BITX40 QRP Transceiver

The BITX40 is an interesting project. It is an inexpensive ($59) QRP 40 meter band SSB (LSB only) transceiver that comes as a semi-preassembled kit. The main boards are built and tested by the manufacturer in India and the end user only has to mount the boards in a case and wire the power, controls, and antenna connections. The radio itself is controlled by an Arduino microcontroller using a version of the Raduino firmware and a digital synthesizer chip provides frequency stability. Due to its simple design it is easily modified and there are dozens of mods on the internet that can be performed to add features. My ham radio club did a group build project of this radio and we had over 20 members put together their own BITX40.

One of the most convenient features of this kit is that the main boards make use of connectors to simplify construction. This also makes the radio very easy to disassemble since nearly all of the wiring can be unplugged. The kit comes with all the parts you need (other than a case, speaker, and knobs), however, I made a few changes. I had a couple of 6mm shaft knobs that I wanted to use that did not fit the potentiometers that were supplied. I also wanted to implement a couple of the simplest and most useful mods. I ordered the following parts from Mouser:

The pushbuttons are useful due to the features added in the modified Raduino firmware. With the new firmware installed, the white pushbutton serves as a Menu button that provides access to the additional features included in the firmware (Multiple VFOs, RIT, Split, USB, CW, frequency calibration, scanning features, and many others). One of the interesting things about the firmware is that if you have it installed and do not add any buttons or other mods, it still behaves like the default firmware. Only when you perform the appropriate mods does the additional functionality become accessible.

Another mod I performed allows the red pushbutton to serve as a Tune button. When pressed the radio automatically switches to CW mode, keys the transmitter, and generates a tone to allow tuneup of an antenna tuner. This functionality actually requires 3 separate mods (PTT Sense, CW Carrier, and TX-RX) which are detailed in the updated firmware’s documentation. They involve soldering a couple of resistors to specific locations on the board as well as a transistor across the PTT line and wiring from these components to the Arduino’s IO points. In order to maintain my ability to easily disassemble the radio by removing the case’s front and rear panels, I used a 2 pin header to create my own connectors for plugging and unplugging some of the additional wires that were added for these mods. The final modification involved soldering a 100pF capacitor in parallel with the inductor L7. This helps suppress the 2nd harmonic to levels that are acceptable to the FCC.

For the speaker I drilled some holes in the top of the case to let the sound out and mounted the speaker.  I left the wires long so that it is easy to remove the top of the case and lay it to the side without having to unplug the speaker cable from the main board. Using a speaker is highly recommended for this radio instead of using headphones. This is due to the fact that the BITX40 has no AGC (although there are mods to add one) and consequently the audio from strong stations is drastically louder than weaker ones. This difference in volume could easily hurt your ears if you were wearing headphones.

An electret microphone element is provided with the kit and I wired it up with a pushbutton in a small enclosure to work as a hand mic. I secured the mic element and the shielded cable using hot glue. This arrangement required me to wire the mic and PTT lines to the same 1/8″ stereo jack on the front panel, even though they have separate connections to the main board. As basic as this setup is it functions well and I have had good audio reports on the contacts I have made using it.

The best word to describe using the BITX40 is funky. After years of using modern complex transceivers, the BITX is almost shocking for how simple it is. You tune around and adjust the volume, that’s about it. Nevertheless it works, as long as you abide by QRP operating procedure:  find the strongest station on the band and weight for them to call CQ or QRZ. It’s pretty impressive how simple this radio is and how little you really need to make contacts.

I put a fair amount of effort to construct this radio carefully, however, some of my ham club’s members who built their own ended up with a spaghetti of wires and their radios still functioned fine. Because all of the complex circuitry is pre-assembled and tested, the hardest part of this project is already done. That is a key part to this being a great project because it allows people of all skill levels to build something and be virtually guaranteed that at the end they will have a working radio. In my club we had people who had never soldered before build this radio (with plenty of guidance) and they were all smiles when we powered up their creation the first time. The combination of affordability and functionality make the BITX40 an amazing piece of technology and a fantastic addition to ham radio.

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