Cable Television networks are intended to be closed systems. Signals that are within the coaxial cable are intended to stay in; signals outside of the cable are intended to stay out. Over the air licensees are authorized to use their assigned frequencies. Cable operators are allowed to use those same frequencies as long as the system integrity keeps those channels contained within the cable. This works well until the cable shield cracks, connectors loosen, other faults occur that allow signals on the cable to leak out (egress) and over the air signals to leak in (ingress). When this happens, signals interfere with each other. Quality of service suffers on cable and over the air communications may not get through.
In the mid 1980's, as cable operators began adding new channels, problems with shielding integrity started to become evident. Interference between cable TV mid-band channels and over the air licensed services, such as Federal Aviation Administration (FAA) communications and navigation channels became an issue. The Federal Communications Commission (FCC) worked with the FAA and the cable industry to find ways to control this interference. Due to the harsh environment in which the cable television plant lives, signal leakage can never be eliminated but only managed.
The FCC arrived at a short list of procedures to mitigate interference and still allow cable operators to continue to use the effected channels. As long as the interference is under control, the cable operators can continue to use the channels. If interference is not controlled, the cable operators risk the loss of those channels and monetary fines. The bandwidth impact is the loss of all channels from 108 to 137 and 225 to 400 Mhz.
The FCC revised 47 CFR 76.611-76.617 to include new requirements in order to use channels in FAA aeronautical bands.
First, cable operators are required to follow frequency offset formulas. In the 108 to 118 MHz navigation band, cable operators' carrier frequencies are required to use odd multiples of 25 kHz. In the 118 to 137 MHz and 225 to 400 MHz communications bands, allowed frequencies are odd multiples of 12.5 kHz. In this manner, the cable channel carrier frequencies fall exactly half way between FAA channel carriers and offer a degree of interference protection. Cable frequency tolerances are required to be +/- 5 kHz in these bands and apply to all channels carried at levels above +38.75 dBmV.1 Initial notification is required to the FCC before carriage begins and then annually thereafter in FCC Form 320.
Secondly, cable operators are required to perform quarterly monitoring and annual measurements. On a quarterly basis, operators shall cover substantially all of the cable plant noting and repairing all leaks found in excess of 20 uV/m at 10 feet distance from the cable plant. The results of this monitoring must be on file for inspection by the FCC or the general public for a period of 5 years.2
Annually, in order to show compliance, the cable operator has the choice of an airborne measurement or performing calculations based on groundbased measurements (Cumulative Leakage Index or CLI). Procedures for each approach are defined in 47 CFR 76.611 of the FCC rules for Cable Television.
Groundbased calculations involve the cable operator dedicating staff and equipment resources to drive out the majority of the cable plant searching for leakage in excess of 20 uV/m. The measurement can be done using an video carrier or a dedicated signal leakage carrier. Measurements are to be taken in the aeronautical band of 108 to 137 MHz or correlated back to that band if measured at other frequencies. Measurements are to taken on an unmodulated carrier or correlated back to an unmodulated carrier if a modulated carrier is used.
When leaks are found, measurements of the leak intensity are taken using a half-wave horizontally polarized dipole antenna at 10 feet (3 meters) from the leakage source. Leaks measured in excess of 50 uV/m at 10 feet are plugged into one of two formulas to determine if the system has a passing or failing score.3
Flyover testing involves equipping an aircraft with a suitable antennas and test equipment and measuring signal leakage in the airspace above the cable TV plant at an altitude of approximately 1500 feet (450 meters). Parallel passes are flown at approximately 1 mile spacing covering the entire cable plant. Signal leakage levels and GPS data are collected in a matrix to associate leaks found with specific locations. Care is taken in selecting the test frequency to avoid interference with known over the air transmitters in the area. Specific offsets or tagging can be used with the test signal to rule out signals from adjacent cable or SMATV systems, if necessary. Finally, the collected data is analyzed and a report is created showing "hot spots" and percentage of data points below the 10 uV/m threshold.
1) A Snapshot in Time
Groundbased measurements can be extremely time consuming, depending on the cable system size. Conditions can change over time and collecting data for a groundbased CLI calculation can take up to 3 months. Changing conditions can have a dramatic effect on leakge levels over time. One advantage of a flyover is that it gives you a snap shot in time of system leakage performance over a period of a few hours rather than a few months.
