44
Determining a Cable's Length or Distance to an Open
A length of coaxial cable open at both ends is equivalent
to a long capacitor, with the two conductors forming the
plates. Every type of coaxial cable has a normal amount
of capacitance per foot, specified in picofarads per foot
(pF/ft). The capacitance per foot values for some
common coaxial cable types are listed in Table 12. The
length of a piece of cable, as well as the distance to an
open, is found by simply measuring the capacitance
between the center and outer conductors and dividing
this total capacitance by the cable's capacitance per foot
value. If possible, measure from both ends of the cable
to more accurately pinpoint the break. In most cases, the
length of a cable can be determined within 1-2%.
To measure the length of a cable:
1.
Zero the LC103 test leads.
2.
Connect the red test lead to the center conductor and
the black test lead to the braided shield outer
conductor of an open (unterminated) cable.
3.
Press the CAPACITOR VALUE test button and read
the total capacitance of the cable.
4.
Divide the LC103 capacitance reading by the cable's
capacitance per foot value. This gives the length of
the cable, or the distance to the break in feet.
50-55 Ohm
Nominal
Nominal
Nominal
RG/U Cable Type
Impedance Cap in pF/ft Inductance
5 B/U
50 29.5
8U
52 29.5
8U Foam
50 26
8A/U
52 29.5
10A/U
52 29.5
18A/U
52 29.5
58/U
53.5 28.5
58U Foam
50 26
58A/U
50 30.8
58C/U
50 26
58C/U Foam
50 26
74A/U
52 29.5
174/U
50 30-30.8
177/U
50 30
212/U
50 29.5
213/U
50 30.5
214/U
50 30.5
215/U
50 30.5
219/U
50 30
225/U
50 30
224/U
50 30
You can also use this test to determine the length or to
pinpoint a break in multi-conductor cable that has 3 or
more conductors. Due to variations in conductor spacing
and noise pickup, however, the accuracy will not be as
good as for coaxial cable. Follow the same procedure as
above, except tie all but one of the conductors together
to form the outer “shield”. Measure the capacitance
between this “shield” and the remaining single wire.
You can determine the capacitance per foot for the cable
using the procedure in the section “Determining
Capacitance and Inductance per Foot.”
NOTES: 1. The accuracy of these measurements
depends on the cable tolerance. The values listed in
Table12 are nominal amounts
which may very slightly
(within 2%) with cable manufacturer. 2. Excessive
crimping or clamping along the cable will change the
total capacitance reading.
Locating a Short in Coaxial Cable
A coaxial cable which has a short between its center
conductor and outer conductor is similar to a very long
inductor. The LC103 can be used to determine the
distance to a short using the INDUCTOR VALUE test.
The amount of inductance per foot of a coaxial cable is
not usually published by the cable manufacturer, and the
amount for the same type of cable may vary significantly
from one manufacturer to another.
70-75 Ohm
Nominal
Nominal
Nominal
RG/U Cable Type
Impedance
Cap in pF
Inductance
uH/ft
6A/U
75 20
6A/U Foam
75 20
11U
75 20.5
11U Foam
75 17.3
11A/U
75 20.5
12A/U
75 20.5
13A/U
74 20.5
34B/U
75 20
35B/U
75 20.5
59/U
73 21
59/U Foam
75 17.3
59/BU
75 20.5
164/U
75 20.5
216/U
75 20.5
90-125 Ohm
Nominal
Nominal
Nominal
RG/U Cable Type
Impedance
Cap in pF
Inductance
uH/ft
5 B/U
50 29.5
62/U
93 13.5
63B/U
125 10
71B/U
93 13.5
79B/U
125 10
Table 12 - Capacitance per foot values for common coaxial cable types.
