Preselector

A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector blocks trouble-causing out-of-tune frequencies from passing through from the antenna into the radio receiver (or preamplifier) that otherwise would be directly connected to the antenna.

A preselector improves the performance of nearly any receiver, but is especially helpful to receivers with broadband front-ends that are prone to overload, such as scanners and ordinary consumer-market AM broadcast and shortwave receivers.

A preselector typically is tuned to have a narrow bandwidth, centered on the receiver’s operating frequency. The preselector passes through the signal unchanged or only slightly reduced on the frequency that it is tuned to, but it diminishes or eliminates off-frequency signals, reducing or eliminating unwanted interference. However, a preselector does not remove interference on the same frequency that it and the receiver are both tuned to.

Extra filtering can be useful because the first input stage (“front end”) of receivers contains at least one RF amplifier, which has a limited capacity (dynamic range). Most radios’ front ends amplify all radio frequencies delivered to the antenna connection. So off-frequency signals constitute a wasteful load on the RF amplifier. “Limited dynamic range” means that the amplifier circuits have a limit to the total amount of incoming RF energy they can handle without overloading, symptoms of which are nonlinearity and ultimately clipping.

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