Sputnik Regenerative Receiver v1.0

Sputnik Regenerative Receiver v1.0

Sputnik regenerative receiver is a simple radio receiver designed for the amateur radio bands. It has decent receiving capabilities. A project designed for beginners in homebrew radio receivers.

Sputnik regenerative receiver is a simple radio receiver for CW / SSB / AM. It has 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 to help improve the knowledge on 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 radio receiver as stable as possible keeping the design simple. This is the last update for v1.0 as I will no longer make more updates.


1. THE ORIGINAL SCHEMATIC

The schematic was found in one of the Romanian magazines published back in 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.

Schematic - Simple Regenerative Receiver

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. I did not had any of those magazines on hand, so I decided to try to improve it the best I could. I have to note that 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

Regenerative Receiver
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 was discussed about 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 of version 1.0. This is the latest and final updated schematic. I wanted a receiver as small and as simple as possible, but with good performance. To keep it simple and small I did not used an audio preamplifier, so the audio output is designed for regular headphones only. 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.

SCHEMATIC WITH SEPARATE BANDPASS FILTER

Updated Design With Separate BPF -Sputnik Regenerative Receiver v1_0

SCHEMATIC WITH BANDPASS FILTER INCLUDED

Updated Design With BPF Included - Sputnik Regenerative Receiver - v1_0

Most schematics I used in the videos down bellow are somehow similar to this one. What I did was just to either remove or replace different stages that I was not happy about during testing.

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 of v1.0 I also added 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.

3. RF COIL DESIGN

Comparing the RF coil made on PVC pipe, I must say that the coil made on a toroid is much more sensitive to temperature changes, making the receiver drift in frequency. The PVC pipe design seems more stabile even if it’s not as small. Here you have the basic design so you have an idea of how the coil should end up. The numbers of turns depends on the band you decide to build it for. 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. 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.

RF Coil - Sputnik Regenerative Receiver v1.0
RF Coil PCB Pins Placement

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. To calculate the resonant frequency of the circuit you can use this online resonant frequency calculator. 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 with ought major modifications to parts values. But if you really want it for the 10m band as well, feel free to experiment. It’s a great learning experience.

PVC PIPE RF COIL DETAILS

For the PVC pipe version of the coil, L3 has 3 turns / 0.4mm enameled copper.

BANDL1L2C
80m56 turns / 0.4mm enameled copper8 turns / 0.4mm enameled copper15pF
40m18 turns / 0.4mm enameled copper5 turns / 0.4mm enameled copper45pF
20m8 turns / 0.5mm enameled copper2 turns / 0.5mm enameled copper45pF

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

Toroid RF Coil

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. As I was saying earlier, they don’t seem to me so stable in frequency when there are temperature changes. I’m still using them because they don’t take as much space as the PVC pipe coil does.

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, L3 has 2 turns / 0.4mm enameled copper.

BANDL1L2C
80m / T50-2Coming SoonComing Soon15pF
40m / T50-6Coming SoonComing Soon45pF
20m / T50-620 turns / 0.4mm enameled copper3 turns / 0.4mm enameled copper45pF

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 v1.0. I’m not a master in design, but I did my best to keep it as small and also as comfortable as possible. This PCB design includes all the parts and stages included in the schematic. You don’t have to built it completely to work. As an example if you don’t have all the parts, it’s possible to skip the band pass filter. For the tone control I only have a 100nF capacitor placed after P4 to ground. A middle point for a good sound, both for SSB and CW. In the files folder you will find the “Print This” PDF file. It contains a print for two PCB boards of the receiver and two boards for the bandpass filter. Just in case you mess up one board, you have an extra print to repeat the process.

PCB DESIGN WITH A SEPARATE PCB BOARD FOR THE BANDPASS FILTER

PCB Design - Sputnik Regenerative Receiver v1.0
Parts Layout - Sputnik Regenerative Receiver v1.0

PCB DESIGN WITH THE BANDPASS FILTER INCLUDED ON THE BOARD

Updated PCB Design With BPF Included - Sputnik Regenerative Receiver v1_0
Updated Parts Layout With BPF Included - Sputnik Regenerative Receiver v1_0

5. THE BANDPASS FILTER

Initially I wanted to design a custom bandpass filter for the Sputnik Regenerative Receiver. I would be able to do simulations in a software to see how the filter performs, but it would not be the same as testing a real filter. Since I don’t have any equipment to test the filters, I decided to use a well known filter design. I like QRP Labs and Hans Summers work, so I decided to use the same bandpass filter design as the one used by QRP Labs.

