This article originally appeared in Outside Plant Magazine,
Volume 14 / Number 5, May 1996.

"Electrical properties of coaxial cable"

By Matt Davis
Broadband Engineer/Instructor
National Cable Television Institute
Littleton, Colorado

Coaxial cable is used by telephone and cable TV companies, as an outside
plant distribution medium, for a variety of reasons, chiefly moderate cost
and high bandwidth. Coaxial cable generally can be installed, in a suburban
area, aerial, at about 80 cents a foot. In rural areas, aerial plant can be
hung for perhaps 65 cents a foot. Underground plant in urban areas can run
$3.85 a foot. That plant, once installed, can transport up to 1 GHz of
spectrum, representing 4 to 5 gigabytes per second of raw bandwidth.

The features of coaxial most important to outside plant technicians and
engineers are the medium's electrical properties, since these dictate the
cable's usefulness as a conduit for video, voice and data signals, as well as
electrical voltage to power amplifiers and customer premises equipment. Among
the primary differences between standard twisted-pair wiring and coaxial
cable are bandwidth potential and DC loop resistance. Dial-up telephone
modems routinely operate at 14.4 kbps or 28.8 kbps over standard 22-gauge
wire. Using new signal compression techniques, it should be feasible to run
data, at distances up to 1,000 feet, at rates as high as 53 Mbps. A coaxial
cable can carry 30 Mbps in 6 MHz of bandwidth, the space normally occupied by
one television signal. A modern hybrid fiber coax network can carry 110 such
channels between a headend or central office and a customer location.

The other area in which coaxial cable differs from twisted-pair wire is its
resistance to the flow of electrical current. A 22-gauge wire, for example,
has a loss of about 19 ohms per 1,000 feet (measured at 65 degrees
Fahrenheit). Since resistance is inversely proportional to the cross-
sectional area of the conductor, a larger-diameter 18-gauge wire has only
7.51 ohms resistance per 1,000 feet. The basic rule is that when the diameter
of the conductor is doubled, the resistance declines 50 percent. The DC loop
resistance of a much-larger half-inch coaxial cable is about 1.34 ohms per
1,000 feet. A one-inch coaxial cable has loop resistance of about 0.40 ohms
per 1,000 feet.


                  Wire Resistance

           (measured in ohms/1,000 feet)

       Type of wire           Size   Resistance
       ---------------------- ------ ----------

       Twisted pair           22ga    19.00


       Twisted pair           18ga     7.51


       Coaxial cable          0.50"    1.34


       Coaxial cable          1.00"    0.40



The electrical properties of greatest importance to outside plant
technicians, in addition to DC loop resistance and bandwidth, are
attenuation, impedance and return loss. Attenuation is the degree to which
the coaxial cable reduces the amplitude, or signal strength, of radio
frequency energy carried through it. Impedance is a way of describing the
resistance of a cable to the flow of radio frequency energy and its power-
carrying capability. Return loss is a measurement of a terminated cable's
ability to fully absorb energy, without reflecting it back down the circuit.
Of the electrical issues, RF signal attenuation is most crucial, since it is
the RF carriers (high-energy waveforms at specific radio frequencies) that
transmit all the customer-desired signals. If the RF energy is attenuated
beyond network design specifications, then the intended signals will be
weakened (analog signals) error-ridden (digital signals), or, in extreme
cases, will not be delivered at all (analog and digital signals).

Signal attenuation

Signal attenuation is a function of the metal out of which the center
conductor is made, the diameter of the cable, cable resistance, dielectric
performance, frequency of the RF signals carried and temperature of the
cable. A coaxial cable features two circular conductors, each centered on a
common internal axis, with a dielectric layer in between. The center
conductor carries the RF energy and AC power, while the outer conductor is a
ground.

About two-thirds of the total cable attenuation is caused by the center
conductor. so a low-resistance material, such as copper, is desirable. All-
copper center conductors are relatively costly, however. Fortunately, cable
designers have found that a copper-clad aluminum conductor works as well as
solid copper, but costs less, because aluminum is a cheaper metal. This is
possible because RF signals travel only on the outside surface of the center
conductor (a phenomena known as "skin effect"). AC voltage travels through
the entire diameter of the aluminum portion of the center conductor. As noted
earlier, conductor size has a direct effect on attenuation.


