This is a Preselector for SW receivers (diy hf preselector) circuit. The entry low capacitance of MOSFET modern, with two gates, will create negative inverse reaction through a uncoupled source resistor. Applied right this technique allows the creation of a entrance RF stage field with a relatively high dynamic range.
Radio amateur working on short wave: there is no need to tell, in the days of our times, the ability to face the signals of high level is a necessity – for preventing overload of the receiver and intermodulations caused by very strong signals.
Unlike of entry sections in many SW high quality receivers, this schematic has no difficulty in handling RF input signals up to 2.5 Vvv. In this case we have 3Vvv on an impedance of 50 Ω.
Adjustment capacitor C1 determines the global amplification, which is mainly because because LC network resonance at the circuit input. The 6 entrance coils are coiled on ceramic dcores of very high quality with diameters of at least 10 mm. Ferrite pearl is located right on the terminal gate 1 of T1, for a preventing parasites oscillations in the VHF and UHF bands. Output coil L7 is 20 turns (A) and 4 respectively turns (B) coiled on a ferrite core ring type G.2-3/FT16.
Frequencies domains of the selector with active input are: