by the braid is enhanced if an extra aluminum foil is added over the braid, particularly for short wavelength and high frequency applications, where high degree of radiation shielding is of prime importance. Replacing braided cable with a semi-rigid cable in such applications where a high degree of shielding is required mitigates this problem.
Semi rigid Coaxial Cable uses a solid tubular outer conductor usually made of copper, so that all the RF energy is contained within the cable. It is also a preferred cabling option for fixed connections such as inter-subsystem connections inside a microwave system.
The semi-rigid cable connections are expected to remain fixed during the normal operation of the microwave system and not disturbed frequently. Special bending tools are available for bending or giving shape to the semi-rigid cable. Conformable semi-rigid cable is in between flexible and semi-rigid cable in terms of their rigidity and conformability. They are a popular choice of hand formable cable used widely in testing laboratories and systems.
Flat ribbon coaxial cable is special variety of ribbon cable, where multiple coaxial cable runs are grouped side by side and placed together inside an insulating jacket. Each coaxial cable consists of a center conductor separated by dielectric material from the outer conductor or the foil. A drain wire connected to the foil runs along the length of the ribbon coaxial cable and offers an electrical ground connection. Ribbon Coaxial Cable combines the advantages of both ribbon cable and coaxial cable. It is advantageous to use this type of cable when multiple high frequency RF signals are required to be transported to another point in the system, such as a panel mounted termination box. It offers a quick and easy installation option for applications requiring simultaneous transportation of microwave signal, radiation shielding and cross talk shielding from each other. The ribbon coaxial cable can be mass terminated with the insulation displacement technique.
Coaxial cable is widely used in applications such as the following:
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