Bandpass Filter Schematic and PCB Design

If you want you can order a Bandpass Filter from QRP Labs instead and adapt it to the Sputnik Regenerative Receiver. You also have the Filter Building Instructions available. If you want to build the PCB design available in the download folder, the instructions are the same. The only difference is the dimensions of the PCB boards of the filter. If you don’t have the parts for the filter available, you can just solder a jumper wire as noted on the PCB Parts Layout and build the filter later on. Don’t forget to remove the jumper wire when installing the filter.

BANDT1, T2C1, C5C2, C4C3
80mT37-2, 3.83uH, 6:30 turns30pF trimmer470pF60pF trimmer
40mT37-6, 3.02uH, 6:30 turns30pF trimmer150pF20pF trimmer
20mT37-6, 1.79uH, 4:24 turns30pF trimmer56pF10pF trimmer

Just a quick note on the bandpass filter schematic I made comparing to the one from QRP Labs. C1 and C2 are reversed in my schematic comparing to the one used by QRP Labs. Also instead of C3 and C4 in parallel in the QRP Labs schematic, I’m only using the trimmer capacitor C3 in my schematic. So C5 and C6 in the QRP Labs schematic, become C4 and C5 in my schematic. Adjusting the bandpass filter is identical as the one described very well by Hans in his instructions. The only difference is that now you can also adjust C3 as well.

6. 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. You can use a 2N2222 or a 2N3904 transistor.

7. REGENERATIVE RECEIVER STAGE

This is a simple regenerative stage. The level of regeneration it’s adjusted using P3. 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. If you decrease the value of R8 down to about 1.5K it will help you get an even smoother regeneration control. Some people also like using a 10K multiturn potentiometer instead.

For a good stability in frequency C3, C4, C5 and C6 should be the NP0 type capacitors. If you have too much capacitance, or not enough and you can’t get the receiver into the 40m band, you can either increase or decrease the value of C6. If you want to increase the sensitivity of the receiver, you can increase the value of R6 to 6.8K. I would not recommend this as I tried many combinations, and the 4.6K was the best choice that doesn’t overload the receiver with strong signals. Feel free to experiment just as I did.

With the help of the trimmer capacitor C7, 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 trimmer resistor R4, 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.

8. 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, then replace the diode tuning with the variable capacitor as the receiver is more stable and there is no frequency drift when the battery voltage goes down.

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 P2, R3, R4, C8, C9, D1 and D2. For D1 and D2 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 C9 goes to C7, and the rotor ( ground ) of the variable capacitor goes to the other spot where C9 connects to D1 and D2. Because D1 and D2 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.

9. 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. Because of this, the audio level on the output is not strong enough to power a speaker. That’s because I wanted to use headphones only, same as the well known Pixie kits do for example.

10. 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. R9 it’s used to lower the voltage to about 9V and stabilized with the Zenner diode D3 for the rest of the circuits powered by 9v. C15 and C16 are used to filter the voltage for the 9v stages. When building the receiver you can measure the voltage between ground and the cathode of the diode. If the voltage is under 9v, you can lower the value of R9 to something around 220 Ohms.

73 DE YO6DXE

11. VIDEO

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12. OLDER VIDEOS

Down bellow you have a few older videos with different versions of the Sputnik Regenerative Receiver schematic. I was learning about regenerative receivers and testing the performance in all sort of configurations. Thank you to all the people that helped me learn and improve this little receiver. It helped me learn a lot about regenerative receivers. Keep experimenting and modifying the receiver as much as you want. It’s a great way and the best way to learn about building radio receivers. There are countless improvements that can still be made. In the last design update of v1.0 I tried to cover all the requests or ideas I had from the people.

1. Sputnik Regenerative Receiver v1.0 – SSB – CW – AM – 40m Band

2. Schematic Update – Sputnik Regenerative Receiver v1.0

3. Final Schematic and PCB Design – Sputnik Regenerative Receiver v1.0

4. Building the Sputnik Regenerative Receiver v1.0 – SSB – CW – AM

5. Quick Tips on Building the Sputnik Regenerative Receiver v1.0

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