COAX CABLE

Coax Cable

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Date Published:	November 21, 2012
Last Modified:	January 31, 2022

1. Coaxial Cable

Coaxial cable (or just coax cable) is a type of cable consisting of single core surrounded by a circular outer shield, with a insulating dielectric sandwiched between the two. The term coaxial comes from the inner conductor and outer shield sharing the same axis (they are concentric with one another). Coax is most commonly used to transmit high frequency signals over small (oscilloscope probes) and large distances (transatlantic phone cables).

Figure 1. The make-up of a standard coaxial cable. Relative dimensions accurate for RG-6 (dimensions taken from http://www.farnell.com/datasheets/1697593.pdf).

RG-6 is one of the most common types of coaxial cable in residual homes due to it’s use with TVs.

Properties of popular coaxial cable types:

Name	\(Z_C\)	Conductor Diameter	Insulator Diameter	Shield Outer Diameter	Jacket Outer Diameter	Capacitance
RG-6	\(75\Omega\)[4]	1.0mm	4.57mm	5.2mm	6.6mm	68pF/m[4]
RG-58	\(50\Omega\)				5.0mm	82pF/m[3]
RG-59	\(75\Omega\)[3]

Name

\(Z_C\)

Conductor Diameter

Insulator Diameter

Shield Outer Diameter

Jacket Outer Diameter

Capacitance

RG-6

\(75\Omega\)[4]

1.0mm

4.57mm

5.2mm

6.6mm

68pF/m[4]

RG-58

\(50\Omega\)

5.0mm

82pF/m[3]

RG-59

\(75\Omega\)[3]

Properties of popular coaxial cable dielectrics[2]:

Name	Propagation Velocity
Solid Polyethylene (PE)	0.659c
Foam Polyethylene (FE)	0.800c
Foam Polystyrene (FS)	0.910c
Air Space Polyethylene (ASP)	0.840-0.880c
Solid Teflon (ST)	0.694c
Air Space Teflon (AST)	0.850-0.900c

Name

Propagation Velocity

Solid Polyethylene (PE)

0.659c

Foam Polyethylene (FE)

0.800c

Foam Polystyrene (FS)

0.910c

Air Space Polyethylene (ASP)

0.840-0.880c

Solid Teflon (ST)

0.694c

Air Space Teflon (AST)

0.850-0.900c

where \(c=3e8ms^{-1}\) is the speed of light in a vacuum.

1.1. Precision Capacitors From Coaxial Cable

Coax cable has a capacitance of approx. 1pF/10mm, as measured between the central conductor and the outer shield. Precision capacitors for prototype/lab use can be from trimmed lengths of coax. cable.

References

[1]: Pro-Power. Cable Coaxial RG6 (datasheet). Retrieved 2021-05-04, from http://www.farnell.com/datasheets/1697593.pdf.
[2]: RF Cafe. Coaxial Cable Specifications. Retrieved 2021-05-05, from https://www.rfcafe.com/references/electrical/coax-chart.htm.
[3] Wikipedia (2020, Dec 19). RG-58. Retrieved 2022-01-31, from https://en.wikipedia.org/wiki/RG-58.
[4] Wikipedia (2021, Sep 7). RG-6. Retrieved 2022-01-31, from https://en.wikipedia.org/wiki/RG-6.