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Many homes have more than one TV set. Others have at least one FM radio receiver and VCR. Consequently, the installer often must use an antenna system that feeds the received signals to two or more receivers. This is called multi-set reception.
Supplying the signal from one antenna to two or more receivers requires the use of special devices that pre- vent problem-causing interaction between the TV receivers. Without the devices, the interaction between receivers will reduce the quality of the pictures displayed by the receivers. Problems caused by such interaction include multiple images (ghosts) and loss of picture detail (smearing).
One method of multi-set reception uses what are called passive devices. The term "passive" means that they do not amplify (increase) the strength of the signals. Passive devices prevent receiver interaction by "isolating" the TV receivers from each other. However, because passive devices cause reduction of signal strength, multi-set reception systems equipped with these devices require more signal strength out of the main transmission line than do antenna systems that feed only one TV set. Typical signal strengths required for multi-set reception vary from a minimum of 2000 microvolts (uV) to over 4000 microvolts depending on the number of passive devices used in the system.
There are two types of multi-set passive devices: couplers and splitters. Couplers are used in multi-set systems equipped with 300 ohm twinlead. Splitters are used in systems that are equipped with 75 ohm coaxial cable. Couplers and splitters perform the same basic function.
Most couplers are designed to distribute signals to either two or four sets. The coupler (Figure 10-1) usually is mounted on or near one of the TV sets, preferably one that is centrally located. Twinlead is run from the coupler's outputs to the antenna input terminals of the TV receivers.
Couplers are the most economical way of feeding two or more TV sets. However, because twinlead is used, couplers present all of the interference pickup and other installation problems associated with twinlead. Be careful where and how you mount couplers.
Most splitters (Figure 10-2) are also designed to feed either two or four TV sets. However, because they are designed for use with coaxial cable, splitters can be mounted just about anywhere without picking up interference or causing other problems that reduce picture quality.
When a four-set splitter is used to feed only three receivers, the unused splitter output must be terminated" to prevent signal problems. "Terminated" means that a small 75 ohm device (Figure 10-3) is connected to the unused output connection of the splitter. This is done to prevent the unused signal from "bouncing back" through the coax and inter- fering with the signals fed to the TV receivers. Terminating devices are available for use with most standard splitters.
Before attempting multi-set reception with passive devices, use a field-strength meter to measure the signal strength at the receiver end of the main transmission line. If it is less than 2000 microvolts (uV) and you need to feed three sets, you will have to increase the signal level. To do this, you will have to use one of the following methods:
* A higher gain antenna
* A mast-mounted preamplifier (preamp)
* Both a higher gain antenna and a preamp
* An Amplified Coupler
When signal levels are weak, a preamplifier (Figure10-4a) should be used as this will give the lowest system noise figure. A distribution amplifier (Figure l0-4B) is a type of amplifier that is mounted indoors, usually in a central location. It is designed to provide amplified signals to a number of TV sets. The term "amplified" means that the strength of the signal is increased by electronic circuits called amplifiers. Seethe Channel Master catalog for models.
The distribution amplifier should be mounted in a relatively central location so that the coaxial cables (through which it feed signals to TV sets) are the same length. This will help ensure that all TV sets receive the same strength of signals.
In areas with very weak signals you will need to use both a mast-mounted preamp and a distribution amplifier. However, be sure the two amplifiers are matched. The term "matched" means that the electronic characteristics of the two amplifiers are similar enough to permit using them together without causing interaction and reducing picture quality. A signal~distribution system that is equipped with a matched pair of amplifiers is called a tandem system. The matched amplifiers are called "tandem units." See the Channel Master catalog for models.
"MATV" means Master Antenna Television (system). A home MATV system is an expanded version of the multi-set reception systems described previously. It is used to distribute the signals from one antenna (or antenna array) to a number of wall outlets located at convenient points in all or most rooms of the house.
The components and distribution network of a typical medium-size home MATV system are shown in Figure10-5. The antenna signals are fed through coaxial cable to a distribution amplifier. This amplifier increases the strength of the signals and then splits them into two or more outputs. The signals at these outputs are fed through coaxial cable to conveniently located splitters. The splitters, in turn, divide the main branches into subbranches, which feed the wall outlets in one or two rooms. Coaxial cable is used throughout the system. Ideally, a home MATV system should be installed while the house is being built, so that the cables can be conveniently run inside the walls. However, a MATV system can also be installed in a finished house by feeding the cables into the wall spaces through holes drilled in the attic floor above the spaces. The cable-entry holes also can be drilled through the main floor from the basement or crawl space.
A home MATV system can be expanded to provide closed-Circuit TV surveillance of various outdoor and indoor areas, including nurseries, playrooms, and house entrances. It also can be used to distribute Cable TV or Community Antenna Television (CATV) signals throughout the house.
MATV is relatively easy to install. Don't hesitate to suggest MATV to your customers. Most probably do not know about MATV and the convenient, high~quality reception that it provides for a relatively small investment. You can substantially enhance your reputation and your profits by providing MATV sales, installation, and service to both homes and businesses. MATV for business is no more difficult to plan and install than that used in large houses.
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