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A QRP minimalist CW transmitter built in less than ten minutes. Only twelve components and an output power of 700mW @ 12V or 500mW @ 9V. A great project for beginners in building amateur radio equipment. I found the schematic in the Minimalist QRP Book. After building the Ten Minutes Transmitter or the Michigan Mighty Mite, I thought I will not find a transmitter simpler than these. Miguel – PY2OHH also built a version of this simple CW transmitter.
THE SCHEMATIC
If I usually end up making a lot of modifications to most of the schematics in order to get the circuit perform better, in this case ( Fig. 1 ) I didn’t changed much and it works great. For that reason I kept the schematic as simple as possible. The only thing I added is a modified Pi Network low pass filter on the output and a capacitor for frequency adjustment.
Rob – KC4NYK was kind enough to measure the harmonics suppression with a TinySA and it seems that the standard Pi Network filter I used in the past didn’t do such a great job. For this reason I ended up modifying the low pass filter and the results are way better now. Please keep in mind that I do not have a TinySA or other equipment to test the harmonics suppression.
Because I could not measure the output of the transmitter, I ended up listening to the harmonics into a receiver. After the modifications I can barely hear the harmonics in the receiver. If I place the transmitter further from the receiver, the signal disappears completely.
Fig. 1 – QRP Minimalist CW Transmitter Schematic
PERFORMANCE
With a simple general purpose NPN transistor, you will get about 700 mW output power @ 12V or 500mW @ 9V. In some cases depending on the transistor, the power output after the low pass filter can be a little lower. I am sure that with a better transistor you may be able to squeeze a little bit more power. It’s always a good idea to try more transistors ( even the same type ).
Even with 500 mW in CW you can still have lots of contacts all over Europe from Romania. The best part when not forcing the transistor, is that it will not overheat as much. Another great thing is that it doesn’t have any chirp. It sounds great and has a beautiful tone. Together with a simple receiver, you can start doing QRP in no time. I can’t wait to finally learn the code and use it together with the Sputnik Regenerative Receiver. It should be fun doing QSO’s this way.
PCB BOARD
Using the toner transfer method, you can make a PCB board ( Fig. 2 ) in less than 10 minutes. I started to find this a little annoying as they don’t always come out right from the first try. So now I prefer to order the boards from PCBWay. Feel free to order the updated PCB boards. The final result looks amazing.
Fig. 2 – QRP Minimalist CW Transmitter PCB Board
In the video you have the homebrew version, made using the original schematic. I used a homebrew inductor, as I didn’t had the 5.6uH used in the original schematic. It didn’t had a low pass filter as I just wanted to make a quick test and see if I like it.
Here in Fig. 3 you have the latest fully built version that also includes a low pass filter and a heatsink for the transistor. I am also using a 10uH inductor as the transmitter seems to perform much better this way.
Fig. 3 – QRP Minimalist CW Transmitter
LATEST UPDATES
In this video I spoke about ideas I had to improve the transmitter. In the latest version I updated the low pass filter for better harmonics suppression. To adjust the transmitting frequency I added C2 ( Fig. 1 ). By adjusting the value of the trimmer capacitor you will be able to adjust the transmitting frequency exactly on 7,030MHz.
I replaced the simple 2N3904 transistor used in the original design, with a TO18-3 ( metalic can ) 2N2222A transistor. This way you can also place a heatsink on the transistor to avoid overheating, especially if you are planning on using the transmitter heavily. Many people recommended this and I think is an excellent idea. Here you have a simulation of the modified low pass filter. There are way better low pass filters of course. I really wanted to keep it simple and minimal as much as possible. This is the reason why I opted for the simple ( modified ) Pi Network filter.
Fig. 4 – Low Pass Filter Simulation
ADJUSTING THE BANDPASS FILTER
The inductor L2 is recommended to be made on a T37-6 toroid. It has 18 turns of 0.35mm enameled copper wire ( calculated for 1uH on toroids.info ). To adjust the filter once you finished building the transmitter is simple. Connect the transmitter to a power meter and spread or tighten the turns on the toroid for the highest output power. Also do not forget to listen to the signal on 14,060MHz and check for the harmonics suppression. If you have a TinySA is easier to check the harmonics suppression.
We cannot always be perfect with the final value of the inductor when hand winding a toroid, unless you have an LC meter to measure the exact value. In my case I ended up with 16 turns and an output power of nearly 1W using the 2N2222A.
I am wishing you good luck building this fun QRP minimalist CW transmitter. It’s really tiny, easy to build and adjust by beginners. Feel free to experiment using other transistors as well. Also by changing the value of the 330pF capacitor, you may be able to increase the power. Just keep an eye on the transistor so it will not overheat. If you keep the key pressed for about ten seconds and the transmitter is not stable in frequency, chances are the transistor is overheating. Also if there is any chirp, it’s usually from the same reason.
Rob was picked up on the reverse beacon network 233 miles north from his location. The power output he had was only 300mW. Congrats Rob for this. I really need to learn the code as well so I can test the equipment and really enjoy what I am building.
MODIFICATIONS FOR OTHER BANDS
I had a request from my friend Vlad – YO5BDL to build a 20m Band version for him. I did tried once with no modifications to the 40m band version ( no low pass filter ) except the crystal. I had really low power under 50mW at that time. So I made a few adjustments that you can see down in Fig. 5 and also in the video down below. With the small crystal used I could only get around 100mW output power using a crystal similar in size like the ones used in the Pixie transceiver kits. Using a larger crystal you can get 700mW just as in the 40m band version.
Fig. 5 – 20m Band Version
In the video you will see a few experiments and tests using different type of crystals and how they influence the power output and the signal generated by the oscillator. One thing I didn’t mentioned in the video, is the number of turns I used for the inductor. Calculated using torioids.info I needed 12.5 turns on a T37-6. With the adjustments made I ended up with 11 turns.
I did not placed here the filter simulation for the 20m band as is very similar to the 40m band as you can see in the video. I will also check the harmonics suppression with the TinySA once I purchase some RF attenuators. I hope you like this tiny but mighty and really simple transmitter. I can’t wait to see what my friend Vlad will do with it. He has a tallend in making QSO’s all over Europe using only 1W. I am sure 100W is enough for him to play.
73, YO6DXE
Thank you very much for this project! I have just finished building one with your files:) Works great! It is my first pcb since school, doing this project is like a teleport to childhood! Thank you! Sputnik receiver will be the next one to build.
I teleport myself in childhood with each project I am working on here for the blog or the YouTube channel…. so I know the feeling hehe. Glad it works and you like it. I am working on finishing mine as well on a proper PCB board as I just received them last week from PCBWay… and I must try a 20m version for my friend YO6BDL as he really loves it and wants one on 20m. Good luck with the Sputnik. I must make a box for mine as I wanted something smaller than the older version.
73, YO6DXE