There is no technical help with the projects. This circuit is shown as an educational example only.

Tunable Very Low Frequency JFET Pre-Amplifier Circuit Project

There is no technical help with the projects. This circuit is shown as an educational example only.

Radio Shack stores have most of the parts. The circuit was designed to use a Stormwise portable antenna having very high output impedance: This is any of our antenna part numbers starting with "C" or "D".

fet preamp

This Junction Field Effect Transisistor (JFET) preamp allows boosting weak signals and makes a great unit to place ahead of the radio's antenna input. It has almost no electronic hiss noise like most bi-polar transistors have. This is very important when listening for whistlers and tweeks and other natural radio emissions that may have weak signal strengths.

The JFET preamp works between 200 Hz to 2 MHz. It allows for narrow band tuning in the ULF through AM broadcast band.

The Stormwise antenna works to convert the radio wave's magnetic field into an electrical radio wave voltage that is a perfect match for this type of high-impedance pre-amplifier.

Input impedance of this preamp is just slightly over 20 Meg-ohms which is physically set by resistors R1 and R2.

This JFET preamp will amplify 200 Hz to 2 MHz. The MPF-102 transistor operates similar to a vacuum tube circuit (but works with 9-volts instead of 200 volts!) and it has very low noise. Do not leave out any parts, proper DC bias is applied through this combination of parts.

The output of this pre-amplifier can go directly to your radio's 50 ohm or 75 ohm antenna jack, or to the line-level input of a tape recorder or audio amplifier. It works well with the RADIO SHACK mini amp which has 5000 ohms input impedance, Radio Shack cat no. 277-1008C, but any battery-powered audio amplifier will work for whistler reception.

Power this JFET preamp with a 9-volt battery.

  • Antenna is one of our LF or VLF antennas.
  • Q1 is an MPF-102 N-channel field effect transistor.
  • R1 is a 100 K-ohms resistor. See note below. Use this value for tuning below 30 KHz.
  • R2 is a 20 Meg-ohms resistor ( Use two 10 Meg-ohms resistors in series to make this value ).
  • R3 is a 5 K-ohms resistor. ( Use two 10 K-ohm resistors in parallel to make this value ).
  • R4 is a 10 K-ohms resistor.
  • VC1 is our part # AMVC-384 a 384 pF variable capacitor ( for tuning the antenna ).
  • C1 is an 0.22 uF polyester film capacitor only or metal film capacitor only. ( input capacitor ).
  • C2 is an 0.22 uF polyester film capacitor or metal film capacitor only. ( input capacitor ).
  • C3 is an 0.22 uF polyester film capacitor or metal film capacitor or electrolytic capacitor ( this part works as a high pass filter ).
  • C4 is an 0.22 uF polyester film capacitor or metal film capacitor or electrolytic capacitor. (output capacitor ).
  • D1 is a 1N914 diode. Protects the JFET from static damage.
  • D2 is a 1N914 diode. Protects the JFET from static damage.
  • SW1 is a pushbutton ON-OFF switch of any kind.

    C3 controls the low frequency cut off of this pre amplifier. Its like a tone control function. The value of 0.22 uF is good down to 990 Hz, and filtering increases greatly below 900 Hz so that power line hum is greatly reduced. To allow operation down to 200 Hz then change C3 and C4 to 1 uF. To allow operation down to 10 Hz then change C3 and C4 to 220 uF. To allow operation down to 0.1 Hz then change C3 and C4 to 2200 uF.

    NOTE: For tuning the AM broadcast band change R1 to 1000 ohms. For tuning LF (30 KHz - 530 KHz) change R1 to 10000 ohms. For tuning below 30 KHz R1 should be 100000 ohms (100 K-ohms) like shown in parts list. Don't worry about the resistor in the signal path. It acts as a low pass filter for removing higher frequencies that are out of band. It also allows narrow band tuning by not loading down the antenna. The JFET is extremely sensitive to micro voltages and the tuned antenna generates a radio frequency voltage (the received signal, measured in microvolts) at the receive frequency. For the most part the JFET does not even know the resistor is there, but do not leave out the resistor as it is required for proper operation of the amplifier.

    There is no technical help with the projects. This circuit is shown as an educational example only.

