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 Antenna Selection tool and review the listing of available channels at your location. If the station you want to view is in the Red, Grey 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 three preamplifiers with different amounts of gain - the CM-7778 has 16 dB of gain, the CM-7777 has 30 dB of gain and Amplify (CM-7777HD) with a 17 - 30dB Adjustable gain setting. In most cases when the channels you are trying to view are in the Red zone, the CM-7777HD on the low gain setting 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 CM-7777HD on the high fgain setting 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.