Twisted pair characteristic impedance calculator

Similar online calculators compute the characteristic impedance of a twisted pair using a simplified formula. The disadvantage of these calculators is that the user is prompted to enter some, taken from nothing, the average permittivity between the permittivity of air and of the wire insulation dielectric. The fact is that the effective permittivity of the space between the conductors er_eff is less than the permittivity of the insulation material e_r, since part of the electric field lines passes through the air. In addition, the simplified formula incorrectly calculates the characteristic impedance in a transmission line with a small distance between the conductors. Well, twisting is not taken into account at all. Let's fix these flaws...

This calculator uses the method with calculation formulas by the link [3]. With this method, er_eff is automatically calculated taking into account the above effect. In addition, the effect of twisting wires is taken into account. Note that the twisting reduces the characteristic impedance of the transmission line. When using wires with enamel insulation and a twisting step of several turns per centimeter, it is possible to get a TL characteristic impedance of 10..20 Ohm, which is for example necessary when creating a TLT to sum the power of transistor RF amplifiers. Do not forget the geometry, from which it is obvious that the distance S is equal to the diameter of the wire with the insulation.

INPUT DATA:

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Select measurement units

International System of Units (SI)US customary units (USCS)

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Wire diameter d:		[mm]
Distance between conductor centers S:		[mm]
Length of one twist p:		[mm]
Insulation material permittivity er:		ManuallyEnamelPolyvinylchloride PVCPolyethylene PETeflon PTFE

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Calculate

RESULT:

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Javascript source code:
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Related links:

1. Twisted Pair impedance calculator 1 - the calculator computes an untwisted two-wire TL by the simplified formula (at S » d), that isn't correct for TL with a small gap between conductors, more about this in our online calculator of the two-wire balanced TL. Therefore, despite the title, this calculator is actually quite unsuitable for calculating the characteristic impedance of a twisted pair;
2. Twisted Pair impedance calculator 2 -  another of the many clones of supposedly twisted-pair calculators, that even computes an untwisted pair incorrectly;
3. Twisted-pair transmission-line distributed parameters - more accurate modeling by T.Kien.Truong [PDF] (used by the calculator on this page);
4. Ethernet cables comparison between CAT5, CAT5e, CAT6, CAT7 Cables;