Leakage 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 "midband," 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 (CLI) 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 low-level leaks. 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 connects, 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.

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 metal buildings, changing the apparent direction of the source. And 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 add and subtract at the leakage detector, producing readings that rise and fall confusingly as the detector is moved around. Systematic leak location procedures and training can reduce this confusion, but practical experience is also valuable.

The "Drive Out"

It is easy to survey large areas of an HFC system in little time by patrolling the system with a vehicle-mounted leakage receiver and a roof-mounted vertical quarter wave antenna. A "drive out" 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 since the antenna requires a good, uncluttered ground plane for maximum sensitivity and an even pattern of reception. The antenna should be sited as far as possible from metal ladders and other metallic obstructions that might block leakage signals or cause lobes or asymmetrical receiving patterns.

Standing Waves

While patrolling the cable system and monitoring for leakage, the operator may encounter a series of amplitude peaks at regular distances. 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.

Tracking Down the Source

Once the general area of the leak is determined, it is necessary to pinpoint its location on foot using a portable leakage detector. Finding the leakage source can usually be done with the "rubber duck" antenna. However, 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 and noting the position of the dipole elements. Alternately, the operator may rotate for minimum strength: sometimes it is easier to establish a "null" than a "peak" reading. Moving to another spot parallel to, and 15-20 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.

Measuring the Leak

For measuring the strength of a leak, a calibrated leakage receiver is required. Instruments such as the Trilithic Searcher Plus, Searcher Plus GT and Super Plus leakage receivers are equipped to read radiated fields directly in microvolts per meter (μV/m). 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 off 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.

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 systems, 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.

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 low frequency modulation added to the carrier used for leakage detection. This modulation has no effect on the channel's video, but causes a distinctive response in Trilithic leakage receivers. The Super Plus, Search Lite, and other Trilithic detectors have specialized circuits that detect the presence of carriers that have been "tagged" by Trilithic's patented Channel Tag (CT-2 and CT-3). These leakage detectors break squelch only if a tagged signal is detected, making them immune to false triggering. Tagging also confers as much as 12 dB greater sensitivity than is possible with untagged detection systems. This extra sensitivity is very useful for detecting the small leaks that warn of big ingress problems.

. . . And when it is not

Some new QAM 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 Searcher Plus GT, an enhanced Searcher Plus containing additional circuitry to discriminate between signals and noise of all kinds. This circuitry, and an offsetting technique for avoiding overbuilds, gives the Searcher Plus GT much of the noise resistance of the Super Plus, though without its enhanced sensitivity.