A simple regen radio for beginners - Charles Kitchin, N1TEV

This simple SW regenerative receiver was presented in QST magazine (issued September 2000), a monthly membership journal of ARRL.

In this version of the receiver, a prototype PC board is used; it is not directly representative of the currently produced board, although they are similar. In this view of the receiver,
the antenna has been removed. The TUNING capacitor is at the left. Immediately behind the capacitor is the coil, L1. Attached between the TUNING capacitor and the VOLUME control potentiometer immediately beneath you can see D1, C4 and R4 as discussed in the text.

Here's a low cost, simple-to-build, portable shortwave receiver. Its design is noncritical and the receiver is easy to get going. With it, you can receive dozens of international shortwave broadcast stations at night - even indoors - using a 39-inch whip antenna. This little radio is perfect for discovering ham-band QSOs, news, music and all the other things the shortwave bands have to offer.

Although this little receiver is quite sensitive, it naturally won't match the performance of a commercial HF rig, and if you've not used a regen before, you'll have to practice tuning the radio - but that's part of the adventure. Most of today's experienced "homebrewers" got their start by building simple, fun circuits just like this one. You'll gain experience in winding a coil and following a schematic. As your interest in radio communication develops, you can build a more complex receiver later.

The little receiver requires only a single hand-wound coil and consumes just 5 mA from a 9-V battery. At that rate, an alkaline battery can provide approximately 40 hours of operation.
The sound quality of this receiver is excellent when using Walkman headphones. The radio can also drive a small speaker. To simplify construction, a low-cost PC board is available from
FAR circuits. You can house the receiver in a readily available RadioShack plastic project box.

Circuit Description

Take a look at the schematic in Figure 1. L1 and C1 tune the input signal from the whip antenna. Regenerative RF amplifier Q1 operates as a grounded-base Hartley oscillator. Its
positive feedback provides a signal amplification of around 100,000. The combination of the very low operating power of this stage, only 30 uW, with the use of a simple whip antenna
makes this receiver easily portable and prevents it from interfering with other receivers located nearby. Regenerative receivers are, after all, oscillators. R2 controls the amount of positive feedback (regeneration). D1 and C4 comprise a floating detector that provides high sensitivity with little loading of Q1. The relatively low back-resistance of the 1N34 germanium diode (don't use a silicon diode here!) provides the necessary dc return path for the detector.

VOLUME control R5 sets the level of detected audio driving U1, an LM386 audio amplifier. C5 provides low-pass filtering that keeps RF out of the audio amplifier. R4 isolates the low-pass filter from the detector circuit when the volume control is at the top of its range. The bottom of the VOLUME control, R5, and pin 3 of the LM386 float above ground so that both inputs of the IC are ac coupled. This allows the use of a 100-k - VOLUME control; this high resistance value prevents excessive loading of the detector. D5 protects the receiver from an incorrectly connected battery.
L1 is wound on a standard 35-mm plastic film can or a 1-inch diameter pill bottle. C1 can be any air-dielectric variable capacitor with a maximum capacitance of 100 to 365 pF. Total frequency coverage varies with the capacitance value used, but any capacitor in that range should cover the 40-meter ham band and several international broadcast bands. If you use a capacitor with a large capacitance range (such as 10 to 365 pF), you'll find that selectivity suffers. That is, it's more difficult to tune in an individual station because there are more stations within the tuning range than when using a capacitor with a smaller capacitance range (such as 10 to 150 pF). Therefore, an optional fine-tuning control (see the inset of Figure 1) is recommended when using tuning capacitors with a wide capacitance range.

Figure 1 — Schematic of the simple regen receiver. Unless otherwise specified, resistors are 1/4-W, 5%-tolerance carboncomposition or metal-film units. Part numbers in parentheses are RadioShack. Equivalent parts can be substituted; n.c. indicates no connection.

