Cable Design Equations
Balanced Pair
| Cable Type |
CAPACITANCE (pF/ft) |
IMPEDANCE (Ω) |
Unshielded
Twisted Pair |
 |
 |
Shielded
Twisted Pair |
 |
 |
Overall
Shielded &
Cabled |
 |
 |
where:
C = Mutual capacitance (pF/ft.)
d = Diameter of the conductor (in.)
D = Diameter over the insulation (in.)
ε = Insulation dielectric constant (see Table II)
f = Stranding factor (see Table I)
ZO = Characteristic impediance
No. of
Strands |
Stranding
Factor
f |
| 1 |
1.000 |
| 7 |
0.939 |
| 19 |
0.970 |
| 37 |
0.980 |
| 61 |
0.985 |
| 91 |
0.988 |
TABLE I |
| Material |
Dielectric
Constant ε (pF/ft) |
Velocity of
Propagation
VP (%) |
| ECTFE (Halar®) |
2.60 |
63 |
| PFA (Teflon®) |
2.15 |
68 |
| PVC |
5.00 |
45 |
| PVC (Semi-rigid) |
3.60 |
53 |
| PVDF (Kynar®, SOLEF®) |
7.70 |
36 |
| Polyethylene |
2.29 |
66 |
| Polypropylene |
2.25 |
67 |
| Polyurethane |
6.50 |
39 |
| Rubber - butyl |
4.0 |
50 |
| Rubber - natural |
5.0 |
45 |
| Rubber - SBR |
4.0 |
50 |
| Rubber - silicone |
3.1 |
57 |
| TFE Teflon® |
2.1 |
69 |
| TPE |
5.0 |
45 |
| Teflon® |
2.10 |
69 |
| Tefzel® |
2.6 |
62 |
TABLE II |
Coaxial Cable
where:
C = Capacitance (pF/ft.)
d = Diameter of the center conductor (in.)
D = Diameter under the shield (in.)
ε = Insulation dielectric constant (see Table III)
f = Stranding factor (see Table I on previous page)
L = Inductance |
| Material |
Dielectric
Constant
ε (pF/ft) |
Power
Factor
(PF) |
| FEP (Cellular) Teflon® |
1.40 |
0.0002 |
| FEP (Solid) Teflon® |
2.10 |
0.0003 |
| PE (Cellular) |
1.56 |
0.0003 |
| PE (Solid) |
2.26 |
0.0003 |
| PE (Semi-Solid) |
1.29 |
0.0003 |
TABLE III |
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