402 Chapter 14
14.2.8 Skin Effect
As the frequency of the signal increases on a wire, the
signal travels closer to the surface of the conductor.
Since very little of the area of the center conductor is
used at high frequencies, some cable is made with a cop-
per clad-steel core center conductor. These are known as
copper-clad, copper-covered, or Copperweld™ and are
usually used by CATV/broadband service providers.
Copper-clad steel is stronger than copper cable so it
can more easily withstand pulling during installation, or
wind, ice, and other outside elements after installation.
For instance, a copper-clad #18 AWG coaxial cable has
a pull strength of 102 lbs while a solid copper #18 AWG
coax would have a pull strength of 69 lbs. The main
disadvantage is that steel is not a good conductor below
50 MHz, between four and seven times the resistance of
copper, depending on the thickness of the copper layer.
This is a problem where signals are below 50 MHz
such as DOCSIS data delivery, or VOD
(video-on-demand) signals which are coming from the
home to the provider. When installing cable in a system,
it is better to use solid copper cable so it can be used at
low frequencies as well as high frequencies.
This is also why copper-clad conductors are not
appropriate for any application below 50 MHz, such as
baseband video, CCTV, analog, or digital audio.
Copper-clad is also not appropriate for applications
such as SDI or HD-SDI video, and similar signals
where a significant portion of the data is below
50 MHz.
The skin depth for copper conductors can be calcu-
lated with the equation
(14-2)where,
D is the skin depth in inches,
f is the frequency in hertz.
Table 14-6 compares the actual skin depth and
percent of the center conductor actually used in an RG-6
cable. The skin depth always remains the same no
matter what the thickness of the wire is. The only thing
that changes is the percent of the conductor utilized.
Determining the percent of the conductor utilized
requires using two times the skin depth because we are
comparing the diameter of the conductor to its depth.
As can be seen, by the time the frequencies are high,
the depth of the signal on the skin can easily be micro-
inches. For signals in that range, such as high-defini-
tion video signals, for example, this means that the
surface of the wire is as critical as the wire itself. There-
fore, conductors intended to carry high frequencies
should have a mirror finish.
Since the resistance of the wire at these high
frequencies is of no consequence, it is sometimes asked
why larger conductors go farther. The reason is that the
surface area, the skin, on a wire is greater as the wire
gets larger in size.
Further, some conductors have a tin layer to help
prevent corrosion. These cables are obviously not
intended for use at frequencies above just a few mega-
hertz, or a significant portion of the signal would be
traveling in the tin layer. Tin is not an especially good
conductor as can be seen in Table 14-1.14.2.9 Current CapacityFor conductors that will carry large amounts of electrical
flow, large amperage or current from point to point, a
general chart has been made to simplify the current car-
rying capacity of each conductor. To use the current
capacity chart in Fig. 14-1, first determine conductor
gage, insulation and jacket temperature rating, and num-
ber of conductors from the applicable product descrip-
tion for the cable of interest. These can usually be
obtained from a manufacturer’s Web site or catalog.
Next, find the current value on the chart for the
proper temperature rating and conductor size. To calcu-
late the maximum current rating/conductor multiply the
chart value by the appropriate conductor factor. The
chart assumes the cable is surrounded by still air at an
ambient temperature of 25°C (77°F). Current values are
in amperes (rms) and are valid for copper conductors
only. The maximum continuous current rating for an
electronic cable is limited by conductor size, number of
conductors contained within the cable, maximum
temperature rating of the insulation on the conductors,D 2.61
f----------=Table 14-6. Skin Depths at Various Frequencies
Frequency Skin Depth in Inches % Used of #18 AWG
Conductor1 kHz 0.082500 100
10 kHz 0.026100 100
100 kHz 0.008280 41
1 MHz 0.002610 13
10 MHz 0.000825 4.1
100 MHz 0.000261 1.3
1 GHz 0.0000825 0.41
10 GHz 0.0000261 0.13