Chapter Eight - TV ANTENNA-MOUNTED AMPLIFIERS
A mast or TV antenna-mounted amplifier (preamp) is used primarily to eliminate "snow" on the TV screen. "Snow" is actually electrical noise that is generated by the TV receiver and other electrical devices. The object of any installation is to deliver to the TV input terminals, signals that are strong enough to override the noise (snow). Traditionally a "weak" signal is defined as one that is not strong enough to override the level of the noise in the receiver.
Update: Preamplifiers today are strill primarily used to deliver a strong digital signal to the receiver. However, snow and interference will not appear in a digitally received TV picture. A loss of picture is now the result of weak signal.
When to Use a TV Antenna Preamp
There is no simple or universal rule that tells you when to install an antenna-mounted preamp. Generally if the level of the received signal is less than 1,000 uV at the receiver input terminals, preamplification is needed. Customer preference is another factor that must be weighed when considering the use of a preamp. Some customers may not agree to the added expense of a preamp and will be content with a picture that is less than perfect. Others may insist on a preamp just because they want only the best. Aside from customer preference, there are many other variables that will influence your decision. For example, a preamp generates a small amount of noise. In areas with extremely weak signals, even the low level of noise in the preamp will be too near the level of the received signal. Because both the received signal and the noise are amplified an equal amount, the difference in their relative levels will never be great enough to permit the received signal to override the noise and eliminate the snow on the screen. Consequently, you might want to consider another method of boosting a very weak signal. Vertically stacking two TV antennas and pointing them in the same direction is probably the best alternative to a preamp. This will provide additional gain without introducing more noise. However, this will not always solve the problem especially if the received signal is exceptionally weak. In extremely deep fringe or otherwise difficult reception areas, you might have to use both TV antenna stacking and a preamp. However, this is very rare
Selecting a TV Antenna Preamplifier
A major consideration when selecting a TV antenna preamp is the possible sources of interference in the area. It is vital that as an installer, you become familiar with the commercial broadcast and other radio frequency signals that are being used in your area. Local FM stations, police and fire frequencies, military communications, and other sources of radio frequency signals can cause interference that should be trapped (filtered out) at the preamp stage. For this reason, preamps are available with almost every conceivable arrangement of traps (filters). Tunable traps can be adjusted to eliminate a specific frequency, while switchable traps attenuate (reduce the level of) an entire band of radio frequency signals. If an installation is experiencing severe interference from one local station (located at 101.5 MHz for example), a tunable trap can be adjusted so that any signals at this frequency are eliminated without attenuating the rest of the FM band. If interference is caused by several FM stations, you will have to use a switchable trap that attenuates the entire FM band. Many customers want to receive both TV and FM signals from one TV antenna. Adding a trap to a preamp will not necessarily make this impossible. In general, any FM signal strong enough to require trapping will also be strong enough to be picked up by an FM receiver even though it is trapped out of the TV bands. (An FM receiver requires a much lower level of signal to operate well than does a TV.) When a switchable trap is used however, some of the weaker, more distant FM stations will be lost.
Although the ideal TV antenna preamp usually is the one with the lowest noise figure and the highest gain, compromises sometimes must be made. If very strong local signals are present, a preamp with a high input capability must be used. A high input capability will prevent the strong local signals from overloading the preamp. However, a preamp with a high input capability has a slightly higher noise figure. Coaxial cable should be used with all preamp installations. Coax is much less likely to introduce additional signal problems. Tunable traps should be adjusted before they are installed. It is easier to adjust them on the workbench than to try to tune them after they are on the antenna. Simply hook up the preamp with the trap to the input of a field-strength meter, or to the TV antenna terminals of an FM radio if the trap is designed to attenuate FM signals. Tune the meter (or radio) to the frequency (or station) to be trapped out and then adjust the trap to the point at which the unwanted signal is attenuated, as indicated by a reduction in the sound level.
Installing a TV Antenna Preamplifier
A preamp consists of two units: a preamp and a power supply. The preamp itself is mounted on the TV antenna boom or on the mast as close to the TV antenna as possible. The power supply unit is mounted indoors. Power is supplied to the preamp unit through the transmission line. The preamp is located as close as possible to the TV antenna feedpoint because the weak received signal must be amplified before it is attenuated by the transmission line, and also before it can be subjected to interference from sources between the antenna and the receiver. Preamps mounted farther from the TV antenna usually amplify (magnify) the interference along with the signal. Preamp units come with U-bolts for easy mounting on the mast or the antenna boom.
Leave enough slack in the transmission line to form a drain loop. This will keep water out of the amplifier housing. Coat the connections with an insulate sealant.
Next, run the transmission line from the output terminals of the preamp to the power supply. Note: The transmission line also carries power to the amplifier. There is no separate power line to run. The power supply unit is mounted indoors, usually near the TV set. You may want to mount it on the back of the TV. If you do, be careful not to block any of the ventilating holes on the rear panel of the set. Most preamp power supply units have mounting holes that can be screwed to any relatively flat surface. Connect the transmission line from the TV antenna to the input terminals on the power supply unit.
Next, run a length of transmission line from the power supply output terminals to the TV. The last step is to plug in the power supply. Because it uses less current than an electric clock, it will not be expensive to leave it plugged in.
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