SW RF Pre-Amplifier by 2N3819
A radio frequency amplifier to boost SW reception. Frequency range approximately 5 to 20 MHz.
The problem with amplifying weak radio signals is that you also amplify the noise. What you can receive depends on how much background noise is present, whether it be man made interference or static. In this design the RF signal is first met by a resistive attenuator, this is necessary as strong signals could otherwise overload your receiver. The transformer T1 is would on a 1 inch diameter ferrite loop. The primary (antenna side) is 2 turns of 22 swg wire. The secondary is 4 turns of 22 swg wire. The 4 turns are spaced to occupy roughly half the coils circumference. The approximate inductance of the secondary is 20uH. To cover 5 to 20 Mhz a capacitor tuning from around 3pF to 200pF is required. A standard capacitor of 400 or 500pF (full mesh) can be used by including a series capacitor, C2 in the above Capacitors. Capacitors in series behave the same as resistors in parallel. The smallest capacitance is just less than the smallest capacitor in series and highest value also less than the highest capacitance.
Read More Source: Circuit : Andy Collinson
http://www.mitedu.freeserve.co.uk/Circuits/rf/swrfamp.htm
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