Sputnik Regenerative Receiver

Sputnik Regenerative Receiver by DX Explorer
Sputnik Regenerative Receiver by DX Explorer

Sputnik regenerative receiver was designed for the amateur radio bands with decent receiving capabilities, the perfect project for beginners in homebrew radio receivers.

Sputnik regenerative receiver is a simple radio receiver for AM FM SSB and CW, with decent receiving capabilities. It was designed for the 80m, 40m and 20m amateur radio bands. But it can be adapted to other HF bands as well. Sputnik regenerative receiver was designed for beginners to help them improve their knowledge in building radio receivers and amateur radio equipment.

It started from a simple schematic from the 80’s and it evolved in time, up to the point I was happy with the results. I wanted a decent performance regenerative radio receiver as stable as possible keeping the design simple. This is the latest schematic and PCB design and also the last update.


1. THE ORIGINAL SCHEMATIC

The schematic I found was published in one of the Romanian magazines from the 80’s called Tehnium. Later on were published more similar schematics with little modifications, but no improvements in the results. Since it was the first receiver designed for the amateur radio bands I’ve built back then, I decided to get back to it. I tried doing my best to improve it. I had a limited knowledge in building circuits. This receiver made me understand many things about how a radio receiver works, especially regenerative receivers. So having that great learning experience thanks to this receiver, I thought it may be a good idea to share it with you all. Maybe it will also help others making their first steps into the radio building hobby.

Simple Regenerative Receiver Schematic

There is a lot to talk about the history of this schematic. It was published in many international publications all over the world, with or without little or major modifications. The receiver it’s actually based on a German tube regenerative receiver design, later on adapted to work with transistors.

THE RECEIVER BACK IN 1997 – ORIGINAL SCHEMATIC

Simple Regenerative Receiver
Simple Regenerative Receiver Closeup

2. THE NEW SCHEMATIC

If you wonder why the receiver it’s called Sputnik, the explanation is somehow simple and also funny. Because of the way the propagation works, after building the very first versions all I’ve heard during the day time were the Russian amateur radio operators. Since I’m a fan of the vintage space era, knowing that Sputnik was the first Russian satellite launched, I decided to call the receiver the same way.

With ought any intention, for some reason it became somehow popular around the world. Either the first version or the second version of the receiver were popular on a QRP Forum in Germany, the SolderSmoke Blog, California QRP Club also asked the permission to use the design for a kit, or had some nice words from Peter Parker ( VK3YE ) on his VK3YE Radio Books Facebook Page. I have loads of emails and photos from the subscribers to the YouTube channel that also built the receivers in all sorts of enclosures.

Down bellow you have the latest schematic design. This is the final update as I will no longer make any changes. I wanted a receiver as small and as simple as possible, but with decent performance. To keep it simple and small I did not used an audio preamplifier, but the audio output is good enough for a small speaker or headphones. It will not draw too much current, so the batteries will last for days.

With ought the feedback sent by the subscribers, or the different ideas to make little improvements, this receiver won’t be as nice as it turned out now. So I would like to thank you all for all the help and support.

SPUTNIK REGENERATIVE RECEIVER SCHEMATIC

Sputnik Regenerative Receiver - Schematic Diagram

Instead of a voltage regulator, I decided to use a Zenner diode. A voltage regulator doesn’t help much when using batteries, as if the voltage drops under 12v, the LM386 IC will end up creating that ugly “motorboat” sound. The only disadvantage using a Zenner diode is that as the voltage goes down, there is also a slight frequency drift when using the diode tuning. If you want to remove the frequency drift, you can use a variable capacitor instead of the diode tuning. This will also increase the stability of the receiver even more.

In this final design I also added a modified Pi Network filter that acts as a bandpass filter. The reason for that are the strong AM broadcast band interferences heard during the evening. Since I just wanted to cover the amateur radio bands, I was not interested in the AM broadcast bands. The receiver works well in most HF amateur radio bands. I decided to concentrate on the most popular: 80m, 40m and the 20m band.

3. RF COIL DESIGN

You can build the receiver using either a PVC pipe s a coil form, or a toroid. After many experiments I didn’t notice any difference between them. The best is the one you have on hand so you can build the receiver. The numbers of turns depends on the band you decide to build it for. The terminal marked as 1 is the beginning of the coil and the terminal as 2 is the end of the coil.

