Cable Leakage Technologies
As I walk the grounds of Texas Christian University (TCU), I can’t help getting hit with a wave of nostalgia at being back at school. Seeing all of those bright eyed kids with their entire lives ahead of them is very reenergizing. But my reason for being here is to help a life long friend of mine, Dan Kennedy, deal with a public notice from the FCC. And topic, signal leakage, just happens to be right in my wheelhouse. The notice, issued on August 16, 2012, informed all noncable multichannel video program distributors to report their use of and prevent and eliminate signal leakage on aeronautical frequencies. Noncable MVPDs include universities, hotels, apartment complexes, etc. And the fines for noncompliance are pretty steep: Up to $37,500 for each violation or day of continuing violation. So I told Dan not to worry. Let’s get to work.
The FCC rules state that we must find and fix and fix all leaks 20 microvolts per meter and higher. With that in mind we set about choosing a test frequency between 108 and 137 MHz. We selected cable channel 99s visual carrier at a frequency of 115.275 MHz. Then it was time to think about what the landscape would be like to be patrolled. This is definitely different than a standard city or town where the ride out of the hard line plant consists of driving down all of the streets and alleyways. At many universities, the standard mode of transportation for staff members is a golf cart.
Campuses are ringed with sidewalks surrounding the dorms and halls. Any thought of using a standard car or truck were deemed to be inadequate because a great deal of the plant would be missed. No problem. We rigged up a power cable for a Wavetracker GPS-based leakage detector with clips to clamp onto the golf carts battery. Next we found that the roof of the golf cart was fiberglass so mag mount antennas had no metal to stick to or create a ground plane for the leakage detection antenna. But we found a way to secure the GPS antenna and the Doppler antenna on top. A standard quarter-wavelength monopole antenna needs a metal ground plane around it approximately equal to the height of the antenna element. The Wavetracker Doppler ADF (automated direction finding) antenna is designed with four vertical elements that make up the antenna array and also four horizontal elements that act as the ground plane around the antenna array. Most Doppler arrays do not have this ability to do without a metal rooftop to act as the ground plane
Next we ran a drop cable from an accessible tap to a freestanding transmit dipole test antenna. We measured it with a freestanding receive dipole antenna connected to a standard signal leakage meter. The two antennas were then placed 10 feet away from each other. The dipole elements were set to 25 inches in length for a total 50 inches end to end, in order to be resonant on 115.275. The formula for element length is 11811 divided by the frequency divided by 4, which provides the quarter-wavelength of that frequency. An important point to remember is that both dipoles are oriented vertically because the RF antenna array on the golf cart has elements that are oriented vertically. If the polarity is not matched, signal leakage readings could be off by 20 dB, or more. So with all this correct we were able to make sure the leakage detection equipment on the golf cart, which was also 10 feet away from the transmit antenna, was reading the same as the hand held leakage meter. All checked out so we were good to go for the ride out. Then we rode around every building that had cable plant connected to it. We also drove by every manhole and hand hole that contained underground plant. After the ride out we uploaded the data to the automated positional leakage analysis software for analysis and work order production.
Our next task was, while using a hand held leakage meter, to walk out every closet, node room, and communication room. We covered everywhere with indoor hard line plant. Amplifiers, taps, splitters, etc, were checked. Some folks might think this walk out is unnecessary but that is not true for several reasons. First, the FCC rules tell us to inspect our hard line plant. Second, there are rooms deep within these buildings that are missed from the sidewalk ride out. Also, rooms on the upper floors could be missed. And third, these rooms are visited by folks working on equipment other than the cable TV network so leaks can be caused by their inadvertent handling of our plant.
Another misconception is that the stubby rubber duck antenna that comes with most hand held RF leakage meters can be used to measure the leak. That is not the case. That is for probing the equipment to find the source of the leak. A dipole is designated for correct measurement. One thing I noticed about these rooms is that they are very tight and cramped. So in some rooms, it is next to impossible to measure signal leakage and then calculate a CLI (cumulative leakage index). The rules about calculating a CLI require that you measure the leak by using a hand held dipole at 10 feet and spin it vertically and horizontally to maximize the leakage reading. Well years ago, I was introduced to the term “letter of the law and spirit of the law.” So I got as close to 10 feet away from the leak as I could. Also, because of the racks and other IT equipment, I was only able to open my dipole vertically. I could not spin it in the horizontal plane.
The aeronautical frequencies are not the only thing we monitored at TCU. There is a rising issue of concern about leakage from a cable plant causing interference to the cellular world of 4G LTE. This is the spectrum between 700-800 MHz. The present FCC rules about interference outside of the 54-216 MHz spectrum limit leakage to a maximum field strength of 15 µV/m at 30 meters. On a free space basis, this is equivalent to 150 µV/m at 10 feet.
