A preselector is a radio technological term for an electronic device that is inserted between the antenna and the receiver, limiting the range of frequencies that can be applied to it. Tuning to the desired frequency keeps the preselector's narrow bandwidth centered at the operating frequency, rejecting or reducing out-of-band unwanted interference signals. A preselector also protects the sensitive receiver input from damage caused by static and voltage transient. It improves the performance of nearly any receiver, but is especially helpful to those with broadband front-ends that are prone to overload, such as scanners or average receivers.
Extra filtering is needed because the front of the receivers, conformed by the RF amplifier and Mixer, have a limited dynamic range. Dynamic range is defined as the amount of RF energy those circuits can handle without overloading. If the front-end overloads, the performance of the receiver is severely reduced or even damaged. In situations with noisy and crowded bands, or where there are strong local stations, the dynamic range of the receiver can quickly be exceeded. Extra filtering limits frequency and power that are applied to the receiver, allowing them to handle the overall dynamic range of signals that are within the desired band.
To best understand the theory and practice of selecting and using preselectors, a search of the Internet will yield much information regarding theory, design, application, and construction. (Keywords: preselect, preselector, receiver tuner, signal preconditioning)