The Sputnik regenerative receiver was designed for the amateur radio bands, the perfect project for beginners in homebrew radio receivers.
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Sputnik regenerative receiver has decent receiving capabilities. It’s a simple regenerative receiver for AM, SSB, CW, FM. It was designed for the 40m amateur radio band. 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. In the video above you have one of the first modified versions. Here in the article is the latest schematic and PCB design updated in 2023. I will soon make a new updated video as well.
THE ORIGINAL SCHEMATIC
The schematic I found was published in one of the Romanian magazines from the 80’s called Tehnium. Later on similar schematics were published with little modifications, but no improvements in the results. Since it was the first receiver designed for the amateur radio bands I’ve built in my childhood, I decided to get back to it. I tried doing my best to improve it, with 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. Maybe it will also help others making their first steps in building radio receivers.
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
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. Different older versions of the receiver are popular on a QRP Forum in Germany and the SolderSmoke Blog. California QRP Club also asked the permission to use it for a kit build, with slight modifications by John Sutter K6JDS. It had some nice words from Peter Parker VK3YE on his Facebook Page. I have loads of emails and photos from the subscribers to the YouTube channel that also built the Sputnik regenerative receiver.
Down bellow you have the latest schematic design. This is the final update as I will no longer make any updates. 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. If outdoors the speaker is not so loud though, so headphones are better.
With ought the feedback sent by the blog readers and the subscribers on YouTube, or the different ideas to make little improvements, this receiver won’t be as nice as it turned out now.
SPUTNIK REGENERATIVE RECEIVER SCHEMATIC
I decided not to use a voltage regulator, as it doesn’t help much when using batteries. If the voltage drops under 12v, the LM386 IC will end up creating that ugly “motorboat” sound. If you want to improve the frequency drift, you can use a variable capacitor instead of the diode tuning. This will improve the stability of the receiver even more. I would recommend a variable capacitor with some sort of reduction drive.
I also decided to get rid of all sorts of filters that I used in some of the older versions. There is no more bandpass filter on the input. I tried to get rid of the AM broadcast band interferences by testing the audio amplifier in all sort of configurations. There is no more CW audio filter either. Instead I opted for a sound that makes both CW and SSB signals sound good. This way the schematic is very much simplified.
RF COIL DESIGN
You can build the receiver using a PVC pipe as RF coil form, or a toroid. The terminal marked 1 is the beginning of the coil and the terminal 2 is the end of the coil. L3 is the pickup coil for the external frequency meter if you want to use one. L3 is optional if you want to keep it simple. 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.
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 40m amateur radio band. You can also calculate it for other HF bands as well. I tested it from 80m up to 15m and it works great.
RF COIL DETAILS
L3 has 3 turns / 0.4mm.
When I started playing around with receivers schematics, I did not know how to calculate the circuits to resonate on the frequencies I wanted. I learned the hard way and at first I discovered how to do it by trial on error. Later I discovered that using some really simple online calculators, can save me a lot of time and also enameled copper wire.
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.
SPUTNIK REGENERATIVE RECEIVER PCB BOARD
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 regenerative receiver designed by N1TEV. This is not an RF preamplifier as many people think. It does amplify the signal but only a little. Is nothing but a buffer stage.
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 C11, C12 and C16 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 C16. I intentionally left two spots on the PCB board for C16. 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 C15, 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.
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 can cover the entire 40m band. One of the reasons I used regular diodes instead of varactor diodes, is that they are really easy to find.
The diode tuning stage is formed from multiturn 10K potentiometer, the 10K trimmer resistor, C13, C14, D2, D3, and R8. For D2 and D3 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 it in the spot designed for C13. Replace D2 with a jumper wire. The value of the variable capacitor should be at least 20pF.
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 headphones. Is also good enough for a speaker if listening in a quiet room. Not loud enough if listening outdoors. Try using a larger speaker as it gives a better sound. If using a smaller speaker replace C3 with a 100uF capacitor instead of 470uF.
This is a simple stage powered by 12v for the LM386 audio amplifier and 10v for the regenerative receiver stage, RF preamplifier and the diode tuning stage. R3 it’s used to lower the voltage to about 10V. C7 is used to filter the voltage for the 10v stages. I used no voltage stabilisation, as I wanted to keep it simple.
If you decide to build it Manhattan style or dead bug style, try to keep the connections between parts as short as possible. Built well with attention to detail it can end up as a really nice receiver. As a radio receiver project, it could be better. I tried keeping it simple for beginners. It was also designed as a portable receiver with a minimum battery consumption. For frequency stability for C16 try to combine capacitors of different types until you get it as stable as possible in frequency.
As example I used a 68pF ceramic capacitor and a 30pF styroflex capacitor. They seem to compensate each other for temperature changes and the receiver i really stable in frequency. Also using a variable capacitor instead of diode tuning ( one with a reduction drive ), can make the receiver even more stable. I hope you like it and you have success in building it.
73 DE YO6DXE