Signal Leakage
Leakage Equipment Applications
Preventive Maintenance
Signal leakage, also called egress, occurs when
RF signals escape from the HFC cable plant and
radiate into the environment.
In the 1970s, cable operators began putting
channels into the ‘’mid-band,’’ a range of
frequencies also used for aeronautical
applications. To ensure air traffic safety, the FCC
established rules strictly limiting the level of signal
leakage at aeronautical frequencies. In March
1989, the rules were amended to require offset
signals in the aeronautical band and to clarify the
method by which cable systems report cumulative
leakage index (CLl) to the FCC.
Since its establishment as a legal obligation,
leakage testing has also proven to be a powerful
preventive maintenance technique. Active
monitoring for leaks and prompt repair of their
causes has been shown to improve overall system
performance and to reduce the number of service
calls. Leakage testing has also been useful for
reducing the level of reverse path ingress.
Sources of Signal Leakage
Significant leaks occur most often where shielding
effectiveness is low and signal levels are high.
Both conditions must be present. For example,
the shielding effectiveness of subscriber cabling
is relatively weak, but subscriber signal levels are
low, so home cabling usually radiates only lowlevel leaks. A high percentage of leaks are from
within the home. Much stronger leaks are found
in the distribution system because the signal
levels are much higher, especially near amplifiers.
The hardware used in distribution systems is
designed for high shielding effectiveness so leaks
only occur if there are workmanship problems,
mechanical defects, or damage. Common causes
include loose covers or connectors on passive
devices or amplifiers, poorly made and defective
connections, and bad splices. Damage to
underground distribution cable may be caused by
road construction, careless excavation, or sinking
posts and power poles where cables are buried.
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The Art of Locating Leaks
Pinpointing sources of leakage is a blend of
art and science. An “ideal” leak would behave
like a point source, and the operator would
only need to move his leakage receiver in the
direction of increasing power to find it. However,
real conditions are very different. RF leakage
currents can run for considerable distances from
their source down guide wires, the sheathing
of hard line cable, and other metal structures
connected to, or near, the source of the leak. The
geometry of the leak site, including the location of
surrounding metallic structures, can influence the
apparent direction of the leak.
Sometimes conditions and geometry will promote
standing waves, which make leak strength rise
and fall in a periodic way as the detecting vehicle
drives by the source. Leakage radiation may
also be reflected from buildings, changing the
apparent direction of the source. Sometimes
the leakage being detected is actually from two
separate sources, or from a single source and
a reflection of that source, again obscuring the
direction of the leakage source. All of these
effects create multiple signals that can appear
stronger or weaker at the leakage detector,
producing readings that rise and fall confusingly
as the detector is moved. Systematic leak
location procedures and technician training can
reduce this confusion, but practical experience is
also valuable.
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Signal Leakage
Leakage Equipment Applications
The “Driveout”
It is easy to survey large areas of an HFC system
in a short amount of time by patrolling the system
with a vehicle-mounted leakage receiver and a
roof-mounted quarter-wave antenna. A driveout
will quickly isolate the general area from which
the leak is radiating and will give the operator
a rough indication of signal strength. When
preparing a vehicle for this use, consideration
should be given to the placement of the
monopole antenna on the vehicle’s roof, as the
antenna requires a good, uncluttered ground
plane for maximum sensitivity and even pattern
of reception (Figure 1). The antenna should be
situated as far as possible from metal ladders
and other metallic obstructions that might block
leakage signals or cause lobes or asymmetrical
receiving patterns.
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Figure 1: Vertical-plane radiation pattern for a groundmounted quarter-wave vertical antenna. The solid line is the
pattern for perfect earth. The shaded pattern shows how the
response is modified over average earth.
Standing Waves
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Standing Waves
While patrolling the cable system and monitoring
for leakage, the operator may encounter a series
of amplitude peaks at regular distances (Figure
2). Often, as the vehicle moves, the peaks will
gradually rise in overall strength and merge
until a steady tone is heard. This phenomenon
is caused by standing waves distributed along
metallic cables or similar structures that conduct
leakage energy away from the source of the leak.
