SIGNAL LEAKAGE PART 2- Practical Signal Leakage Abatement Techniques

Transcription

1 SIGNAL LEAKAGE PART 2- Practical Signal Leakage Abatement Techniques IN THE MDU ENVIRONMENT by John Manning & Steve Hegge In a previous article, we examined the FCC requirements for measuring signal leakage and reporting the results. The focus of this article is on practical application what are the nuts and bolts of compliance? Creating an effective signal leakage program starts with an appreciation for the benefits derived from compliance. When the FCC rules were first mandated for franchised systems, some operators responded with avoidance while others sought ways to observe the minimal requirements with the least commitment. After all, very few airplanes had crashed on their pole lines, regardless of the disrepair of their plant. Today, most experienced operators would agree that the benefits of a good program more than justify the cost. Particularly when combined with periodic system sweep, leakage abatement emerges as the most effective preventive maintenance available. Small problems are identified before they become large headaches. The passive extension of the system gets as much scrutiny as the headend and amplifiers. Your technicians correct problems before your subscribers are forced to call them to your attention. The predictable results are fewer trouble calls and better subscriber retention. The Cumulative Leakage Index (CLI) is a mathematical model of the signal leaking from a given system. It can be derived by flying over the plant and measuring the radiated emissions, but this approach has its drawbacks. First, many private cable systems are not architecturally suitable for fly over. They may be too vertical, too discontinuous. Second, if the CLI from a fly over fails, the leaks must be identified and corrected on the ground anyway. (Advice to the optimistic: If you have not been running a conscientious leakage program, your chances of generating a passing CLI in your first fly over are roughly equal to the odds of winning the lottery without buying a ticket in a state where it s illegal.) In short, there is no substitute for ground-based measurements. There are as many ways to skin the leakage cat as there are millionaires at Microsoft, but they all resolve into five basic steps. 1. Detection 2. Location 3. Measurement 4. Correction 5. Remeasurement We will examine each of these steps in turn, but first we should consider system documentation. CLI calculations require precise knowledge of plant mileage, both vertical and horizontal. It is important to know both the total system mileage and the mileage tested for leaks at any given time, as the percentage of tested plant is a factor in CLI. In other words, accurate system maps are a prerequisite for any program.

2 Detection Leaks must be found before they can be fixed, so detection is the first step. To minimize the time invested in detection, the means should be automated and the results unambiguous. You don t want your technicians patrolling the plant with a dipole and a spectrum analyzer. Automating the means implies a system that provides an alarm when a leak is detected. The technician carries the detector with him, which beeps or vibrates whenever a leak is encountered. There are many such systems available at reasonable cost. Certainty of detection is established by inserting a unique carrier at the headend, which the detector will recognize to the exclusion of any random free space RF energy encountered. Virtually all detection systems have these two components the carrier generator and the detector. If your system is in close proximity to your competitor s plant, it s a good idea to confirm that you are using different detection systems, or at least different carriers. In today s competitive environment, it s surprisingly easy to waste time chasing your neighbor s leaks. Larger systems may want to equip all technicians installers, maintenance techs, and even contractors with detectors. This approach delivers fast, wide area coverage and reveals a lot of leaks in a hurry. The technicians do not attempt to locate or repair the leaks; they simply report where the detectors fired, and Dispatch prepares a work order for the leakage tech. Though no measurements are made in the detection phase, hot spots in the system can be quickly identified. If all the installers report that their detectors go bananas two blocks away from a given property, that property becomes the next priority for the leakage tech. Location The next step is to identify the precise origin of the leak, and it is arguably the most difficult of the five steps. The dirty secret of signal leakage is that most leaks are caused by your best friend, the subscriber. Like the evil twin of Toolman Tim, he is forever installing homemade fittings, splicing flat lead onto coax; hooking unshielded house amplifiers into the home drop system, and otherwise wreaking emissions chaos within the walls of his castle. In a dense MDU environment, it will not be unusual to encounter multiple leaks emanating from several adjacent apartments. The time-tested method for isolating drop leaks is simple disconnection. If the leak disappears when the drop is separated from the tap, the problem will usually be found inside the subscriber premise. It may be necessary to disconnect all drops and then reconnect them, one at a time, to identify all units generating drop leaks. (Addressable taps and Interdiction units display exceptional virtue in these conditions.) Some of the more complex leakage detection systems include various types of directional sensors, but a simple hand-held yagi antenna, coupled with a reliable signal level meter sensitive to -60 dbmv, can be extremely useful in establishing the origin of a leak. Unfortunately, a yagi cut to the midband channels where the measurements must be made ( MHz) is a fairly cumbersome device. The wavelengths are long, and so are the antenna elements. Technicians muscling a mid-band yagi down the hallway of a toney high-rise have been known to outrage dowagers. A better approach is to use a high frequency channel to aid in leak location. As we learned with detection strategies, it is important that the carrier be recognizably distinct from any off-air channels and equally dissimilar from carriers on neighboring systems. Dedicating an unmodulated carrier above 400 MHz for this purpose is a favored tactic. A cut-to-channel quarter wave yagi at these frequencies can be added to the technician s hand tools without renting a trailer. The directionality of the yagi allows the tech to establish the source from which the leak energy proceeds. The leak level, as displayed on the signal level meter, increases when the yagi is pointed at the leak. Of course, there is a blend of art and science in the use of these techniques. RF leaks radiate into free space,

