Low Frequency Radio Receiver circuit project

low frequency radio receiver

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

This radio receiver built on a piece of wood uses only 5 transistors but will receive longwave beacons and VLF beacons, depending on the antenna unit connected. Uses our AMVC-384 capacitor and one of our "C" antennas.

This page is still under construction as of 07-08-2011. Our LOUDEST receiver yet. Features low noise FET transistors and improved AM detector. More photos to be added of completed unit.

This 5 transistor low frequency radio receiver allows you to hear longwave beacons, broadcasts, and other signals in the range of 10 KHz to 530 KHz. It works just like an AM radio, a transistor demodulates AM signals.

The receiver is designed for use with any of our high impedance antennas starting with the letter "C" in the model number, in the range of 10 KHz to 530 KHz. The antenna's tuning adjustment (via a variable or fixed-value capacitor, sold separately) is what selects the receive channel.

This is a direct detection amplified "crystal" radio receiver. It does not generate RFI or use regeneration. It has no AGC circuit or noise-blanker, making it excellent for research applications (such as lightning detection). Receiver will easily hear the lightning static that is always present in the lower frequency bands.

This radio will drive any common audio amplifier (like the Radio Shack mini-amp) loudly to room-filling volume.

If you don't want to use an external amplifier, it also powers a crystal earphone directly from the output jack.

Power the radio with a rechargeable 6 volt lantern battery. The radio will run a -very- long time, it only draws @ 10 mA, making it excellent for research uses where it has to run on solar or battery power only. The radio will also work on 9 volts without any problems.

Power the radio and audio amplifier only with battery power to prevent AC power buzz noise.


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

The parts and values are shown on the diagram above on this page. Standard values used. General parts available at any electronics store. Antenna and tuning capacitor available here at WWW.STORMWISE.COM

VC1 is STORMWISE part # AMVC-384. Variable capacitor.

Antenna is one of our high impedance narrow band tuned antennas. Select from models on our web site, depending on which tuning range you need.

The transistors Q1 and Q2 are Field Effect Transistors (FET) MPF-102. These are very high impedance transistors, similar to a vacuum tube in actual input and output performance and circuit design (but of course are solid-state devices not needing any filament heater or high voltages!). Two FET's are shown, enough to boost the received signals for the 2N3904 transistor AM detector - giving loud volume into an audio amplifier. An additional FET rf pre-amp could be added in (just ahead of the AM detector) for even more gain, but this could be too much sensitivity - producing overload and distortion.

Q3, Q4, Q5 transistors are 2N3904.

All transistors are available from Radio Shack stores or other electronics parts store.

R1X is approximately 470 K ohms. This resistor will be different in each receiver depending on the initial sensitivity of the meter driver transistors. If the value is too little then the meter will indicate when no signal is present. Adjust meter with R1X so it barely indicates with no signal present (short the antenna leads together so no signal is received). But first you will also need to set the full scale reading with the 4.7 K-ohm variable resistor in the meter's line. Shorting the first meter driver pre-amp's BASE to ground will cause meter to read full scale - you can even add a battery test function by installing a momentary pushbutton switch here.

NOTE: You can omit the meter and meter drivers if you don't want a meter, the receiver will work fine without them.

METER is MOUSER ELECTRONICS Part # 541-MSQ-AVU-AW. Large VU Meter. (This meter has 120 ohms coil movement). Polarity marked part.

How to build it: Print out the above circuit diagram. Scale it down on your printer or photoshop program so that it is the right size, large enough for the electronic parts to fit the dots. This size is about 8 inches wide and 5 inches high. The wood board should be cut 8.5 inches X 5.5 inches.

Do not leave out or bypass any of the parts, all parts have a required function.

How to build: See photos. (will be added).

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

Obtain some safety goggles, you'll be soldering and hammering nails and cutting a piece of wood (use a hand saw only). Solder is molten metal and can spatter into your eyes. Do not solder without full eye protection.

A 30 watt soldering iron was used.

If you are not familiar with soldering practices, then it -IS- possible to use wire-wrap methods to build the receiver. You can also build the receiver on a solderless breadboard. Ask at Radio Shack store for this item. You'll need the one with the grounded shield base, receiver -will- oscillate on these if not connected to the shield base. You will still need the safety goggles when clipping part's leads or working with the wires.

A square piece of wood should be obtained that is slightly larger than your print-out.

Obtain some copper plated weather-strip nails from the local hardware store. Cut out the paper circuit diagram print-out and tack it to the wood, one tack going into each dot in the diagram. Place the parts on the tacks. Solder parts to the tacks. Make sure tacks do not short to ground shield except where indicated.

Place the transistors and the diodes last after all the other parts have been soldered, to prevent static damage or heat damage. It is best to just let the transistor sit on top of the tacks and solder it to the top of the tacks with a small drop of solder. Do not bend the transistor's leads too much.

Parts may contain LEAD or other chemicals so wash hands after handling. Solder contains LEAD or other chemicals so wash hands after handling.

The 10 meg was changed to "RX1" and is @ 470 K ohms.

The output audio signal is several millivolts, enough to drive a small audio amplifier loudly. Use Radio Shack's Mini Amp, Radio Shack part number 277-1008C.

You should easily hear static from thunderstorms 1000's of miles away at night with just the antenna only. No external wire antenna needed. To hear some types of beacon stations (that don't send AM or morse code) you will need to input the same frequency as the station from a local oscillator ( an audio generator that goes from 20 KHz to 530 KHz) to allow mixing of the signals to get a "beat note". You can input the mixing signal directly into Q3 base junction point via an additional 10 pF capacitor.

The antenna should not be placed more than 2 feet away from the receiver. The receiver requires very high impedance input provided from our narrow band high impedance antennas, so long runs of cable will load down the antenna which prevents good reception. Cable capacitance is a problem with long cables, the capacitance will cause antenna to tune lower in frequency than the intended range for 0 to 384 pF.

The receiver worked perfectly on the first try, except for having to change R1X value from 10 Meg down to 470 K to adjust the meter. Do not allow the receiver to oscillate (squeal or howel) from feedback.(Try touching the 2nd MPF-102, you'll see what it does). If it does not stop then add grounded shield. This can be insulated wires strung over the unit around this sensitive point. This unit is not designed to be a regenerative receiver or to emit any RFI when operating, it is simply a tuned direct detection RF receiver. It works by boosting the received RF up to a level that can be demodulated by the AM detector, basically an amplified crystal radio.

NOTE: If receiver does not work then try 9 volts. If this solves the problem then change out the FET's. Some don't work on 6 volts and should be considered "defective" parts. In designing several FETs were swapped in on the solderless breadboard (another way to build it), and a few were found that did not work at 6 volts.

Certianly a PC board can be etched and the receiver built that way, for those who are advanced, but the wood way works great for beginners so it's up to you.


VLF Receiver with Signal Strength Meter

ELF Magnetic Field Detector

VLF Whistler Receiver

Copyright 2011
All rights reserved