Ask any cable technician where they spend the majority of their time, and chances are the response will be the same. Irrespective of geographical area, the last 150 feet in the cable TV system continues to be the cable industry's Achilles heel.
The majority of all CATV service calls, have historically been related to some compromise of the service drop or house wiring. Because of the frequent activity relating to service upgrades and downgrades, the customers drop is one of the most handled and mishandled portions of the CATV plant. Add to this the fact that many customers are installing their own outlets, often using inferior equipment, and it is easy to see why 70% to 80% of subscriber complaints result from problems between the directional tap and the television set.
While the distortions seen in a cable system carrying analog service are objectionable, a total loss of service does not always result. When excessive noise or reflections are introduced into the same plant transporting data and digital TV, the result can be more severe. Impulse noise, microreflections and ingress can result in a large number of bit error rates (BER), which can cause a picture to freeze up, or in the case of digital music, objectionable popping and sometimes a complete loss of service results.
The characteristic 75 ohm impedance of coaxial cable must remain constant, or downstream signals will be reflected back in the reverse direction. Several things can occur during installation which can change the impedance of coax.
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1) Stretching. Although the coax supplied by most manufacturers is fairly rugged, it is still possible to place too much stain on it when it is being pulled through conduit or wrapped around corners.
2) Bending. Care must be taken not to bend the cable in less than a 2.5 inch radius or the equivalence of 10 times the outside diameter of the cable. This is most likely to happen when the cable is being routed around corners.
3) Compression. This used to be an especially large problem when staple guns were used during installation. Cable clips which are designed to hold the cable firmly without smashing it have helped to alleviate this problem
Many of the above problems can be avoided through the use of the correct type and size cable for every job. Coaxial cable which contains a flooding compound and is designed for underground use is the only type of cable that should be buried. Aerial or regular house cable that gets used for an underground installation will prematurely fail. For aerial installations, only messengered cable should be used. This will insure the proper support of long drops, and will assist in the prevention of stretching in wind and snow loading conditions. Using the right size cable for the particular application is also important. RG-6 coax should only be used for relatively short drops, the length of which should be determined based on the highest frequency being used. For longer Drops , an RG-7 or larger cable should be used.
Splitters
Splitters are obviously a necessary part of most installations. There are however several factors to keep in mind when they are used in either analog, or digital systems.
Isolation
A typical CATV splitter with a bandwidth of 5 MHz to 1,000 MHz has its outputs isolated from each other by 20 dB to 25 dB. It is the isolation between these output ports which prevents the local oscillators (LO's) in TV's, computer modems, and converters from interfering with each other. If outlets exist which are not being used, they should be terminated with a 75 ohm resistor, or a smaller size splitter should be used. This will prevent the unused outlet from acting like an antenna, and back-feeding interfering carriers into the other outlets.
Return Loss
Return loss is defined as; the difference between the power incident on, and the power reflected from a discontinuity, it is given in dBs. The average quality splitter used in most systems has a return loss (RL) of 16-18 dB. While these numbers are acceptable in an analog system, they might not be high enough for digital signals. It is interesting to note that TV's and converters have the lowest RL of the equipment used in most cable systems, they average only around 10dB.
Shielding
The EMI shielding of splitters can range anywhere between 70 dB to 140 dB. Shielding and construction of splitters go hand in hand, and as in most other areas, "you get what you pay for". The need to keep signals contained in a "closed system" in order to keep leakage to a minimum, has made it necessary to pay more attention to shielding. FM band radio, cellular phones, pagers, CB radio and Amateur radio are just a few of the signals that can get into a system unless good shielding is present.
Connectors
There are many good connectors on the market today, if properly installed, most will provide 80 dB of shielding. The key here is proper installation. The connector in a cable system is undoubtedly the single component which is responsible for the highest majority of all service calls. Proper training ,and the use of appropriate installation tools will keep connector problems to a minimum. All connectors which will be exposed to outside weather conditions should be properly sealed against moisture.
Training
This is probably the most important element in reducing the number of service calls in any network. If properly trained and outfitted with adequate tools, an installer will be the most effective front-line defense against customer complaints.
FireWire
And if all this drop maintenance stuff is too much of a pain, consider an alternative to traditional coax for the home. The IEEE 1394 digital interconnect standard better known as FireWire could someday replace other more common networks in the home. FireWire promises to deliver speeds of 200 Mbps between interconnected consumer electronics in the home, and is work is progressing towards speeds which will probably exceed 1 Gigabit per second. So if it's speed, bandwidth, and easy maintenance your interested in, fasten your seat belts and take a look at this developing technology. The consumer electronics industry is predicting that 25 million home electronics such as VCRs, TVs, DVDs, etc. etc. will be sold with the FireWire plug in them by the end of 1999! (More on this in future columns).
Richard Campbell works for TCI Cablevision of Nevada, Inc. as an advanced technician. Any opinions or statements expressed in this column are solely his and not those of TCI. If you have any suggestions or comments, contact Richard at soup9034@aol.com.
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