A mast or antenna-mounted amplifier (preamp) is used primarily to overcome the loss in the coaxial cable between the antenna and the TV set. In areas distant from TV transmitters, overcoming this loss is necessary since the signals are already weak. In areas close to TV transmitters, a preamplifier is usually not needed, and can actually cause problems as it may cause the signal levels to be too high.
The only sure way to determine if a preamplifier is needed is to measure the received signal levels with a field strength meter. Since this is not usually available for most home installations, determining the correct preamplifier can be a challenge. One good way to determine if a preamplifier is needed it to enter your address into the tool at www.antennaweb.org and review the listing of available channels at your location. If the station you want to view is in the Red, Blue, or Purple zones, you most likely will see an improvement by installing a preamplifier.
Another possible need for a preamplifier will be on installations with long coaxial cable runs from the antenna to the TV set. If there is 50 feet or more of cable between the antenna and TV set, a preamplifier may provide an improvement.
One more situation that may benefit from use of a preamplifier is when multiple TV outlets are being fed from the same antenna. Since the splitter used to connect multiple TV sets cause signal loss, a preamplifier at the antenna can help overcome these losses by increasing the signal levels at the antenna.
Channel Master provides preamplifiers with two amounts of gain - the CM7778 has 16 dB of gain and the CM7777 has 30 dB of gain. In most cases when the channels you are trying to view are in the Red zone, the CM7778 should have sufficient gain, even when feeding up to four TV sets, as long as the coaxial cable lengths are not excessively long. For Blue zone, and especially Purple zone, channels, or in installations with very long cable runs and a high number of TV sets, the CM7777 may be the better choice.
For Red, Blue, and Purple zone channels, it is preferred to use a preamplifier at the antenna instead of a distribution, or drop, amplifier in the house. The reason for this is to ensure the amplifier is receiving sufficient signal levels. If the signal levels are already bad at the input to the amplifier, they will not be improved by amplifying them.
In some instances, it may be necessary to use both a preamplifier at the antenna and a distribution amplifier inside the house, but in most installations, there is no advantage to using both at the same time.
In Yellow, Green, Light Green, and some Red zones, signal levels are usually strong enough at the antenna that a preamplifier is not required. However, for multiple outlet installations, a distribution amplifier is recommended to ensure that signal levels to each outlet are maintained at high enough levels to prevent drop out of the TV signals.
Installing a Preamplifier
A preamplifier consists of two units: a preamplifier and a power supply. The preamplifier itself is mounted on the antenna boom or on the mast as close to the antenna as possible. The power supply unit is mounted indoors. Power is supplied to the preamplifier unit through the coaxial cable.
The preamplifier is located as close as possible to the antenna feed point because the weak received signal must be amplified before it is attenuated by the coaxial cable, and also before it can be subjected to interference from sources between the antenna and the receiver. Preamplifiers mounted farther from the antenna usually amplify (magnify) the interference along with the signal.
Preamplifier units come with U-bolts for easy mounting on the mast or the antenna boom. After the preamplifier is attached to the antenna or mast, run a length of coaxial cable from the antenna output connector to the input connector on the preamplifier. Leave enough slack in the transmission line to form a drain loop. This will keep water out of the amplifier housing.
Next, run the coaxial cable from the output terminals of the preamplifier to the power supply. The coaxial cable 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. Most preamplifier power supply units have mounting holes that can be screwed to any relatively flat surface.
Connect the coaxial cable from the antenna to the input terminals on the power supply unit. Next, run a length of coaxial cable 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.