UNAOHM
NG500 (Rev. 0)
13
Figure 5 Channel amplifier response taken as measuring example
7.2.
MEASUREMENT OF DISTANCE TO FAULT (OPEN CIRCUIT OR SHORT
CIRCUIT) ON A CABLE
As mentioned previously, it is possible to find the distance at which a transmission line is
strongly mismatched (up to the case of either a shortcircuit or an open circuit).
To carry out this measurement it is necessary to use two instruments: the NG500 generator and a
FSM together with a
"T"
coaxial connector and the connections are made as shown in Figure
6A.
Connect the line in question to the
"T"
connector. If it is mismatched, standing waves are
created in it which are visible on the screen of the FSM with half-sine oscillations as shown in
figure 6B.
The measurement consists of finding out the space, or frequency range between two minima (or
maxima), which we will indicate as "
Δ
F".
This "
Δ
F" frequency may be used to calculate, given the type of coaxial cable (PE / PEE -
compact or expanded polyethylene), the distance "L" in meters from the measuring point to the
point of open circuit (or shortcircuit) along the cable by means of the simple formulas :
L = 100/
Δ
F
(for dielectric cables in hard polyhetylene PE)
L = 120/
Δ
F
(for dielectric cables in expanded polyhetylene PEE)