C1 - 150 to 350 pF (maximum value) air-dielectric variable capacitor; see text
C2, C3 - 0.001 µF, 50 V (or more) disc ceramic (RS 272-126)
C4, C10, C11, C14 - 0.01 µF, 50 V (or more) disc ceramic (RS 272-131)
C5 - 0.002 µF, 50 V (or more) disc ceramic (use two RS 272-126 connected in parallel)
C6, C9 - 0.047 µF, 50 V disc ceramic (RS 272-134)
C7 - 10 µF, 35 V electrolytic (RS 272-1025)
C8 - 220 µF, 35 V electrolytic (RS 272-1017)
C12, C13 - 47 µF, 35 V electrolytic (RS 272-1027)
C15 - 5 to 10 pF, 50 V (or more) mica (RS 272-120)
D1 - 1N34A germanium diode (RS 276-1123); don't use a silicon diode here.
D2-D5 - 1N4148 or any similar silicon diode (RS 276-1122)
D6 - 1N4003 silicon diode (RS 276-1102)
J1 - 21/8-inch, three-circuit jack (RS 274-246)
L1 — See text.
Q1 - 2N2222A NPN transistor (RSU11328507) or MPS2222A (RS 276-2009)
R1, R3 - 1 k (RS 271-1321)
R2, R5 - 100 k potentiometer, linear taper (RS 272-092)
R4 - 22 k (RS 271-1339)
R6 - 10 (RS 271-1301)
R7 - 150 k (RSU11345287) or use series-connected 100 k (RS 271-1347) and 47 k (RS 271-1342) resistors.
R8 - 100 k audio-taper potentiometer (RS 271-1722); connect so that clockwise rotation increases the voltage at the junction of the potentiometer arm, R9 and C14.
R9 - 1 M (RS 271-1356)
S1 - SPST miniature toggle (RS 275-612)
U1 - LM386N audio amplifier (RS 276-1731)

PC board (see Note 1);
39-inch whip antenna (RS 270-1403);
8-pin DIP socket for U1 (RS 276-1995A);
9-V battery clip (RS 270-325); three knobs (RS 274-402A);
project box (RS 270-1806);
#6-32 screws and nuts, rubber feet;
9-V battery, Radio Shack 22-gauge solid hook-up wire.

RSU items in the RadioShack catalog need to be ordered (delivery in approximately 7 to 10 business days).

Building the Receiver

Finding the Parts
Air-dielectric variable capacitors can be purchased from several suppliers. You can also find them at ham flea markets or salvage one from a discarded AM radio. All the other components
are available from RadioShack and Digi-Key. PC boards are available from FAR Circuits (see Note 1).

Winding the Coil
Some would-be builders are intimidated by the idea of winding a coil. Actually it's quite easy to do. Sometimes, having a second set of hands helps. For the coil winding, use 22-gauge solid-conductor insulated hook-up wire. Before you start winding the coil, drill a mounting hole in the bottom of the film can or pill bottle. Then, drill two small holes in the side of the coil form, near the top, where the winding starts. (By winding from the top of the coil form to the bottom, the winding bottom is kept well above the PC board, preventing any circuit loading that could decrease the receiver's selectivity.) Feed one end of the coil wire through the first hole to the inside of the form, then out through the second. Tie a knot at the point in the wire where it enters the form—this keeps the wire in place and prevents it from loosening later on. Be sure to leave a two to three inch length of wire at each end of the coil
so you can make connections to the PC board. You can wind the coil in either direction, clockwise or counterclockwise. Tightly wind the wire onto the form, counting the turns as you go. Keep the turns close together and don't let the wire loosen as you wind; this takes a little practice. To make the coil tap, wind 11 turns on the coil form. While holding the wire with your thumb and index finger, mark the tap point and remove the insulation at that point. Solder a two to threeinch piece of wire to the tap. Continue winding turns until the coil is finished (13 turns total). Keep the free end of the wire in place using a piece of tape and drill two more holes in the coil form where the winding ends. Feed the wire end in and out of the coil as before and tie a knot at the end to hold the winding in place. When the coil is finished, remove the tape then carefully solder the three wires from the coil (bottom, tap and top) to their points on the PC board keeping the wire lengths as short as possible. For best performance, the floating detector must be wired using short, direct connections. Therefore, these components are not mounted on the PC board. Mount the VOLUME control, R5, close to the TUNING capacitor, C1. Connect D1, C4 and R4 in series between the hot side of C1 (the stator) and the top of the VOLUME control.