L3 is the pickup coil for the external frequency meter if you want to use one. There is no preamplifier circuit for the pickup coil, so if your frequency counter doesn’t have one, you may have to build a preamplifier for it. Playing around with the coil and also the LC circuit will help you learn a lot about how tuned circuits work.

PVC Coil - Sputnik Regenerative Receiver
PCB Pins Placement - Sputnik Regenerative Receiver Coil

To calculate inductances if you want to use a toroid, I would recommend toroids.info website. If you decide to build the coil on a PVC pipe as I prefer, then you can use Coil32 Software to calculate the desired inductance and also the resonant frequency. There should be a balance between the LC circuit capacitors and the inductance of the coil to have the receiver very stable in frequency. If you go to high with the inductance and too low with the capacitance, the receiver will drift a lot. Even though a higher inductance allowed me to tune the entire 300Hz on the 80m band with only two 1N4004 diodes. Using a varactor diode will help you have more capacitance swing. That will allow you to have smaller inductances in the coils and more stability in frequency. It’s up to you to experiment.

Down bellow you have the RF coil details for the most common HF amateur radio bands. I also tried with the 10m band, but the receiver is not as stable in frequency and no longer as sensitive.

PVC PIPE RF COIL DETAILS

For the PVC pipe version of the coil, L4 has 3 turns / 0.4mm.

BANDL2L3C15
80mComing SoonComing SoonComing Soon
40m21 turns / 0.4mm5 turns / 0.4mm100 pF
20mComing SoonComing SoonComing Soon
Toroid Coil - Sputnik Regenerative Receiver

Here you have the image of the toroid version of the RF coil. For the 80m band you should use a T50-2 toroid. For the 40m and the 20m band a T50-6 toroid. They don’t take as much space as the PVC pipe coil does, so for that reason I use a toroid.

With a little effort picking your capacitors carefully, you can build the receiver to be extremely stable. It takes a little bit of experimenting and testing. But after two days of tests and experiments, I did managed to get the receiver to be extremely stable in frequency, drifting only 2Hz from 8PM until 8AM.

TOROID RF COIL DETAILS

For the toroid version of the coil, L4 has 2 turns / 0.4mm.

BANDL2L3C15
80m / T50-2Coming SoonComing SoonComing Soon
40m / T50-6Coming SoonComing Soon100pF
20m / T50-6Coming SoonComing SoonComing Soon

The value of “C” represents the total capacitance of C3, C4, C5 and C6 all together.

4. PCB DESIGN AND PARTS LAYOUT

Here you have the PCB design and also the parts layout of the Sputnik Regenerative Receiver . I’m not a master in PCB design, but I did my best to keep it as small as possible. The “Print This” PDF file found in the downloaded folder contains a print for two PCB boards of the receiver ( PCB Design and Parts Layout ). Just in case you mess up one board, you have an extra print to repeat the process. Read the “License and Info” file.

SPUTNIK REGENERATIVE RECEIVER PCB BOARD

Sputnik Regenerative Receiver - PCB Board Design

5. THE BANDPASS FILTER

I wanted something really simple since the receiver is designed for beginners. For this reason I quit using complicated bandpass filters that usually are hard to tune with no proper equipment. I opted for a simple Pi Network low pass filter that was slightly modified to act as a bandpass filter. Pick the values of the components from the table down below, depending on the desired band you build the receiver for.

BANDL1C21, C22C23, C20
80m / T37-21.11 uH ( 17 turns )1012 pF2226 pF
40m / T37-60.56 uH ( 14 turns )512 pF1125 pF
20m / T37-60.28 uH ( 10 turns )256 pF563 pF

6. ADJUSTING THE BANDPASS FILTER

The inductor that I made for the bandpass filter was made on a T37-2 toroid. It has 15 turns of 0.4mm enameled copper wire. To adjust the filter once you finished building the transceiver, simply find a signal somewhere in the middle of the 40m band and adjust the windings on the toroid by spreading them or tightening them until you get the loudest audio coming from that signal.

In my case the filter helped me get rid of most AM broadcast band interferences. I may still hear some now and then, but really low in the background and you can barely notice them.