3 but they also travel beautifully down the coaxial shield. If there is a significant leak in each of four apartments connected to a single tap, the combined energy of the leaks may appear as a high-amplitude locus at the tap. Intuition is an asset in this game, and it can be developed with patience and persistence. Figure 1 Sample Door Hanger Address We don t want to INTERRUPT YOUR SERVICE... but our technicians have detected a signal radiation originating inside your home. The Federal Communications Commission prohibits leakage at these levels and requires us to repair the leak or disconnect your service. To avoid service interruption, call (800) and ask for a CLI Technician. We will be happy to make an appointment at your convenience for the necessary repairs. If we cannot make this appointment within five days, we regret that we will be forced to disconnect your service. XYZ CABLE TV Most signal strength meters are not sensitive to 60 dbmv, but their shortcomings can be overcome with a portable (battery-powered) amplifier of known gain. The more elaborate leakage systems incorporate this feature in their designs, but effective alternatives have been devised (for example) with a 12-volt line extender and a motorcycle battery. (No need to waste. The motorcycle may prove useful in other applications.) Leak location, then, is a dual function of direction-finding and isolation through drop disconnection. When the leaks are determined to be behind the closed doors of your subscribers, an administrative solution is employed. Pre-printed door hangers are helpful. They should mention that the entire program is mandated by the federal government, and they should make it extremely clear that failure to provide access will result in disconnection of service within a certain number of days. Measurement Once the leak is located, it must be measured. The rules say that all leaks greater than 20 microvolts per meter (µv/m) must be repaired to a level below 20 µv, and any leak greater than 50 µv/m needs immediate repair. Commercial leakage systems make these readings easy, typically providing a special carrier at the headend, a dipole of the right size and a special meter that reads in µv/m and is adjustable for distance variations. But perfectly acceptable results can also be obtained with a few simple variations on the humble equipment used to locate the leak in the preceding paragraphs. Table 1 End-to-End Length of Dipole Correspondence Table MHz.5 20 µv 50 µv wavelength For FCC reporting purposes, the measurement must be made on a carrier between 108 and 137 MHz, using a halfwavelength dipole. Again, it is important to select a carrier that can be easily recognized as exclusive to your system. Table 1 illustrates the end-to-end length in inches of a reference dipole tuned to channels in the measurement band. (To calculate the wavelength of a given carrier in inches, divide 11,811 by the frequency in MHz.) The technique is to face the reference dipole to the leak at a distance of 3 meters from the leak (about 10 feet). The dipole elements should also be at least that far away from earth ground and other conductors.

