A pre-amplifier, often called a "booster" or "preamp", is a mast or antenna mounted amplifier that is used to boost the weak incoming signal at the antenna before the signal has had a chance to weaken through the coax cable running between the antenna and the TV.
Digital signals tend to be much weaker than the old analog signals. You either get a perfectly clear picture, "tiling" (where the signal is on the border of coming in or not coming in), or nothing at all. Most of the time the second two are caused by a weak digital signal which a pre-amplifier can greatly help with.
To determine if you need a pre-amplifier, first make sure that your antenna is pointed in the direction of your local TV station transmitters. Then connect only one TV to the antenna and make sure there aren't any splitters between the antenna and the one TV. If you still can't receive all of your local TV stations, then there is a good chance that you need a pre-amplifier to help boost the weak digital signals.
Preamplifiers consist of two parts: the actual amplifier that is mounted right at the antenna, and the indoor power supply. The indoor power supply sends low voltage through the coax cable to power the amplifier, so a separate power cable going to the amplifier unit is not required.
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