700 mW Minimalist CW Transmitter With One Transistor

A very simple and minimalist CW transmitter built in less than ten minutes, using only ten components and with a power output of 700mW.
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A minimalist CW transmitter built in less than ten minutes. Only ten components and a power output of 700mW. 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, I thought I will never find simpler than that. Miguel – PY2OHH also built a version of this simple and cute CW transmitter ( thanks John for the link ).

THE SCHEMATIC

If usually I end up making modifications to most of the schematics, in order to get the circuit perform better, in this case I did nothing. I could not believe that it worked perfectly and didn’t need any adjustments. For that reason I kept the schematic as simple as it was. The only thing I added is a simple Pi Network low pass filter on the output. Please keep in mind that I do not have an oscilloscope to test the harmonics suppression. 

To be honest I am wondering if it does transmit any harmonics with no low pass filter. I guess I will find out the answer in the future. An oscilloscope is way too expensive for me right now. I did end up doing a simulation for the low pass filter, together with the 330pF capacitor from the output. From experience I noticed that the output capacitor will influence the filter.

I cannot test this, but I was told that if used through an antenna tuner, there is no need for a low pass filter. Technically if the tuner is good and has a narrow bandwidth it should work. I don’t have so much experience on this, but one day when I can afford an oscilloscope I will.

MINIMALIST CW TRANSMITTER SCHEMATIC

PERFORMANCE

With a simple 2N3904 you will get about 700 mW output power. In some cases depending on the transistor, the power output after the low pass filter is around 500 mW. I am sure that with a few adjustments you may be able to squeeze a little bit more power.

I don’t plan on doing that as even with 500 mW in CW you can still do 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 at all. It sound great and has a beautiful tone. Together with a simple receiver, you can start doing some QRP work in no time. To be honest, 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.

The version in the schematic is for the 40m band, but in the download folder you also have details for the 80m version in the parts list. The power seems to be the same. I also tried 20m but I couldn’t get it to work so well. Maybe I will manage to do that in the future.

PCB BOARD

Using the toner transfer method, you can make a PCB board in less than 10 minutes. I started to find this a little annoying as they don’t always come out right from the first time. So now I prefer to order the boards from PCBWay. This was my first design in Eagle. Now I you can also order the PCB boards.

In the video you have the homebrew version, made following the original schematic. I also used a homebrew inductor, as I didn’t had a 5.6uH inductor in my junk box. It didn’t had a low pass filter as I just wanted to make a quick test and see if I like it.

Here you have the final version that also includes the low pass filter. For the final version I used a 6.8uH inductor as I noticed a slightly bigger output power.

ADJUSTING THE BANDPASS FILTER

The inductor L2 is recommended to be made on a T37-2 toroid. It has 18 turns of 0.35mm enameled copper wire. 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.  

So all I can say now, is to wish you good luck building this fun minimalist CW transmitter. Is really tiny and 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, is usually from the same reason. 

73 DE YO6DXE

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2 Comments

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    • Thanks John. Ohhh I had this one confused. I thought is the CURUMIM CW Transceiver initially. These names get me confused ( ha ). I just opened the link and is the same transmitter yes. I just changed the value of the inductor and the output capacitor to increase the power a little and minimize the chirp. But I think this depends a lot on the transistor used also.73, YO6DXE