BASIC OPERATIONS
67
The wave impedance of all combinations of wires should also be the same. The wave impedance
of most round cables is 300 - 700 Ohm.
These cables are often used to transmit low-frequency
signals (telephone lines, burglar and fire alarms, etc.).
In multi-wire flat cable,
the wires of the same length (matching the cable length) are
arranged in a row. Capacitive coupling pairs formed by adjacent wires will be greater than other
pairs of wires. The smallest capacitive coupling will be at the wires located at the edges.
This arrangement determines the different wave impedance in different combinations of wires.
Multi-wire cable, consisting of several twisted pair.
Each pair of wires is twisted with a different pitch. This avoids spurious emissions,
compensates for interference, and reduces signal transmission loss. Due to the different pitch of
the twist, the lengths of wires of different pairs are different. These lengths are always greater
than the cable length.
The wave impedance of twisted pairs is ≈100 Ohm. The wave impedance of the combinations
formed by wires from different twisted pairs will be significantly more than 100 Ohm. The
"twisted pair" type cables are designed to transmit RF signals over a large (up to 100 meters)
distance.
Since the REF output has a wave impedance of ≈50 Ohm, an input cable inconsistency leads to
losses of the probing and reflected signal. The most consistent with the REF output are twisted
pairs.
Influence of Shortening Factor on measurement of distance to heterogeneity.
An important characteristic of the cable is the Shortening Factor, the value of which is
determined for each type. Factor shows how much the propagation speed of the electromagnetic
signal in this type of cable is less than in vacuum.
The Shortening Factor affects the correctness of determining the distance to heterogeneity.
The Shortening Factor is specified in the cable specification or on manufacturers' websites.
For round and flat multi-wire cables, (regardless of the set Shortening Factor), the measured
lengths of pairs without defects will be the same. If an incorrect value of the Shortening Factor is
set, the measured length will differ from the real one.
Shortening Factor of the cable consisting of twisted pairs.
The speed of the electromagnetic signal in the cable, consisting of "non- twisted" pairs, will be
the same. The speed of the electromagnetic signal in different pairs of cable, consisting of
"twisted" pairs, will be different.
In "linked" pairs, the signal propagation speed will be less than in "non-linked" pairs.
Thus, with the same Shortening Factor set, different distances to heterogeneity (or the end of the
cable) will be measured:
distances to heterogeneities in "non-linked" pairs are the same;
distances to heterogeneities in "linked" pairs are different.
Results of testing
The results of testing a multi-wire cable are presented in tabular form. Knowing the features of
cable structures without defects it is possible to detect and identify the anomalies existing in them.