Fine-Tuning Control
You can add a fine-tuning control to the receiver using the circuit shown in the inset of Figure 1. D6 functions as a poor man's Varactor (voltage-variable capacitor). As the voltage from FINETUNING control R8 is increased, the diode is reverse biased and its capacitance decreases. This fine-tuning control is cheap and easy to add, but its added capacitance somewhat reduces the maximum frequency range of the receiver. You can compensate for this by removing turns from L1 if necessary.
Two-Band Option
If you'd like a two-band receiver with noncritical tuning, use a 150-pF capacitor for C1 and install a miniature toggle switch with very short leads to add an additional 250-pF fixed-value mica capacitor in parallel with C1. With the capacitor in the circuit, the receiver will then tune the 80-meter band.

Packaging the Radio
The recommended RadioShack project box includes metal and plastic tops. Use the metal top as a large front panel by mounting it to one side of the box using two small screws and nuts through two of the four predrilled holes. Then drill the control mounting holes and mount the three controls and the ON/OFF switch on the metal panel. The radio is easier to operate if you mount the TUNING capacitor and the regeneration (REGEN) control on opposite sides of the front panel. The VOLUME and REGEN controls are best mounted near the bottom of the front panel to keep their connecting wires to the PC board as short as possible. You can use the RadioShack hook-up wire for the VOLUME and REGEN control connections if you twist the wires closely together and keep their lengths very short. Otherwise, use shielded wires for these connections. You can mount the ON/OFF switch last, in any convenient location. Use one of the two remaining holes in the metal front panel to attach a wire connecting the panel to the PC board ground. Attach the PC board and the coil to the bottom of the project box using small screws. Mount the headphone jack on the box rear, close to the PC board and the LM386. Attach the RadioShack 39-inch whip antenna to one of the back corners of the box using a small screw and nut. If you use the RadioShack jack specified for J1 (RS 274-276), connect together pins 2 and 5 and attach that common lead to C8. Ground pin 1 of the jack. If you intend to use a small speaker, connect it between pins 1 and 3. Then, when headphones are plugged in, the speaker will be disconnected automatically.

Testing and Operating the Receiver

Set the VOLUME and REGEN controls to midrange, plug in the headphones, extend the whip antenna, attach the battery and turn on the receiver. You can check to ensure that the audio stage is
functioning by placing a finger on the center lug (wiper) of the VOLUME control and listen for a buzz. If the audio stage is working, adjust the REGEN control until the set produces a sound, indicating that Q1 is oscillating. If Q1 is not oscillating, carefully check the wiring and measure the voltages labeled on the schematic using a high-impedance DVM or multimeter. Common problems are Q1 being wired backwards (emitter and collector connections reversed) and the wires from coil L1 connected to the wrong places on the PC board.
Use two hands when operating the receiver: one for tuning, the other for controlling regeneration. For international broadcast stations or AM phone operation on 40 meters, carefully adjust the REGEN control so that Q1 is just below oscillation. For CW and SSB, increase the REGEN level so that the set just oscillates providing the required local oscillation for these modes. This receiver picks up lots of stations with just its whip antenna, although using a ground connection will greatly reduce any hand-capacitance effects. To pull in more stations during daylight hours, a 10 to 15-foot (or longer) length of insulated hook-up wire can be used as an external antenna. Simply wrap the end of this wire a couple of times around the whip antenna.
If you operate this receiver close to another radio, the regen's 30-uW oscillator might interfere with it. Those who are interested in building a higher-performance regen receiver for serious CW and SSB reception should read my article "High Performance Regenerative Receiver Design". You can also see the project at http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm

A similar design made by BH1RBG can be found here https://sites.google.com/site/linuxdigitallab/rf-ham-radio/aamazing-regen-receiver.

The PDF file containing everything about this receiver can be found on ARRL website (not available anymore, but archive.org has a backup of it).

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