Bird Technologies SA-6000EX - Manual (Page 66)

Bird Technologies
50
Setting the Frequency Span
For a given number of data points (238, 475, 949), entering a fre-
quency span causes the Site Analyzer to calculate and enter the maxi-
mum measurable distance and, conversely, entering a maximum
distance causes the Site Analyzer to calculate and enter the required
frequency span.
There are two methods you can use to determine the frequency span
(for both methods, you need to know the maximum distance, velocity
of propagation, and number of data points). Instructions for both
methods are in Step 2 below:
•
Use the Fault Location Distance Tables, page 140.
•
Calculate the frequency span.
1. Select Measure Match Mode (refer to Select the Measure Match
Mode, page 18).
2. Determine the frequency span. Choose how many data points you
want to measure then, from the appropriate Fault Location
Distance and Resolution table (page 140), choose a velocity
propagation percentage that is closest to the cable in your system.
Read down the Vel Prop percentage column to determine the
frequency span or the maximum distance. When choosing a
distance, choose one that is approximately 25% more than your
tower height (or length of the cable system to be tested).
For example, for a cable with a dielectric constant of 2.296, the
velocity percentage is 66%. If the cable system is 100 feet long, and
you choose 238 data points, then the Site Analyzer should be set to
a frequency span of 700 MHz. The resolution of the graph will be
5.6 inches.
Note: To avoid potential false “spikes” (aliases) on the
trace, be sure to select a distance that is at least 25% greater
than the length of your cable system.
If you know your cable’s velocity of propagation and its length but
they are not on one of the tables, you can either enter the max dis-
tance into the Site Analyzer or you can determine the approximate
frequency span required by using the following equation:
Freq Span =
150 ( ) relative velocity (%) # points 1 – ( ) Ч Ч
test length (m)
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where (# points - 1) is 237, 474, or 948
If you enter a span greater than the current settings permit, the
span will be set to the maximum allowable value.