How to Find Leaks using the new ComSonics Digital Leakage Equipment
How to Find Cable Leakage
The ComSonics’ QAM Leakage solution set allows detection of cable TV leakage in an all-digital system (all QAM channels) without the need for an analog carrier in the downstream broadcast. It is a leakage detection system made up of three general parts: The first is a “marker” signal source that is installed at a headend or hub of a CATV system. The second is a fleet-based GPS leakage detection platform. The third are the handheld devices.
· The QAM Marker is a unique signal source typically installed at a headend or hub site, configurable to support up to three co-located CATV systems in an overbuild area, to distinguish leaks between the CATV systems.
· The Genacis QS platform is a vehicle-based solution that is best used in a fleet type application. It uses a fully automated “black box” approach requiring no technician interface for broad-coverage leakage detection / monitoring.
· The QAM Sniffer and QAM Shadow are handheld cable leakage detectors used for locating and isolating the leakage source. The QAM Sniffer is oriented towards the maintenance technician’s use, while the QAM Shadow is better suited for the installer.
The QAM Marker and QAM Sniffer products work together to form a reliable leakage detection system in an all-digital channel cable system that detects very low-level leakage signals. ComSonics’ solution uses a unique low level “marker” signal that is injected at the headend or hub directly between two adjacent QAM signals. The handheld detection units are then programmed to detect the corresponding marker signal.
There are 2 primary tools needed to for a comprehensive leakage management program:
1. Automated Fleet Application: In order to provide continuous leakage detection and obtain good plant coverage on a regular basis, a fleet-based platform is recommended that typically uses a mix of maintenance and installation trucks. Typically about 30% to 40% of vehicles should be outfitted with a mobile detection capability to provide a high percentage of coverage every 3 months.
An example of the leaks found using the mobile application is shown in Figure 1 and be used to manage and prioritize leakage repair.
2. Handheld Leakage Detectors: The second category of equipment needed for leakage management is handheld leakage detection devices that can provide a combination of directionality and leakage level information. This category can be further broken down into two subcategories: Maintenance and Installation. The maintenance technician generally needs a leakage detection device that provides directionality information that will allow him/her to track down leaks in the outside plant. For this reason, a device such as the QAM Sniffer is recommended as it has a built-in directional dipole antenna. The installation technician is generally working inside a home or building and thus may not require the same level of directivity to find leakage due to their limited working area. The recommended device for this work group is the QAM Shadow which has a built in near-field antenna. This will help the technician zero in on a leak once they are in close proximity to the leak source.
The two different device types are shown below: The QAM Sniffer is on the right has the retractable di-pole antennas and the QAM Shadow on the left contains the near-field antenna.
How to Find a Leak Using the QAM Sniffer
In this section of the document, the technique of finding a leak using ComSonics’ leakage detection equipment will be discussed. The first step is to understand the antenna radiation pattern of the QAM Sniffer dipole antenna. This is important as the radiation pattern is the fundamental mechanism that provides the device’s directionality.
The area of the pattern that provides the maximum antenna gain also provides the maximum signal reception. As shown in Figure 4 below, this area revolves around the front, back, top and bottom of the unit. The “Null” or minimum gain portion of the antenna pattern is from the right and left sides of the unit; the end of the antenna elements.
The technician can use the maximum gain and null points of the antenna pattern to provide directional information to track down a leak. This is done by using these recommended practices:
1. After a leak has been detected or a general location provided, the technician unfolds the antennas on the QAM Sniffer and extend the elements to the correct length to match the frequency of interest. For 612 MHz range, the antenna elements should remain recessed. For the 138 MHz range, the antenna elements should be pulled out to full length.
2. The technician points the QAM Sniffer in the general direction of the existing cable plant. A good starting point is usually a visible amplifier or tap point.
3. The technician must then find the maximum or minimal signal level of the detected leak by orienting the QAM Sniffer correctly. The technique involves two steps: The technician can slowly turn around standing in place and note where the reading is highest or lowest. The technician can also rotate the QAM Sniffer to orient the antennas from a horizontal position to a vertical position. As the QAM Sniffer is slowly maneuvered from both perspectives, the orientation producing the highest signal level generally points to the location of the leakage source. As a general rule, the technician should follow the directional information from the QAM Sniffer that points towards some part of the cable plant.
4. Two directional methods can be used to locate a leak: (1) use the maximum leakage level or (2) use the minimum leakage level (null point). If the technician prefers to use the maximum leakage level to find the leak, they will then follow the direction the QAM Sniffer is pointing which maintains the highest signal level possible. If the technician chooses to use the null point (lowest level) to find the leak, the antenna element will point in the direction of the leak. Note that using the null point actually provides more accurate directional information as the antenna pattern at the null point is much narrower than the maximum gain point. Both the maximum and null point information can be used in combination to provide a double confirmation that the direction is correct. The technician will then start walking in the direction of the cable plant using the information provided by the QAM Sniffer.
5. As the technician approaches the cable plant, the detected level of the leak should increase. This should be continuously monitored/checked (every 20 to 30 feet or so) while closing-in on the leakage source location. Sometimes the detected leak can come from a reflection from a building or house, so the direction of the leak may suddenly change once a direct line-of-sight to the leak occurs. Once the technician reaches the cable plant, it is recommended to walk a short distance up and down the cable plant to validate the leakage location. This will also determine if multiple leaks are present and can help rule out possible reflections.
6. If the level does not increase as the technician initially walks towards the leak, this is an indication that the leak was likely a reflection. The technician should stop and redo the direction readings to get a new bearing.
7. This process continues until the technician closes in on the leak. There may be several leaks in the general proximately which can make the process a bit more challenging. Once the technician gets to within several feet of the leak, the antennas can be closed which will convert the QAM Sniffer to be used for a near field detector. This pattern is shown in Figure 5 below.
Using the near field configuration, the technician can locate the source of the leak to within a few inches. At this point, the source of the leak should be either visible if it is caused by external damage or the technician will have narrowed down the possible culprits to the most common problems such as loose connectors on a tap plate.
To find leaks using the QAM Shadow, the technique is fairly straight forward. The antenna operates in a near-field mode only and is activated by flipping open the lid on the device. The antenna is located inside the lid. Using both the audible tone and the visible display readings, the technician will move in a direction that produces the largest signal level reading. There is not a definitive null point on this antenna, so only the maximum signal level can be used to localize the leakage source. As the technician nears the leakage source, the QAM Shadow can be used to pinpoint the leak source to within a few inches. Once the general location of the leak is found, the technician should be able to reasonably determine the cause of the leakage. If the leakage source is not visibly obvious such as a damaged cable, then a course of tightening and replacing components is the usual course of action to fix the leak.
Finding leaks is not always a straight-forward science, and often requires an element of “art” as well. Leakage signals can be reflected, absorbed, and combined to give the technician misleading information causing them to look in the wrong direction. This is where the technician’s experience and understanding of how leakage interacts with other objects come into play. Radiation patterns emitting from a leakage source are not uniform in nature, which can also require the technician to move around the area to find where the leak is radiating at its maximum level. All these factors combined make leakage hunting a bit more complex to master. There are many instances where a leak was thought to be coming from a certain location, but upon investigation a much larger leak at a distant location was found.
If a technician is equipped with the right equipment, a basic understanding of radiation patterns, and training on how to use the equipment, they will be outfitted with the core fundamentals required to track down leaks. Only experience and trial and error will round off the technician’s ability to more efficiently to track and find cable leakage.