A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver, to prevent trouble-causing out-of-tune frequencies from passing through to 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 receivers.
A preselector typically is tuned to have a narrow bandwidth, centered on the receiver's operating frequency. The preselector passes the signal it is tuned to, but attenuates other signals, diminishing unwanted interference. Additionally a preselector may also protect a sensitive receiver from damage caused by static input, voltage spikes, and strong signals from other, nearby transmitters. However, a preselector does not remove interference on the same frequency that it and the receiver are tuned to.
Extra filtering can be useful because the first input stage (front end) of the receivers that contain the RF amplifiers and mixer, have a limited dynamic range, and most RF amplifiers amplify all radio frequencies delivered to the antenna connection. So off-frequency signals constitute a wasteful load on the RF amplifier. The amplifier circuits also have a limit to the amount of incoming RF energy they 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 range and power demands that are applied to all later stages of the receiver, allowing it to handle only the dynamic range of signals within the desired band.
Bandwidth vs. signal strength trade-off
With all preselectors there is some loss at the tuned frequency; usually the loss is in the tuning coil (the ‘inductor’). Tuning the preselector for narrower bandwidth (or higher , or greatest selectivity) increases this loss.
Most preselectors have separate settings for an inductor and at least one capacitor. So with at least two adjustments available to tune to just one frequency, there are often a variety of settings in its middle-range that will tune the preselector to the same frequency.
For the narrowest bandwidth (highest ), the preselector is adjusted for lowest capacitance and the highest in-tune inductance, but this produces the greatest loss. For lowest loss, the preselector is adjusted for the highest capacitance and the lowest in-tune inductance (and the lowest ), which allows some interference through from nearby frequencies.
Different from an antenna tuner
Although a preselector is placed in the same location as an antenna tuner, it serves a different purpose. An antenna tuner is used to smoothly transfer the signal from the radio transmitter into the antenna's feed cable; when properly adjusted, it prevents transmitted power from being reflected back (‘backlash’). Some circuits are designed for both antenna tuning and preselection, for example the Series Parallel Capacitor tuner (SPC tuner) and most circuits for balanced line tuners (BLT).
Some simpler types of antenna tuners have limited preselection function, such as the common Hi Pass Tee network. By adjusting for high operating , the Hi Pass Tee will block frequencies below the operating frequency, but not as much above; the complementary Lo Pass Pi network can similarly be adjusted to block frequencies above the operating frequency, but not as much below.
- Stanley, John, K4ERO, “The Filtuner,” ARRL. Antenna Compendium, vol 6. Newington, Conneticut: American Radio Relay League
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