October 1, 2003
Stop That Leak
Easy-to-Implement Signal Leakage Strategies
By Ken Eckenroth, Cable Leakage Technologies
Controlling signal leakage is not just about complying with Federal Communication Commission (FCC) regulations. It’s about protecting aircraft in our skies. And it’s working. The inevitable improvement of the RF electropollution cloud since 1990 is the combined effort of thousands of individuals. The United States is now a better place for the safety of the flying public.
An aircraft accident caused by RF interference is highly unlikely in America. Thanks to the FCC’s creation of signal leakage rules, the cable industry has reaped enormous benefits in the form of improved picture quality and superior cable modem service. Fixing egress (leakage) corrects most of the ingress problems on the return path.
What exactly are our requirements? The FCC requires cable operators to patrol the majority of the cable plant in each of their systems four times a year for leaks of 20µV/m and higher. Four times a year may seem excessive, but, on the bright side, cable operators can use this opportunity to visually inspect and record any obvious problems with their plants. These are the solid basics for demand and preventive maintenance. This program can be implemented while satisfying the FCC’s signal leakage requirements.
In addition to quarterly monitoring, the FCC requires a yearly test called the cumulative leakage index or CLI. This test yields a figure of merit that relates to the system’s total signal leakage, and can be thought of as a snapshot of system performance. A system is usually defined by an area serviced by a single headend. Engineers can perform ground-based measurements or an aerial flyover. A flyover measures leakage at an altitude of 1,500 feet above ground level. Ninety percent of the measured leaks must be below 10 µV/m to pass. If the system fails, you must fix the leaks and perform the flyover again.
Ground-based measurements also can satisfy the yearly CLI requirements. There are two formulas; the I of 3000 and the more commonly used I of infinity. Each leak 50 µV/m and higher is used in the ground-based formula. The results of the I of infinity formula must be less than 64. A single leak of 1600 µV/m can cause a failing score. However, all the cable operator must do is fix the leak and enter the repair results in the CLI exhibit B report, and that leak can be left out of the CLI calculation.
Over the years, I have noticed that some cable operators fix large leaks and do not report the results. I believe they are afraid that it might trigger an FCC audit just like a tax red flag might trigger an IRS audit. The fact is, the FCC knows that all plants can have large leaks. A leak on the output of a line extender with its high signal levels can be a common scenario for high-level leaks. What the FCC wants is for these leaks to be found and fixed. The Commission loves that. And I, as a member of the flying public, love it too. The questions the FCC will ask will be about engineering practices and documentation.
An unfortunate reality in searching for signal leakage from our plants is the presence of erroneous RF. The frequencies we use are shared by others. You could have an airplane overhead with a pilot keying his mike producing large amounts of RF that splashes onto the cable operator’s leakage channel. Aviation navigation aids, like a VHF omnidirectional range (VOR), are also in our patrolled spectrum and can be confused with a cable leak. Other sources of erroneous RF include: ham radio, a competitor’s cable system and power line interference.
Electrical interference may be produced by power lines. It is not necessarily from high-voltage lines. In fact, it is more common to see it on the lower voltage secondary lines. The power companies use electrical conductors to transmit voltage from point to point. The voltage flowing through the conductors is surrounded by a magnetic field. The conductor is supported at the pole by an insulator. If the insulator has a crack, the magnetic field can arc across this gap producing a very disruptive broadband signal, which can splash across our patrolled cable aeronautical frequencies. If the interference is close to a headend tower, it can interfere with received over-the-air TV channels.
VHF channels 2 through 6 can have severe sparkles on their pictures as a result of electrical interference. A cable operator with the right equipment can locate the source of the interference and ask the power company to correct the problem. Another source of interference can be loose pole hardware. A magnetic field also can arc across this gap. Power line interference is most evident on dry days and appears to vanish when it rains because the moisture in the air shorts out the gap.
There is a method for ignoring erroneous RF called channel tagging. Channel tagging cannot be overemphasized for its importance and value to the cable operator. It is the most reliable way I have ever seen to differentiate cable leaks from erroneous RF. It could easily be said that it is 99.99 percent accurate when installed and used correctly.
A common misconception is that tagging is only needed for overbuild situations. Channel tagging allows the user to go deep into the noise floor and reliably detect leaks of 2 and even 1 µV/m. This deep detection is important when monitoring leaks in backyard easements. After a 20 µV/m leak travels 100 feet, its measured strength is only 2 µV/m. So if a cable operator is ignoring 2 µV/m leaks at the street, it may not be finding all the leaks it’s required to find.
