Cable Design Equations

Balanced Pair

Cable Type CAPACITANCE (pF/ft) IMPEDANCE (Ω)
Unshielded
Twisted Pair
Cable Design Equations Cable Design Equations
Shielded
Twisted Pair
Cable Design Equations Cable Design Equations
Overall
Shielded &
Cabled
Cable Design Equations Cable Design Equations

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

Measure Equation UNITS
CAPACITANCE Cable Design Equations pF/ft
IMPEDANCE Cable Design Equations

Ω

INDUCTANCE Cable Design Equations H/ft
VELOCITY OF
PROPAGATION
Cable Design Equations %
TIME DELAY Cable Design Equations nsec/ft
CUTOFF
FREQUENCY
Cable Design Equations GHz

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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