4 The strict interpretation of these provisions may not be possible to implement in the MDU environment. It may not be possible to isolate the dipole from electrical conductors in the ceiling, for example. Still, a leak is a leak. Even if you can t meet all the specific measurement standards in the FCC language, it s a waste of time to ponder whether the leak energy is somehow intensified in the closed environment. To paraphrase a popular cheer, JUST FIX IT! If you re using a reference dipole and a standard measurement device (signal level meter or spectrum analyzer), you will want to convert the µv/m requirements to dbmv. The conversion factors are frequency dependent. Table 1 shows the correspondence between µv/m and dbmv in the measurement band, and Table 2 extends the data set to 1 GHz. The dbmv levels can also be calculated with the formula: L = 20 log (E/21. f) where L is the level in dbmv, E is the voltage in uv/m and f is the frequency in MHz. Let s say you re measuring a carrier at MHz. From Table 1, the length of the reference dipole should be 51.2 inches, and the maximum allowable level is 41.7 dbmv. Between the dipole and your signal strength meter is a line amplifier with 22 db of gain. Your meter reads -16 dbmv. Subtracting the gain of the amplifier, the leak level is -38 dbmv. Record this level on the leakage work order or CLI spreadsheet or on a napkin from the bar of your choice, but write it down. This is a leak that needs repair, but not necessarily immediately. (The threshold for immediate repair is 33.7 dbmv at this frequency.) The recorded level of the leak becomes the baseline for repair. A detection system that is working properly should go into alarm in the presence of leaks that may be below 20 µv level. If you re using a higher frequency to locate the leak, you will have a preview of the measurement you will make at midband. As you can see in Table 2, tolerance for leaks decreases as the frequency rises. This natural effect works in favor of your program. A leak located at MHz and measuring 50 dbmv must be repaired, even if the recorded measurement at midband is below 20 µv/m. The MDU environment poses some special challenges to accurate measurement. Grubbing through the inside wiring of a high-rise apartment, with an unhappy subscriber looking over your shoulder and a shitzu clamped in a death grip on your ankle, you may be tempted to wonder whether a leak of 30 µv/m is still 30 µv outside. You are sidling up to the slippery slope of perdition. Unless you invest in a second career as a window washer, you will never know. Take courage: Leakage repair improves system quality, which will always be Job 1.

5 Table 1 Maximum Leakage Levels The following table gives the maximum leakage levels at the given channels and voltage levels. Ch# Visual 20 µv/m 50 µv/m Ch# Visual 20 µv/m 50 µv/m Ch# Visual 20 µv/m 50 µv/m Carrier (dbmv) (dbmv) Carrier (dbmv) (dbmv) Carrier (dbmv) (dbmv) T T T T T T T

6 Table 2 Sample Leakage Log for a Single Property W.O. ADDRESS UNIT BEFORE AFTER CAUSE Riverside Dr Bad RG-6 fitting at tap Riverside Dr Customer twisted center conductors together Riverside Dr 12th Floor Closet NA Cracked splitter housing Riverside Dr Customer installed amp in attic on flatlead Riverside Dr Bad customer fitting Riverside Dr Bad RG-6 fitting at tap Riverside Dr Bad customer fitting Riverside Dr Bad RG-6 fitting at tap Riverside Dr Dog ate outlet jumper Riverside Dr Leakage tech locked dog in closet Correction Once the leak is located and measured, the repair is often remarkably simple. First on the hit parade is OUTRAGEOUS SUBSCRIBER PRACTICES, as mentioned above. A more bitter pill is the historical and statistical fact that your technicians will be responsible for the second largest incidence of leakage drop and outlet fittings. You will find the occasional true plant problem a tap collar broken at an HTH connector, some cracked hardline but the obscure problems, like a line terminator with a manufacturing defect, are certifiably rare. Leakage techs that follow recommended practice in location and measurement are usually surprised at how obvious (and needed) the repair is. Remeasurement The final measurement proves the effectiveness of the repair, and it is the one that goes into the final CLI calculation. It should be performed with the same equipment, at the same location, as the initial measurement, and the results should be recorded in the same journal (though you may have a new napkin by now). If the leak still measures higher than 20 µv/m, it simply has not been repaired. That does not mean it is beyond repair; it is usually a symptom of incomplete diagnosis. In your first leakage program, the leaks will overlap, compound and confound. Your tech will measure a nasty leak at a tap, disconnect the drops and find a rat s nest of regrettable connectors, poor weatherproofing, drop corrosion, and a range of technical difficulties. After replacing the tap, the drops and all the connectors, only to discover a leak still vibrant in the same location. Probably it radiates along the shield from the units served. The game is yet afoot. Were the initial repairs necessary? Of course. Are we done yet? Nope. After a building has been cleared of known leaks, it deserves a walkout in the paths not taken the roof, stairwell, accessible perimeter. Leakage is insidious; it persists despite our best intentions. Pump up the detection carrier by 3 db at the headend and take a walk. See what arises. Meet it and defeat it.

7 Sources The FCC s Web Site is a very useful source of information. There are several pages designed to make compliance as painless as possible. The following pages summarize the process: Tells you how to comply Is a rules summary Is a signal leakage calculator