The Importance of
Reference Distances
Although regulations specify that
leakage measurements must be taken
at 3 meters (10 feet) from a leak
source, this is not always possible
because portions of a cable system
may be inaccessible. For example,
cables may be situated in backyards,
easements, inside a home, or the dis-
tance to the cable may be unknown.
While the optimum approach is to
measure at 3 meters (10 feet), some-
times the technician may be required
to estimate longer distances.
Estimating the distance from the
antenna to the leak source can be diffi-
cult. Reliable distance estimations
require practice. One way to learn how
to estimate distances is to measure
known distances and use them as a
reference. For long distances, a more
accurate way to determine distances is
to use a range finder. Many surveying
companies use range-finding instru-
ments to determine the distance
between two predetermined points.
Reading a leakage meter for RF level
without correlating the reading to the
distance involved may result in a dis-
astrous situation for the cable opera-
tor. A miscalculation in distance can
seriously skew leak magnitude esti-
mates. If a leak is determined to be
greater than 20 µV/m at street side,
but the actual cable plant is located
in a distant rear easement, the leak
will most certainly be a large one.
Let’s work through an example in
order to clarify this point. Assume an
average sized leak is measured in a
patrol vehicle to be 70.29 µV/m on
channel C (133.2625 MHz). While the
leak is greater than 50 µV/m and must
be included in the FCC Cumulative
Leakage Index (CLI) test, it is certainly
a manageable sized leak. On a typical
workday, this leak would be logged
and repaired within a few days,
depending on your company’s specific
policies.
Now let’s factor in a distance correla-
tion and assume that the leak is locat-
ed in the cable plant in a rear ease-
ment 230 feet (70 meters) away.
The equation for correlating signal
levels from a known distance to 10
feet (3 meters) is:
µV/M (at 10 feet) = µV/M (at y feet) • (y
÷10)
Let’s insert our new values into the
equation:
µV/M (at 10 feet) = 70.29 µV/M (at 230
feet) • (230 ÷ 10)
µV/M (at 10 feet) = 1616.67 µV/m
As you can see, the new calculation
resulted in a much more serious leak-
age problem. Miscalculations like
this could cause any cable system to
fail the CLI test!
Note: A simple method that can be
figured in you head when you get out
of the truck to chase a leak is: Every
time you are half the distance you
double the leak intensity.
In Truck on
the Street 10µV/m 20µV/m 40µV/m
3-12
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