Adding 80m to the BITX40
A common question for people have about the BITX40 is “Can I make it work on [insert band here]?”. The answer is that yes, you can make it run on other bands, and we’ll show you how to add 80m to the BITX40. Radio wizards know that, but many of us are just learning, and to us, these things are mysteries to be revealed. This week I was able to uncover one such mystery for myself, with the help of Orin Beebe VE7BEE on the BITX40 Facebook group. He documented bypassing the 40m Band Pass Filter (BPF) and modifying the analog (non DDS) VFO to work on 80m. I wasn’t really understanding how it worked, so I experimented a bit, and now have my BITX40 working on both 40m and 80m with a flick of a switch, and it was far easier than one would imagine.
First things first: What is a Band Pass Filter (BPF)? A BPF is a filter that effectively limits what frequencies get past it. It’s used in the BITX40 receiver to reject unwanted interference and noise.
Secondly: What is a VFO? A Variable Frequency Oscillator effectively tunes the radio to a specific frequency by mixing the Beat Frequency Oscillator (BFO) and the incoming radio signal, and you get audio. The analog VFO on the BITX40 only covers 7.1-7.3mhz, but I don’t have that- I am using the QRP Labs VFO kit. This mod should work with the Raduino that’s packaged with the BITX40 as well, but the specifics are unknown to me. If you know it, comment below.
The Nitty Gritty: Adding 80m to the BITX40
So if you want to receive 80m on a 40m receiver, you have to allow the frequency past the BPF. So you have to bypass the 40m BPF.
You’ll be opening the receiver wide open for interference, because you’re basically eliminating the BPF. So you do get more noise, but you also get any band you want to listen to. If you look at the circuit, you can see how you can bypass it from L1 to C6 like so (to the right). By putting a switch inline with the bypass that I diagrammed to the right, you can selectively bypass the BPF, and you can receive on 80m. It’s really that simple. You can also receive on any other bands, and if you tune to 2.5mhz you can even hear WWV. Pictures of my own modification are at the end.
What about Transmitting on 80m?
This is where we get into the Low Pass Filter. As far as I understand, the LPF suppresses harmonics (which are always of a higher frequency). So it already allows lower frequencies on transmit, and works fine with 80m. No other modifications are necessary! Just tune your VFO down to 80m, bypass the BPF, and now you have a dual band radio. The best way to go about this would be to add an 80m BPF that you could switch in and out as needed, but this works and costs not a dime and a bit of soldering. Give it a shot, and let us know how it goes in the comments. 73!