Standing wave effects diminish with distance
from the source as the energy is attenuated by
conduction loss. The simplest way to deal with
standing waves is to drive at a constant pace (to
make the standing wave pattern recognizable)
and observe the fluctuations until they reach
their peak, then begin to decline in strength. The
source of the leakage will be found near the point
where the peak of the fluctuation was found.
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Figure 2
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WEAKER
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Signal Leakage
Leakage Equipment Applications
Tracking Down the Source
Once the general area of the leak is determined,
it is necessary to pinpoint the location on foot
using a hand-held leakage detector (The area
that needs to be investigated on foot can be
greatly reduced if the driveout data is analyzed
first by Trilithic’s Leakage Analysis Workshop
software). Finding the source of a leak can
usually be done with the ''rubber duck'' antenna,
affixed to the leakage detector. If the location of
the leak cannot be determined by simply "walking
it down,'' the operator may use a dipole antenna
to triangulate the source. On foot, in the vicinity
of the leakage source, the operator simply rotates
the dipole antenna for peak leakage detector
readings while noting the position of the dipole
elements (Figure 3). Alternately, the operator
may rotate for minimum detector readings;
sometimes it is easier to establish a "null" than a
“peak” reading. Moving to another spot parallel
to, and 15 to 20 feet away from the cable plant,
the operator takes a second bearing on the
leak. The leakage source will be near the point
where the bearings from the two measurements
intersect.
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Figure 3: Dipole antenna radiation pattern.
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The procedure for making calibrated
measurements for the Cumulative Leakage
Index (CLI) calculation as prescribed by the FCC
requires that the operator take his measurements
using a horizontal dipole antenna, elevated three
meters above the ground, and positioned three
meters from the source of the leak. Procedures
require the dipole antenna to be rotated in the
horizontal plane to maximize the leakage signal,
giving the highest reading on the calibrated
leakage measurement receiver.
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Measuring the Leak
For measuring the strength of a leak, a calibrated
leakage receiver is required. Instruments such
as the Trilithic Seeker Lite², Seeker, and Seeker
GPS leakage detectors are equipped to read
radiated fields directly in microvolts per meter
(μV/m).
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Signal Leakage
Leakage Equipment Applications
False Triggering and Noise Sources
‘’False triggering “of leakage detectors is caused
by electrical noise and, in some cases, overbuilt
HFC systems. Besides complicating leakage
measurements, false triggering can obscure
real leakage problems in the same area. False
triggering caused by electrical noise can have
many sources, including electric motors, faulty
power line hardware, and even neon signs.
Interference can also be generated by a vehicle’s
electrical system, including those of the vehicle
carrying the leakage detector. The operator
can reduce noise from his own vehicle through
improved grounding and shielding of suspect
electronic devices and the use of resistor spark
plugs to combat ignition noise.
all kinds. This circuitry gives the Seeker series
additional noise resistance, as compared to
competitive leakage detectors.
When Signal “Tagging’’ is the
Answer…
When leaks from overbuilt HFC systems or high
local noise levels routinely cause the leakage
detector to false alarm, the operator may want to
consider signal tagging. A tag is a special lowfrequency modulation added to the carrier and
used for leakage detection. This modulation has
no effect on the channel’s video, but causes a
distinctive response in Trilithic leakage receivers.
The Seeker Lite², Seeker, Seeker GPS, and
Seeker BB-2 have specialized circuits that detect
the presence of carriers that have been tagged by
Trilithic’s patented CT-2 channel tagging device.
These leakage detectors break squelch only if
a tagged signal is detected, making them more
immune to false triggering.
…And when it’s Not
Some new digital terminals and baseband descrambling set tops are intolerant of tagging
modulation. In those cases, the operator may
wish to use another technique to improve noise
immunity. The best choice is the to rely on the
noise discriminating function of current detectors
such as the Trilithic Seeker series, coupled
with the CT-3 channel tagger. The Seeker
leakage detectors contain additional circuitry
to discriminate between signals and noise of
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