2) Third-party Verification
A flyover provides an outside audit of a cable system's ground based monitoring work. How effective is your in-house signal leakage monitoring program? A flyover provides the credibility of a 3rd party review, similarly to that of an outside financial audit. Trust but verify.
3) Cost Effective and Minimal Impact to System Operations
Using resources to measure leakage and calculate the Cumulative Leakage Index diverts personnel from service calls and maintenance affecting customer service. Performing a flyover requires minimal system resources; generally limited to a technician installing and setting up the headend signal generator (leakage signal source). The cost per mile of a flyover on most systems is less than the cost per mile of a ground-based measurement and calculations.
4) A Useful Tool
Beyond meeting the FCC requirement for annual signal leakage measurment and reporting, a flyover can be a useful tool for locating signal leakage in the cable plant. Hot spots found during the flyover can be correlated to a half mile radius on the ground to help isolate leak locations. After major natural dissasters such as tornados, hurricanes, or ice storms, many cable systems perform flyovers to determine the extent of plant damage and to prioritize repair resources based on where the strongest leaks are. The Frequency Finders will be happy to discuss how we can meet your specific needs.
Even when using a flyover, it is important to remember that the annual report is not the only signal leakage requirement. 47 CFR 76.611 requires quarterly monitoring on the ground to keep signal leakage under control. The easiest way to comply with the quarterly monitoring requirement is to equip technician vehicles with signal leakage equipment designed to alert the technicians to leaks as they are driving from location to location in the course of their normal duties. When a leak is detected, the location can be noted and turned into to dispatch to generate a work order. Other detection systems are available incorporating a GPS to automatically log the leak level and location with no intervention required by the technician.
Controlling signal leakage is a two part approach; monitoring and testing. Think of the quarterly monitoring as "homework and studying" and the flyover as the "final exam" showing how well you prepared for the test. The better you perform the quarterly monitoring and associated repair work, the better your flyover scores will be.
With the advent of additional cell phone frequencies, interference in the 700 and 800 MHz LTE bands is a new issue. Leakage in these bands often does not correlate with leakage in the aeronautical band. Leakage in one part of the LTE band does not necessarily correlate to leakage in other parts of the band.
While present FCC rules do not require signal leakage reporting in these bands, rules exist requiring leaks be below 15 uV/m at 300 meters and that no harmful interference will be generated no matter what the signal level is.
While the FCC rules for flyovers in the LTE bands are not presently in place, The Frequency Finders have been working diligently with ARCOM to create viable methods for detecting and measuring UHF leakage from QAM signals.
The Frequency Finders offer nation wide flyover service in both analog and digital formats. Formed in 2008, we continue to expand our services and client base. The Frequency Finders currently perform flyovers for Comcast, Time Warner Cable, Cablevision and Adams Cable among other providers. Operating a variety of aircraft appropriate to geographic system location and size has allowed us to offer competitive pricing and efficient flyovers. The company has focused on utilizing the most appropriate techniques for aerial measurements. In doing so, we employ the latest industry leading equipment.
In 2014, The Frequency Finders launched a new analog product, increasing the accuracy and location reporting of leakage. We are also nearing completion of our digital solution, partnering with ARCOM Digital and their award winning QAM Snare technology. We believe this technology is the best solution for digital flyovers. While confident in our product and technique, we have under gone a third party review by Johnson Telecom, insuring only the best techniques are employed. Our website offers secure access to flyover results as well as future schedules for client reference. Each staff member is highly trained; our aircraft utilize custom antennas and state of the art navigation equipment. We maintain an excellent relationship with both the FCC and FAA, allowing us to operate within special use airspace and among the nation’s busiest airports.
|Signal Leakage Receiver||ComSonics Sleuth 2 and QAM Snare|
|GPS Receiver||Various brands and models|
|Aircraft Antenna||360 Antenna, Yonkers, NY|
|Calibration Antenna||360 Antenna, Yonkers, NY|
|Signal Filtering||Trilithic Band Pass Filter|
|Headend Signal Source||HP Signal Generator model 8647A-1E5|
The Frequency Finders can help you with your signal leakage compliance needs in a reliable, timely, and cost effective manner. Additionally, the flyover reports make an excellent tool to help find leaks on the ground, based on the airborne readings. Contact The Frequency Finders to see how we can help you with compliance and interference issues.
1 - 47 CFR 76.612 Cable television frequency separation standards
2 - 47 CFR 76.601(e)
3 - 47 CFR 76.611