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 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. A preselector can also be engineered so that it will protect a sensitive receiver from damage caused by static input, voltage spikes, and overload from 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 receivers contains RF amplifiers and mixer, and those have a limited dynamic range. 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.
Tunable antenna preamplifiers (preamps) often incorporate a front-end preselector circuit to improve their function. The integrated device is both a preamplifier and a preselector, and may correctly be referred to with either name. This ambiguity sometimes leads to confusion. Passive preselectors work quite well with modern receivers with no power and a small loss to the signal, and untuned preamps do not always need a preselector front end.
Bandwidth vs. signal strength trade-off
With all preselectors there is some loss at the tuned frequency; usually, most of 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. It also requires re-tuning the preselector more often, while searching for signals, to keep the preselector pass-through frequency closely matched to the receiver’s working frequency.
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. It also reduces the need for re-tuning the preselector, while tuning the receiver, since the same setting for the preselector will pass many 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 signal power from the radio transmitter into the antenna's feed cable; when properly adjusted, it prevents transmitted power from being reflected back (‘backlash’ current). 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 , a side-effect for the Hi Pass Tee will block frequencies below the operating frequency, but only a little of the higher frequencies. The complementary Lo Pass Pi network can be similarly adjusted to block frequencies above the tuned frequency, but only a little of the lower frequencies.
- Stanley, John, K4ERO, “The Filtuner,” ARRL. Antenna Compendium, vol 6. Newington, Conneticut: American Radio Relay League
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