UNAOHM
14
NG500 (Rev. 0)
Figura 6 Connections for distance to fault calculation
NOTE the maximum cable length which may be measured depends on the attenuation itself; if
for example the cable has 20 dB attenuation for 100 meters, possible mismatch will not provoke
substantial standing waves. We therefore recommend, since the coaxial cable attenuation
increases with the frequency, carrying out the measurement in the VHF band (the result does
not change).
The maximum measureable distance of the cable depends on the bandwidth of the meter which
must be, in any case, much lower than
Δ
F.
It is evident that for small mismatches it is not possible to carry out a reliable measurement since
the standing waves are not clearly marked.
Example
Suppose that in the installation detailed in Figure 3 the signal is not present down column A, at
the user’s outlets 3, 4 and those that follow.
We can safely say that the failure may be found in the D1 or D2 shunters or in the cable length
marked “A”.
It is therefore necessary to disconnect the "A" cable from shunter "D1" and to make the
connection as indicated in figure 6A. Some oscillations will then be noticeable on the FSM.
Figure 6 legenda: -Misuratore di campo = field strength meter (FSM)