7. RF ATTENUATOR

The RF attenuator will help you to adjust the signal level entering the receiver. It will also stop the RF generated by the regeneration stage of the receiver leak back into the antenna. This RF attenuator stage was borrowed from another great regenerative receiver designed by N1TEV. Between the bandpass filter and the RF attenuator you have the D6 and D7 diodes ( 1N4148 ). These are not really needed unless you are using the receiver close to a transmitter. They are there just to prevent overloading Q2 while transmitting.

8. REGENERATIVE RECEIVER STAGE

This is a simple regenerative stage. The level of regeneration it’s adjusted using the 10K regeneration potentiometer. I wanted to bring the receiver to a stage where the regeneration control is really smooth and easy to use. It can also be used as fine tuning. Some people like using a 10K multiturn potentiometer instead.

For a good stability in frequency C12, C13 and C15 should be the NP0 type capacitors. If you have too much capacitance, or not enough and you can’t get the receiver into the desired band, you can either increase or decrease the value of C15. I intentionally left two spots on the PCB board for C15. This way you can combine values from two different capacitors. This is also useful if you want to place a temperature compensation capacitor to help the receiver be even more stable in frequency.

With the help of the trimmer capacitor C14, set the receiver in the higher part of the band ( ex 7.2MHZ ), with the multiturn potentiometer all the way at the end. Then with the help of the 10K trimmer resistor, set the receiver at the beginning of the band ( ex 7.00MHz ), with the multiturn potentiometer all the way at the beginning. Or if you want you can simply set the receiver just for the CW or the SSB part of the band.

9. DIODE TUNING

The diode tuning was used in the design to replace the variable capacitor. These days is hard to get your hands on a good variable capacitor. If you do have a variable capacitor, replace the diode tuning with the variable capacitor as the receiver is more stable and there is no frequency drift when the battery goes low.

With the two 1N4004 diodes I am trying to cover the 300Hz used in most HF amateur radio bands. I did not want to use only one diode, as I would have to increase the inductance of the coil and that will create the receiver to be unstable in frequency. Unfortunately I had no varactor diodes to test.

The diode tuning stage is formed from Multiturn 10K potentiometer, the 10K trimmer resistor, C17, R10, D4, D5 and C16. For D4 and D5 I’m using 1N4004 diodes. You can always replace them with a proper varactor diode. If you want to replace the diode tuning with a variable capacitor, you can connect the stator of the capacitor to the spot where C16 goes to C14, and the rotor ( ground ) of the variable capacitor goes to the other spot where C16 connects to D4 and D5. Because D4 and D5 are not needed, replace one of the diodes with a jumper wire. The value of the variable capacitor should be around 20pF, even better with gear reduction.

10. AUDIO AMPLIFIER

Initially the intention was to use an audio amplifier based on transistors only. The LM386 amplifier can be anytime replaced with another audio stage if you want to. I wanted to use the LM386 because of it’s simplicity and because the IC it’s easy to find. The receiver presented here does not include an audio preamplifier, but the audio level on the output is strong enough to power a small speaker or headphones.

I also placed a very simple CW audio filter ( R5 and C7 ). For SSB and AM you can use the receiver with the switch taking out R5 from the circuit. For CW when the switch is on, R5 is present in the circuit and the filter will cut most frequencies higher than 720 Hz making the CW tones more pleasant to listen to.

11. POWER INPUT

This is a simple stage powered by 12v for the LM386 audio amplifier and 9v for the regenerative receiver stage, RF preamplifier an the diode tuning stage. R4 it’s used to lower the voltage to about 9V and stabilized with the Zenner diode D3. C10 is used to filter the voltage for the 9v stages.

73 DE YO6DXE

12. SPUTNIK REGENERATIVE RECEIVER

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13. OLDER VIDEOS

The receiver started simple and evolved in time. This was a learning process for me to understand how different stages work together, making many modifications. I started this before having my amateur radio license and is a project close to my heart, because it brought me back into the hobby.

If you have the time you can also watch the older videos and see how this simple but really nice receiver evolved and improved in time. The newest videos are on top and the older ones on the bottom. I renamed all “Simple Regenerative Receiver” not to create confusion with the latest version of the receiver.