The channel tagger is fast and very sensitive. The cable operator can look for low-level leaks that may be causing ingress and cable modem problems. The tagger causes the modulation of the channel to rise and fall typically 20 times a second. This is definitely a man-made occurrence and is difficult to confuse with erroneous RF. Those folks that have been in cable for a while remember the old Cuckoo systems on the FM bands. With an extremely low-level leak, one could barely hear the warbling Cuckoo signal, but it was still there. The tagger has the ability to help us find low-level leaks and automates the process through hardware and software.
Knowledge of antennas is essential for an effective leakage program. Let’s start with the element length. Free space wavelength in inches can be found with the formula 11,803/frequency in MHz. For example, Ch. 17’s free space visual carrier wavelength is 11,803/139.25 MHz = 84.76 inches. Next, divide by two to get the free space length of a 1/2 wave dipole or divide by four to get the element length of a 1/4 wave monopole. In practice, actual antenna dimensions will be about 95 percent of the free space value.
If this length is off by more than 1 or 2 inches, the resonance will be wrong, and the antenna will receive less than the total amplitude of the leak. The next thing to be concerned with is the ground plane around the antenna. A 1/4 wave vertical monopole needs a ground plane around it equal to the height of its element. Vehicle metallic rooftops make a suitable ground plane.
A 1/2 wave dipole is ideal for use away from the vehicle. It is generally used by a technician to measure a leak at 10 feet after the leak is located. It does not need a ground plane, although its gain will be affected by proximity to the earth or nearby objects. A global positioning satellite (GPS) antenna, while it is mounted on the roof of a vehicle, does not need a ground plane around it. (See Photo 3 at right). It is generally a double dipole or patch antenna tuned to the frequency of 1.575 GHz.
Another type of antenna array used is a Doppler antenna. Four 1/4 wave vertical monopole antennas are spaced evenly in a square. A ground plane around it in all four directions is necessary.
Software used for signal leakage monitoring and maintenance is varied and widely available. Two popular programs are LES and CLIDE. These programs provide for all types of reports as well as the CLI formulas. Other types of software that combine GPS tracking with geographical information systems (GIS) digital mapping also are widely available. These software packages overlay the leakage measurement results, which are pinpointed by GPS coordinates, onto a digital map.
A question that I get frequently concerns the coordinate system used in GIS mapping. GIS coordinates are given in decimal degrees. These are different than the standard coordinates derived from GPS, which are degrees and decimal minutes. The coordinates from plotted maps are given in degrees, minutes and seconds. Let’s look at an example.
The GIS map coordinates for Dallas, Texas, are decimal degrees:
- Latitude: 32779893
- Longitude: - 96789918
The GPS coordinates for Dallas, Texas, are degrees and decimal minutes:
The plotted map coordinates for Dallas, Texas, are degrees, minutes and seconds:
Further confusing things is the minus sign in front of the 96. That minus sign indicates west longitude.
The formula is simple. For the latitude, remove the first two digits, in this case, 32. This is the degrees. Next, multiply the rest of the number, 779893 times 0.6 = 467936. Remove the first two digits, 46, to obtain the minutes. Then, multiply the rest of the number, 7936, times 0.6 = 48, to get the seconds.
The same process is applies to the longitude. However, if the location is on the West Coast, the longitudinal degrees will have three digits, so the first three digits must be removed before multiplying by 0.6. Usually, the format for an FCC site location report is in degrees, minutes and seconds. Note that some GPS receivers allow the user to change the displayed coordinates between degrees-decimal minutes and degrees-minutes-seconds.
GPS and GIS software is advancing. What was once common PC-based mapping software is available as a web-based protocol retrievable by any Internet browser. This has immense logistical value for a local cable operator as well as a corporate MSO wishing to keep track of its systems.
Controlling signal leakage doesn’t just impact your plant’s performance. It impacts lives. Technology for finding and documenting signal leakage has improved dramatically. Having a solid leakage control plan in place will make the FCC happy, as well as your customers, who will benefit from better picture quality and digital services performance.
Ken Eckenroth is vice president of technology for Cable Leakage Technologies. Email him at firstname.lastname@example.org.