This FET preamp allows boosting weak signals and makes a great unit to place ahead of the radio's antenna input. It adds very little hiss noise unlike bi-polar transistors do.
The FET preamp works between 3 KHz to 2 MHz. It allows for narrow band tuning.
Input impedance of this preamp is @ 0.5 Meg-ohms which is physically set by parallell resistors R1 and R2.
This FET preamp will amplify 3 KHz to 2 MHz. The MPF-102 transistor operates similar to a vacuum tube circuit (but works with 9-volts instead of 200) and it has very low noise. Do not omit the source or drain load resistors and capacitor (R3, C3) else the circuit will not work. The FET requires this resistor and capacitor so that proper DC bias is applied.
Need more gain? Just stack another FET preamp at the output of the first one.
Power this FET preamp with a 9-volt battery.
Connect antenna directly to input, unless the antenna instructions specifies that it can be remotely tuned through long length of coaxial cable.
For the capacitors, do not use tantalum capacitors. They are somewhat lossy in some cases. Non polarized audio grade capacitors are fine to use.
How to build it: Print out the above circuit diagram. Scale it down on your printer or photoshop program so that it is large enough for the electronic parts to fit the dots. This size is @ 5.0 inches wide by @ 2.3 inches high. 112 dpi. Get some copper tacks from the hardware store. Get a small piece of wood. Cut out the paper circuit diagram printout and tack it to the wood, one tack going into each dot in the diagram. Place the parts on the tacks. Solder. Do not bend the FET's leads too much. Place the FET last after all the other parts to prevent static damage or heat damage. It is best to just let the FET sit on top of the tacks and barely solder it to the top of the tacks. HAVE FUN!
VLF and LF signals penetrate walls of homes. The antenna and pre-amp should do very well indoors. It can be built into a wooden or metal box with jacks and switches. Power it with a 9-volt battery.
The amplifier can also placed remotely and even be solar powered. An electronic varactor diode can be used for tuning by remote control, or a gear-box motor can be used to drive the tuning capacitor to tune remotely. Have fun!