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.
When the front-end overloads, the performance of the receiver is severely reduced, and in extreme cases can damage the receiver. 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 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.
A preselector can be engineered so that in addition to attenuating interference from unwanted frequencies it will limit input signal voltage, to protect a sensitive receiver from damage caused by static discharge, nearby voltage spikes, and overload from nearby transmitters’ signals. It can also incorporate a small radio frequency amplifier stage to boost the filtered signal, although pre-amplification typically isn’t needed. None of these extra conveniences is a necessary part of preselection.
Tunable antenna preamplifiers (preamps) often incorporate a front-end preselector circuit to improve their performance. 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 that have no power and no internal amplifier work quite well with modern receivers with negligible loss to the signal, and preamps do not benefit from a preselector when fed from a narrow-band source, such as a tuned-resonance loop antenna.
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 Q , or greater 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 that will tune the preselector to a frequency in its middle-range.
For the narrowest bandwidth (highest Q ), the preselector is tuned using the highest inductance and lowest capacitance for the desired frequency, but this produces the greatest loss. It also requires retuning the preselector more often while searching for faint signals, to keep the preselector’s pass-through frequency closely matched to the receiver’s working frequency.
For lowest loss (and widest bandwidth), the preselector is tuned using the lowest inductance and highest capacitance (and the lowest Q , or least selectivity) for the desired frequency. The wider bandwidth allows interference through from more nearby frequencies, but reduces the need for retuning the preselector while tuning the receiver, since any one low-inductance 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 into the transmitter (‘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 also produce some preselection. The common Hi Pass Tee network can be adjusted for high operating Q , which will strongly block frequencies below the operating frequency, and can attenuate frequencies above the operating frequency by as much as 20 dB. The complementary Lo Pass Pi network can be similarly adjusted to strongly block frequencies above the tuned frequency, with as much as 20 dB attenuation below the tuned frequency.
- George Cutsogeorge. (2014). Managing Interstation Interference with Coaxial Stubs and Filters, (2nd ed.); (1st ed. 2009). Aptos, CA: International Radio Corporation.
- Stanley, John, K4ERO. (1999). The Filtuner. ARRL Antenna Compendium, vol 6. Newington, CT: American Radio Relay League.
- Stanley, John, K4ERO. (2015-09). Technical Correspondence: Antenna Tuners as Preselectors, (In) Wolfgang, Larry, (Ed.). QST, pg. 61, (September 2015). Newington, CT: American Radio Relay League.