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Archives
 July 1999 Issue
Hranac - Notes for the Technologist
by Ron Hranac
New Look at an Old Problem
Satellite Leakage and Ingress
What do you think of when you read or hear the terms "signal leakage" and "ingress?" If youre like most folks in the cable TV industry, you probably think about damaged shielding integrity that results in signals inside the cables leaking out, or over-the-air signals leaking in. You might also think of midband cable channels: aeronautical radionavigation and communication frequencies, cumulative leakage index (CLI), signal leakage logs, leakage detectors, resonant half-wave dipole antennas, Part 76 of the Federal Communications Commissions rules, and maybe even reverse path interference.
All of these are things most of us know well and deal with on a day-to-day basis.
Heres a new one for next weeks technical staff meeting: satellite equipment leakage and ingress.
"Cmon Hranac, there arent enough hours in a day as it is," you groan. "We dont need more goblins to chase."
OK, I promise its not as bad as it sounds. It is a problem, though, and one that deserves a closer look than what weve given it in the past.
L-band leakage
This months column evolved from e-mail correspondence and subsequent telephone conversations with Sierra Satellite Technologys Jeff LaRoche. LaRoche is a satellite antenna installation specialist and has several years of experience working with Simulsat and multibeam antennas.
He has found that many satellite reception problems can be traced to leakage and ingress in the 950 MHz to 1,450 MHz L-band, a common low noise block converter (LNB) output frequency range. Another problem is crosstalk interference.
To deal with this, Jeff has assembled an L-band leakage detection package consisting of a small UHF antenna and DX Communications 950 MHz to 1,450 MHz line amplifier that he uses in conjunction with an ATCI model TE-887B satellite/CATV analyzer and monitor (Antenna Technology Communications Inc., , fax , www.atci.net).
He switches the TE-887B to satellite receiver mode and from a few feet away points the antenna at suspect cables, connectors and other equipment. If leakage exists, voilą!, watchable pictures appear on the TE-877Bs display.
He told me that an estimated 60 percent of the systems he has done work for have L-band leakage problems. More often than not, when peaking the satellite antenna and adjusting LNB cross-polarization dont resolve digital reception troubles, busy backgrounds in the pictures, or as LaRoche puts it, "just plain unexplainable intermittent situations," he does a little bit of careful sniffing around with his test equipment and finds horrible L-band leakage. The problems LaRoche has seen are not unique to any particular type of satellite antenna installations, either.
Connectorization
Outside at the dish, LaRoche has seen crosstalk between 4 GHz low noise amplifiers (LNAs) and LNBs, and problems with N-connectors on some LNAs. The N-connectors themselves arent necessarily to blame; frequently the trouble is with how they were installed. Often there is no strain relief provided for the interconnecting cables, and damage to the cables center conductor or shielding occurs inside the connector.
Aside from loose, corroded (you did weatherproof them, right?) or improperly installed F-connectors on the backs of the LNBs, one of the biggest sources of outdoor L-band leakage is near the base of the antenna.
"The next point I check is the metal junction box at the base of the antennas," LaRoche comments. "Everybody loves to break the cable here and switch to a hard-line cable. Often the test equipment is set up, and when this metal box is opened, leaks become very obvious. With my analyzer set to satellite receiver mode and the line amp turned on, its not unusual to see clean, unscrambled pictures from several feet away."
In the headend
His next test point is the power inserters and splitters in the headend. The cheap ones generally leak like a sieve, and low-cost cable TV drop splitters with their back covers simply glued on are especially bad at L-band frequencies. Not only can these components leak, but when placed too close to other satellite feeds, crosstalk may occur, especially when different satellites use the same frequencies and polarities.
LaRoche recommends the use of dedicated high quality L-band splitters. They cost more, but they are specifically designed for 950 MHz to 1,450 MHz or higher frequencies.
"Loose connections, conventional crimped connectors and unterminated ports are favorites of my test equipment," he adds. "This important satellite connection matrix is often jammed into the rack or under the floor, and most of the cables are unsupported.
"In some ways, the satellite signals are like water. When you switch between many types of cables and have bad valves and fittings, the correct pressure will never reach the receivers. Many systems then use poor quality line amps to make up for the losses they have already incurred. It is not uncommon to view relatively clear video on my test set caused by L-band leakage from the headends satellite antenna cabling, splitters and power inserters."
Trouble can sometimes happen when a lot of signals are brought into the headend in common conduit or when the wiring and components are hidden away haphazardly to keep the headend looking superficially neat.
Many of these problems boil down to a handful of causes: too many splices or connectors in the cable between the LNB and satellite receiver; poor quality splitters, or the use of low-cost cable TV drop splitters; unterminated splitter ports; loose connectors; use of the wrong cable; and craftsmanship.
Some immediate solutions
LaRoche suggests the use of quad-shield coax whenever flexible cabling is necessary, along with premium sealed F-connectors. This includes cabling between the splitters, power inserters and satellite receivers. Where possible, run a single continuous piecethat means no splicesof quad-shield 11-series coax all the way from the LNB output to the splitter array. Unless the distance is greater than a couple hundred feet, 11 series will work just fine for most applications.
If hardline cable must be used, take extra care when installing the connectors. Remember, youre trying to make this perform well in the 950 MHz to 1,450 MHz range. Provide strain relief at all connections so the cables weight is not supported by the connector and so that any flexing that may occur will not affect the cable-to-connector interface.
Tighten and weatherproof all outdoor connectors, even those not directly exposed to the elements. Try to provide as much physical separation between cables as possible, especially cables from different satellite antennas. The same recommendation applies to splitters, power inserters and L-band line amplifiers.
Terminate all unused splitter ports, and make certain the splitters and power inserters really were designed for L-band operation. While many passive component manufacturers make drop splitters that work fine up to 1 GHz (1,000 MHz), their performance can fall off quickly above that frequency. As was mentioned previously, use dedicated L-band splitters and components. Some of the better splitters cost $20 to $30 each, but theyre worth every penny.
Make your own test set
Whats next? Why not do what LaRoche does and put together your own L-band leakage detection equipment? If you have a spare satellite receiver available, use it as the leakage receiver. Connect it to a portable video monitor. Track down an L-band line amplifier, fabricate an antenna that will work at L-band frequencies (a half-wave dipole resonant around 1,200 MHz will be about 4.7 inches from end to end), hook the equipment together, and sniff around for leakage.
While a spectrum analyzer might work just fine, determining just what youre looking at may be a little harder. At least with the satellite receiver setup, a leak will allow you to see pictures from one or more unscrambled satellite channels on the video monitor. You just might be surprised at what you find.
If you have any questions or would like more information on coping successfully with satellite leakage and ingress, I encourage you to contact Jeff LaRoche directly. His e-mail address is , and Sierra Satellite Technologys Web address is www.satinstallpro.com. Alternatively, LaRoche can be reached via telephone at . - CT
Ron Hranac is vice president of RF engineering for High Speed Access Corp. and senior technical editor for "Communications Technology." He can be reached via e-mail at .
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