The Bitx40 transceiver – mini review If you want something small and low drain for SOTA or parks portable operating, it’s hard to go past the Bitx40 designed by Ashhar Farhan VU2ESE. It’s a monoband 40m SSB transceiver capable of 7 watts output. It comes as a preassembled board and external components. All you need to do is solder external parts eg volume and tuning controls, antenna power, speaker and microphone sockets and you are on the air. An hour all up. You supply the box, speaker and battery. If you’re clever with the casing it can be made fairly small and only 4 or 5cm thick. Details are at http://www.hfsigs.com/
My review of it is at https://www.youtube.com/watch?v=1EvA6Q6C2rE The first version cost $US45 and had an on-board free-running VFO. It was woefully unstable. The version of the Bitx40 available now costs $US59 and comes with a DDS VFO which fixes this problem. But if you want to retain the ultra-low power consumption of the original you could improve stability through other means. The IF is 12 MHz. Which means that if you use a 4.92 MHz ceramic resonator you can cover the most popular section of 7 MHz. Switching in extra capacitors gives additional range – almost 100 kHz all up from 7.068 to 7.163 MHz in my version. This is good for all the popular SOTA and parks frequencies. Details in my video at https://www.youtube.com/watch?v=10109ueIEO0 The improvement in stability and ease of tuning is dramatic. Let me know if you don’t have a 4.92 MHz resonator as I have some spares. There’s some other modifications you can do. If you are using the Bitx40 mainly with headphones you can remove the chip capacitor between pins 1 and 8 of the LM386 audio stage (C 113). This makes the audio less harsh. Good transmit audio quality is essential and makes the difference between being detected and being understood. You want it to be a little peaky and sharp with emphasis around the 2kHz area. My Bitx40 started to tail off around 1.5 kHz which is too low. Changing the 12 MHz carrier oscillator’s frequency (ie raising it a few hundred Hz) helped a lot. I did this by removing the 47pF at C102 and replacing it with 27pF to shift the crystal’s frequency. Details in my video at https://www.youtube.com/watch?v=IELJwapv5qs To conclude the Bitx40 is a great value rig ideal for portable QRP. Doing the above modifications will make it truly practical and a pleasure to use.