SS72012 datasheet(122/144 Pages) ACIT

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Coaxial Cable Formulae

The following formulae can be used to calculate performance characteristics of coaxial cables with various

dielectric and conductor materials.

Characteristic Impedance:

Coaxial cables are typically 50Ω, 75Ω, or 95Ω impedance. Cables with 50Ω

impedance are the most common, because they offer the best balance between maximum power

transmission and minimum loss. Where minimum attenuation is the most important consideration, such

as in CATV systems, 75Ω cable is more widely used.

Attenuation:

Losses occur in coaxial cables both from conductor loss and dielectric inefﬁciency. PTFE has

become the most commonly-used dielectric in MIL-C-17 and other coaxial cables because it combines a

low dielectric constant with good mechanical stability through a wide temperature and frequency range.

VSWR (Voltage–Standing Wave Ratio):

VSWR is one of the most important characteristics of a coaxial

cable, because it is the measure of the cable’s overall efﬁciency in transmitting a signal at a given frequency.

It is expressed as the ratio of the cable’s mismatch to a perfect match, i.e. 1.25:1. Advanced, consistent cable

manufacturing techniques minimize not only the overall VSWR, but can also minimize or eliminate VSWR

spikes at speciﬁc frequencies.

Cutoff Frequency:

The cutoff frequency of a coaxial cable is the frequency at which it no longer transmits

its TEM (Transverse Electromagnetic Mode) signal.

Dielectric Constants

Air . . . . . . . . . . . . . . . . . . . . . . 1.0

E Glass . . . . . . . . . . . . . . . . . . 6.0

ETFE. . . . . . . . . . . . . . . . . . . . . 2.6

Expanded PTFE. . . . . . 1.4–2.0

FEP. . . . . . . . . . . . . . . . . . . . . . 2.0

Foamed FEP . . . . . . . . . 1.5–2.0

Mica Glass. . . . . . . . . . . 1.2–3.0

Nylon . . . . . . . . . . . . . . . . . . . 4.5

PFA. . . . . . . . . . . . . . . . . . . . . . 2.0

Polyethylene . . . . . . . . . . . . 2.3

Polyimide . . . . . . . . . . . 3.0–3.5

Polyimide/FEP ﬁlm . . 2.2–2.3

Polypropylene . . . . . . . . . . . 2.3

Polysulfone. . . . . . . . . . . . . . 3.1

PTFE . . . . . . . . . . . . . . . . . . . . 2.0

PVC . . . . . . . . . . . . . . . . . 4.5–5.8

PVF . . . . . . . . . . . . . . . . . 3.0–8.4

Silicone Rubber . . . . . 2.1–3.5

Urethanes . . . . . . . . . . . 6.7–7.5

Coaxial Cable Formulae

Capacitance (C) =

Picofarads per foot

7.354 x E

Log10

ohms

D + a

d x f

()

Reﬂection Coefﬁcient = I =

Zr – Zo

Zr + Zo

VSWR – 1

VSWR + 1

VSWR =

1 + Á

1 – Á

Impedance (Zo) =

Ohms

138 Log

D + a

d x f

()

√

Inductance (L) =

Microhenries per foot

.140 Log10 (D / d)

Velocity of Propagation =

% of speed of light

Time Delay =

Nanoseconds per foot

1.0167 x E

√

100

√

Cutoff Frequency =

GHz

7.50

(D + d) √

d =

Outside diameter of inner conductor,

in inches.

D =

Inside diameter of outer conductor,

in inches.

E =

Dielectric constant of insulation

(see below).

a =

Nominal shield correction factor

(1/2 of the diameter of an

individual shield wire).

f =

Correction factor for stranded conductors:

Solid conductor: 1.00

7 strands: .93

19 strands: .97

37 strands: .98

A =

Reﬂection coefﬁcient.

Log =

Logarithm to base 10.

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