Presently the FCC has a notice of proposed rule making (NPRM) out on cable technical standards that asks about signal leakage above 400 MHz, which includes LTE. I wonder if after information has been gathered and new rules are made if they will contain something about monitoring close to the cellular towers. Obviously, the closer a leak is to the tower, the greater the potential to interfere. I remember some years back when TV interference or electrical noise was a concern for CATV operators, especially in the grade B contours where a weak VHF local channel was being received by the headend tower. If a source of TV interference was close to the tower, it could cause sparklies in the picture of the retransmitted VHF channel. So patrols were done within about a mile around the headend tower. What I have seen is that AT&T and Verizon will search for interference around their towers if there appears to be a problem. Instances of cable leakage on LTE frequencies are being found and the local cable companies are being contacted to fix the leakage. The problem is that most CATV companies are only equipped with leakage equipment to monitor the aeronautical bands so they cannot tell for a fact that the high frequency leak is fixed after they work on it. Much testing has been done the last two years and it has found that leaks in the aeronautical bands do not correlate with leakage in the LTE band. So fixing one may not necessarily fix the other.
We used a Wavetracker C Lite meter. This is a low cost, hand held (combo) spectrum analyzer and leakage meter for monitoring in the LTE band
as well as the aeronautical frequencies. The C Lite also measures wifi signal levels. Spectrum analyzers are traditionally very large and expensive. This hand held spectrum analyzer is about the size of an iPhone. We also used a hand held Yagi antenna tuned to the LTE band. We were able to clearly see the leaking 6 MHz wide QAM signals with the recognizable dips on either side, between the surrounding QAM signals. What we saw was definitely a cable leak in the LTE band. With it, we were able to check every room on campus as well look for leaks during the sidewalk ride out using a monopole antenna tuned to the LTE spectrum. This is important to a university that just discontinued landline phones to all the dorms. Every kid has a cell these days. Also, TCU has a cell network called a distributed antenna system. And they go to great lengths to see that there is good cellular signal on every inch of the campus. I remember the old commercial where a guy sits in every seat in the football stadium to check signal. Well, that is real life scrutiny at TCU. So in the interest of total spectrum clearing leakage detection, we patrolled for leaks in the aeronautical and the LTE bands.
The leakage tasks at TCU were divided into three categories.
1) initial set up and first patrol.
2) annual CLI.
3) quarterly monitoring
The initial set up and first patrol is where most of the leakage will be cleaned up as all connectors are tightened and defective equipment and connectors are found and corrected. The first initial sweep is the most extensive and will take more time than the subsequent quarterly monitoring.
The annual CLI is where a ride out and walk out of the plant is performed and all leaks 20 µV/m and higher are found. Then all leaks 50 µV/m and higher are identified and a CLI calculated and reported to the FCC. The CLI formula provides a figure of merit that must be less than 64. And all you have to do to pass the FCCs CLI requirement is to fix the largest leaks. Once they are fixed, they are taken out of the calculation. A single leak of 1600 µV/m will cause a cable system to fail. But fix it and you pass. Also if you have several large leaks in the neighborhood of 500 µV/m, a system could fail. So the point is, don’t be scared of finding high level leaks. Just fix them. The FCC knows all systems have at least some high level leaks. What the Commission cares about is detecting and fixing the leaks to prevent interference. Of course, good engineering practices and good documentation will help you if you get into trouble. I was asked about the issue of when the university filed their CLI paperwork or electronic filing, will there be a penalty for having this cable system all these years and never filing? Well, the answer is, not if you don’t get caught before now. What they care about is that you are filing now. But remember, you ignored the leakage rules that came out in 1984. Ignore it now at your own peril because the Commission has issued a new warning in 2012. And the FCC field offices are mobilizing and are adding noncable MVPDs to their lists.
The third item is to make this part of your maintenance schedule by doing quarterly monitoring essentially all of the cable plant for leakage four times each year, and repairing any leaks that are found. This is the cornerstone of having a superior performing plant. Two kinds of maintenance are done at most cable systems: Preventive maintenance and demand maintenance. Quarterly monitoring is an example of preventive maintenance and the dividends will come back to you in the form of a better performing plant. The noise allowed into your system from loose connectors and breaks is harmful ingress. At TCU, problems with blocks of digital channels pixelating are disappearing after the leakage was corrected. This was because noise was entering the plant as ingress. An example of demand maintenance is if you neglect the problems in your system and they cause some of your channels to pixelate and customers call in complaining about it generating a work order.
Once again I’m reminded of things I’ve seen over the years. TV interference, ham radio and aircraft communication are a few examples of what is called “erroneous RF” by techs that are only concerned with finding the RF leaks from their own plant. There are many hostile RF ghosts in the over the air environment that out there and can enter your cable network. At TCU, I looked at the Sid Richardson building. The cable headend is here as well as distribution plant feeding the building. This building is the home of the science and chemistry departments. I can only imagine what a chemical analysis spectrometer can produce in the way of noise. Sid Richardson is also the home of the Monnig meteorite collection. I’m pretty sure there is no RF noise there in the cable spectrum but you never know.
But anyway, the dividends are huge for cleaning up your leakage. Already service calls are going down at TCU. And also peace of mind from not getting visits from the FCC. TCU’s aggressive leakage program has brought under control its RF electropollution footprint. And that benefits all.