Coaxial cables were developed to carry radio frequency and microwave signals from one point to another with minimal signal loss. A typical microwave coaxial cable consists of an inner conductor and an outer conductor separated by a polymer dielectric. These three layers are then encapsulated in a polymer shell. The outer sheath protects the cable and the signal inside it from any external interference.
Coaxial Cable Specifications:
Velocity of Propagation (Vp): The speed at which electrical energy is transmitted in a cable of a given length compared to the speed of light, expressed as a percentage, is called the velocity of propagation.
Bend Radius: The radius measured from the inner curvature of the cable by which you can bend the cable without damaging the cable or changing the electrical characteristics of the signal passing through it is called the bend radius.
Attenuation (insertion loss): is the loss of signal power from the input end of the cable to the output end. Attenuation is measured in dB loss per cable (eg 19 dB/50Ft.). Attenuation increases with frequency.
VSWR (Voltage Standing Wave Ratio): The ratio of the maximum effective voltage to the minimum effective voltage measured along the transmission line. The lower the VSWR, the better. Higher VSWR means more reflections at the cable interface.
Dielectric: The dielectric separates the inner conductor from the outer conductor. The type of dielectric used determines the electrical characteristics of the cable, namely the level of attenuation, velocity of propagation and the frequencies at which it can operate.
Transmission Line Impedance or Characteristic Impedance: Characteristic impedance is the relationship between capacitance per unit length and inductance per unit length. The diameter ratio of the inner and outer conductors and the dielectric constant in a cable define the parameters involved in determining the characteristic impedance. Coaxial cables are typically available in 50 and 75 ohm impedance values, with 50 ohm cables used for data signal applications and 75 ohm cables used for video signal applications.
Shield: A conductive sheath made of wire or foil that covers the dielectric and center conductor of a cable.
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