Bird Technologies SA-6000EX - Manual (Page 65)

Fault Location Mode
49
What You Need to Know About Setting the Frequency
Span and Max Distance
Note: Changing the frequency span or the max distance
will automatically turn calibration off. Always set the fre-
quency span or max distance before calibrating the unit.
The maximum distance you can measure is determined by the fre-
quency span, the velocity of propagation of the cable (V
p
), and the
number of data points to measure (238, 475, or 949). These factors are
discussed in the following paragraphs.
Max Distance
When you specify a frequency span, the Site Analyzer
calculates the maximum measurable distance. If you know the maxi-
mum distance you want to measure, you can enter it instead of the fre-
quency span; the Site Analyzer calculates the frequency span for you.
If you manually enter a maximum distance, be sure to select the data
points before entering the max distance
Note: If you select a maximum distance less than the total
length of the cable system, the trace might show a spike that is
not really a fault, but rather an echo from the portion of the
cable that is beyond the length you chose. Such a spike is
known as an alias. To avoid aliasing, always set the maximum
distance to a value that is a few feet or meters greater than the
entire length of your cable system.
Data Points If you increase the number of data points you will increase
the measurable distance (475 points results in twice the distance of
238 points, and 949 points results in four times the distance of 238
points) (refer to Equations, page 140). When you change the number of
data points measured, the software automatically recomputes the new
maximum measurable distance. If you manually enter a maximum
distance, be sure to select the data points before entering the max dis-
tance. Changing data points does not turn calibration off.
Frequency Span
If you narrow the frequency span you will increase
the measurable distance (refer to the Fault Location Distance Tables,
page 140).
Velocity of Propagation
The velocity of propagation is a characteristic
of the cable and is expressed as a percentage. Contact the cable manu-
facturer to get the velocity of propagation value for the cable you are
using.