Preselector

[b] Extra filtering can be useful because the first input stage ("front end") of receivers contains at least one RF amplifier, which has power limits ("dynamic range").

[2] In situations with noisy and crowded bands, or where there is loud interference from nearby, high-power stations, the dynamic range of the receiver can quickly be exceeded.

Extra filtering by the preselector limits frequency range and power demands that are applied to all later stages of the receiver, only loading it with signals within the preselected band.

[3] Automatically switched filter banks can likewise be incorporated into various broadband, general purpose receivers.

On the other hand, when an antenna preamplifier (preamp) is actually needed, it can be made "tunable" by incorporating a front-end preselector circuit to improve its performance.

The adjective is redundant, but emphasizes to those only familiar with tunable preamplifiers that the preselector is normal, and has no internal amplifier, and requires no power supply.

Turning up the inductance gives the preselector a narrower bandwidth (or higher Q, or greater selectivity) and slightly raises the loss, which nonetheless remains very small.

Circuit of a very simple preselector. For any one frequency, using a larger tuning coil results in a narrower bandwidth , i.e. greater rejection of out-of-tune signals.
Preselectors become increasingly helpful at lower shortwave and mediumwave frequenies, where noise of all kinds becomes drasticly louder. This log-log plot is taken from ITU CCIR Report 322; it shows typical ranges for atmospheric and human-made radio noise. [ 1 ] The graph shows that for frequencies below 20 MHz, human-caused noise and natural noise (" static "), both grow at least exponentially louder as frequency drops below about 3 MHz.
Frequency response curves for a simple preselector tuned by a capacitor set to 10, 30, 100, or 300 pF ; the inductor is near 160 μH .