Propagation ifactor2/28/2023 R DC is the overall DC resistance of the cable, W /m, If there are two shields (triaxial cable), use the DC resistance of only the inner shield.ĭCR CENTER is the resistance of the center conductor, W /m,ĭCR SHIELD is the resistance of the center conductor, W /m, and These parameters are generally listed on the cable datasheet. R DC is calculated as the sum of the DC resistance per meter of the center conductor and the shield. Parameters Z, v, and R DC generally appear directly in a coaxial cable datasheet. For most coaxial cables a reasonable value for w is 10 MHz. What follows is a discussion of how to compute the metallic-transmission model parameters for coaxial cables (See Section 3.1, "Signal Propagation Model"). Channel specifications (which include connectors and patch cords) are always worse (see Section 7.2, "SNR Budgeting"). Please note that these are cable specifications, not overall channel specifications. I've selected only a small number of representative cables for study. Coaxial cables are not subject to rigid standardization, as are UTP and some varieties of fiber cables, so you will find many, many types and varieties of coaxial cabling. Fifty ohms is most popular for test equipment applications 75 W is most popular for audio-visual applications.Įlectrical specifications for selected Belden coaxial cable types appear in Table 10.2. For example, the surface roughness on the inside face of a braided shield will increase the skin effect loss of the cable, but not the impedance.Ĭoaxial cables are readily available with characteristic impedances near 50, 75, or 93 ohms. If, however, the perturbations are small and closely spaced compared to the wavelength of the signals used, and if the imperfections are uniformly distributed, the impedance remains unchanged. Mechanical perturbations in the cross-sectional geometry of the cable can affect its characteristic impedance. The onset of the skin effect happens at about 100 KHz in RG-58 and related coaxial cables. R is the dielectric constant (real part of relative electric permittivity) of the insulating material,ĭ 2 is the inside diameter of the shield, m, andĭ 1 is the effective diameter of the signal conductor, m. The following equation shows the impedance obtained in the skin-effect region, with signal current flowing only on the facing surfaces of the inner and outer conductors. The dielectric exists to hold the center conductor in the middle of the structure, keeping it from flopping over to one side and shorting against the shield.Ī good coaxial cable presents a nearly uniform impedance at all frequencies above the onset of the skin effect. Epilogue Why Ethernet 10BASE 5 Picked 50 WĬoaxial cable is a single-ended transmission medium, meaning that the signal current flows through the main signal path (the inner conductor) and then returns to its source through a low-impedance, hopefully zero-voltage-drop return path (the outer shield).Article first published in EDN Magazine, January 4, 2001.Article first published in EDN Magazine, September 14, 2000.
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