    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 JFET's leads too much. Place the JFET last after all the other parts to prevent static damage or heat damage. It is best to just let the JFET 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. Use battery power only to prevent extra hum noise.

    Keep the wire leads short length between the pre-amp and the antenna. Coaxial cable has about 20 pF capacitance per foot so this figure will add into the tuning capacitance value across the antenna. The amplifier can also placed remotely and even be solar powered.

    Some carbon-zinc 9-volt batteries create a random crackling sound in the receiver, (even without the antenna connected) by generating noise in the battery itself. Use a 1000 uF capacitor across the power terminals to eleminate any battery jitters that could occur.

    LISTENING WIDE BAND INSTEAD OF TUNED: Add a bandwidth switch!

    You will find it more pleasing to listen to VLF whistlers, tweeks, dawn chorus, and other natural radio emissions in a wideband receive mode, instead of the narrow tuned mode. (Use antenna part # C10T-1K-2K-BC). To do this just place a 500 K-ohms resistor directly across the pre-amp's input terminals. This will broaden the antenna's bandwidth and greatly reduce the tuned ringing effect. The tuning capacitor will still work and the bandwidth will be much wider and the sound more pleasant. You can use a switch to turn this feature ON and OFF by switching the 500 K-ohms resistor in or out of the circuit. To make the 500 K-ohms resistor use two 1 Meg ohms resistors in parallel.

    This graph shows the approximate frequency response of this amplifier based on frequency response of various C3 capacitor values from 0.1 uF to 1.0 uF. The starting cutoff point is at 750 ohms which is 6 times lower than the 5000 ohm source resistor.

    The point on the graph above 750 Hz (the red line) is the filter cutoff region. The point below the red line (below 750 ohms) is the operating range response.

    C4 should always be equal or larger than C3 for the strongest output signal.

    A 1.0 uF capacitor for C3 is good down to @ 210 Hz.

    A 0.47 uF capacitor for C3 is good down to @ 460 Hz

    A 0.22 uF capacitor for C3 is good down to @ 990 Hz.

    A 0.1 uF capacitor for C3 is good down to @ 2100 Hz.

    Based on the numerical progression of the measurements in this graph the following can be roughly calculated:

    A 2.2 uF capacitor - good down to @ 99 Hz.

    A 10 uF capacitor - good down to @ 21 Hz.

    A 22 uF capacitor - good down to @ 9.9 Hz.

    A 100 uF capacitor - good down to @ 2.1 Hz.

    A 220 uF capacitor - good down to @ 0.99 Hz.

    A 1000 uF capacitor - good down to @ 0.21 Hz

    A 2200 uF capacitor - good down to @ 0.099 Hz.

    A 4700 uF capacitor - good down to @ 0.046 Hz

    A 10000 uF capacitor - good down to @ 0.021 Hz.

    Capacitor to use for 30 KHz and up (to filter out everything below 20 KHz): A 0.01 uF capacitor - good down to @ 21 KHz. Use this value for C3 if you only want to operate at 30 KHz and higher.


    TUNING A RANDOM LENGTH LOOP ANTENNA FOR VLF and LF RECEPTION

    The Stormwise antenna works great by itself. It also allows tuning random wire loops to allow powerful reception of weak signals.

    The above diagram shows how to tune a random length loop antenna for listening for whistlers and other natural emissions, and will also work for tuning longwave. The portable antenna by itself works great, but if you want to go for the very weakest signals then you can use a larger loop antenna and couple it into your receiver like shown in the above diagram. Aim the loop and the portable antenna for lowest hum level. Connect a ground rod to the loop antenna to drain away static electricity.

    For tuning long wave (100 KHz - 530 KHz), use only a few turns around the portable STORMWISE antenna. You must use a portable STORMWISE antenna made for the desired frequency range.

    How it works: The radio waves have a magnetic field that makes a small current flow in the large loop antenna. The 20 windings around the STORMWISE antenna concentrate the magnetic field (produced by the radio signal current) into the STORMWISE antenna. The STORMWISE antenna transforms this magnetic field into a radio wave voltage across the tuning capacitor, and this goes into the pre-amplifier and finally into your receiver.

    There is no technical help with the projects. This circuit is shown as an educational example only.


    There is no technical help with the projects. This circuit is shown as an educational example only. Copyright 2013
    All rights reserved