       Coaxial Cable Center Conductor
            Size and Attenuation

       Diameter of cable    Attenuation (dB) at
       (in inches)          600 MHz per 100'
       -------------------- -------------------

            0.500                 1.80

            0.625                 1.59

            0.750                 1.36

            0.875                 1.20

            1.000                 1.08


There also is some loss in the dielectric material, which can be either a
foamed polyethylene material, or an "air dielectric," where plastic discs are
used to maintain the proper spacing between the center and outer conductors.

Each type of dielectric has a specific "velocity of propagation," a measure
of how fast RF travels through the cable, compared to its speed in free
space. As the velocity of propagation increases, attenuation decreases. An
air dielectric cable allows RF to travel at 93 percent of the atmospheric
rate, while a foam dielectric allows RF to travel at only 87 percent of
atmospheric rate. For that reason, at 600 MHz, 0.750-inch foam dielectric
cable has attenuation of 1.26 dB per 100 feet, while air dielectric cable of
the same size has loss of 1.11 dB per 100 feet.

Also, cable attenuation is frequency dependent. As a rule, a four-fold
increase in frequency will cause a doubling of attenuation.

A typical design goal for an untapped length of 0.750-inch cable is a maximum
of 22 decibels of loss. If that cable has 1.43 dB of loss for each 100 feet,
at the highest frequency carried on the network, then signals can travel for
1,538.5 feet (0.29 miles), or less than a third of a mile, before the signals
must be repeated (amplified).

Changes in cable temperature also affect attenuation performance. Generally,
a 10-degree change causes a one percent change in attenuation. Performance
increases one percent for a 10-degree temperature drop, and decreases by the
same amount for each 10-degrees of temperature increase. Cable temperatures
for aerial plant can be 10 to 20 degrees higher than atmospheric temperature,
especially when the sun is out and when the cable has a black plastic
coating.

Moisture that gets into the cable and mechanical damage also will impair
attenuation performance from nominal, or expected, levels. In most cases,
water vapor, kinks, dents and holes in coaxial cable will tend to cause
excessive attenuation at some, but not all frequencies.

RF energy carried on outside plant coaxial cables will tend to be no higher
than 31 dBmV (35.48 millivolts) to 40 dBmV (100 millivolts).

Loop resistance

Where cable attenuation is a measure of the loss in signal amplitude of the
RF signals, the DC loop resistance is a measure of the loss of AC power as it
travels through the cable. The lower the loop resistance, the less power is
converted to heat and rendered useless for network powering applications. The
DC loop resistance of a coaxial cable decreases as the diameter of the cable
is increased.

Impedance and return loss

The coaxial cables used by hybrid fiber coax networks have a 75-ohm
impedance, selected for use because it offers the lowest attenuation profile.
If power transmission alone were the concern, then a 30-ohm cable would have
the best performance. But power carrying requirements for RF signals are
relatively modest, so the minimal power-carrying capability of a 75-ohm
conduit is not a problem. A cable carrying 78 TV channels at a signal level
of 48 dBmV, for example, requires only about 0.10 watt.

Impedance is important because all of the connectors, terminators, splices,
couplers and splitters used on the network must be electrically matched at 75
ohms, plus or minus two ohms. Variations from the designed impedance will
cause energy to be reflected at each transition from one device to the next,
creating a source of signal interference. One of the forms of interference
created by impedance mismatches is the voltage standing wave, which can cause
voltage levels to be higher than designed for, or lower than designed for.
Return loss is a measurement of the strength of an intended signal voltage to
the reflected signal voltage.

Conclusion

Attenuation of the radio frequency energy launched down a coaxial cable is
the single most-important concern for outside plant engineers and
technicians, since this dictates the distance signals can be carried before
they must be repeated. DC loop resistance historically has been important
because it determined the placement of power supplies in the network, and is
a more-crucial consideration in the modern network because of requirements to
provide ringing voltage for customer telephones. Impedance matching is
important because it determines the amount of unwanted signal reflections
within the network. Impedance mismatches will cause impairments such as
"ghosts" in video, and data errors for digital traffic.