BITX20 Project

I was listening to the SolderSmoke podcast and Bill, N2CQR, the host was talking about his 'BITX' radio. 'BITX' is a design by VU2ESE of India, it uses the same basic amplifier block throughout, and uses a 'bidirectional' architecture. A lot of the circuit is shared between RX and TX. The beauty and appeal of the BITX design is that it uses very common (cheap!) parts and there's lot of troubleshooting information available at the BITX Yahoo group.

The BITX design appealed to me, because, for the last few years, I've had the desire to have a transatlantic QSO with a home made radio via voice. I haven't actually managed a contact to North America with a home brew radio, although I've done it many times on CW with rigs built from kits. I contacted Bill, N2CQR, and asked his opinion as to whether a common or garden ham like myself could build a BITX. The answer was simple - go for it!

I intended to build my radio with a traditional VFO, true to the VU2ESE design. Recent episodes of SolderSmoke have featured Pete Juliano, N6QW. Bill had copied Pete in on the emails and Pete put forward his case for a DDS VFO. At first I resisted, but then having thought about the benefit of stability, especially as my rig would be used outdoors, I decided that the DDS VFO would actually be a very sensible idea.

I'd seen a presentation by Paul M0XPD at Rishworth convention about the use of Arduino in ham radio in October 2013, I came away feeling inspired. Microcontrollers and programming is not an area of electronics that I'm particularly interested in, but I could see the benefits of Arduino. soon after the presentation, I ordered an Arduino Uno and AD9850 DDS module. I managed to get the Arduino Uno and DDS module running, but then put them in the drawer, like you do!

Pete, N6QW, told me that instead of using an Arduino Uno board, for more permanent applications, an Arduino Pro Mini or Arduino Nano is a cheaper and smaller solution. There's a lot of stuff packed on to an Arduino Uno board, most of it is not actually used once the sketch (Arduino code) is up and running. The Arduino Pro Mini is a very basic Arduino board, it consists of the microprocessor chip and crystal plus voltage regulators. The Arduino Nano is similar to the Pro Mini but has an on-board USB programmer. To program a Pro Mini, you have to use an external programmer. The benefit of the Arduino Pro Mini is that you only have the hardware that you need to run the code, this makes it very cheap and also very small.

I ordered an Arduino Pro Mini clone board via Ebay from a UK seller, they're cheaper from overseas, but at £3.35 posted to your door within a couple of days, I think you can't go wrong. I also ordered another AD9850 module. The price of the AD9850 modules seems to be steadily going up, this is probably because supply is drying up.

Pete N6QW kindly sent me his Arduino sketch and also a wiring diagram for interfacing the DDS to the Pro Mini. It took a few weeks to get things working, I had trouble with my first external programmer, it had been fitted with a fake FTDI chip and FTDI had sent out some code via Windows update to 'brick' fake chips. The programmer worked for a few minutes and then the computer failed to recognise it. I obtained another programmer board, this time fitted with a CH430G chip, this one has worked flawlessly. At some point, I'd managed to copy a schematic mistake from one of the web resources regarding the connections for I2C from the Arduino into my lab book. I battled for ages trying to get my display working, it turned out that I had A4 and A5 (I2C output) transposed. I learned some useful stuff whilst getting the LCD working, so I'm thankful for the initial mistake!

By late October 2014, I had the Arduino controlled DDS up and running, complete with a cool looking white on blue, four line by twenty character, display.

Bill, N2CQR put a picture of his layout diagram on the BITX Yahoo group, I used this to start my BITX build. I replaced the VFO section with my Arduino VFO module. I used the original BITX schematic, starting with the microphone amplifier in the bottom right corner of my single sided copper clad 10"x8" board. I chose to use 2N3904 transistors. I built a few sections, including the BFO, balanced modulator and IF amps, before applying power for the first time. I checked that the oscillator was working and also tested the mic amp and balanced modulator by hooking up a microphone temporarily. My Tektronix 465 scope was very useful at this point. All checked out OK, so I proceeded to add the crystal filter.


Next to be added was the second IF amplifier and balanced mixer. Pete N6QW's Youtube channel came in very useful, it helped a lot whilst winding the trifilar transformers. I fixed the Arduino DDS module in place with M2 countersunk screws and stand off sleeves. With the addition of the LM386 audio amplifier and another IF amplifier, I was in a position to try the receiver for the first time. Some signals were heard, so at that point I was happy. It wasn't until later that I discovered that I had put a resistor in the wrong place in one of the IF amps (This was my only mistake in the build, and I found it straight away whilst sweeping the crystal filter to measure it's response.) By the end of November 2014, I had a working BITX receiver.

I used the original version BITX band pass filter, the later versions use different designs, but I wanted to go with something simple, I wasn't looking for ultimate performance. I used the excellent blog site of Edwin Groot, PA1ED for information about the band pass filter. After the addition of the BPF, I terminated the end with a 50R load and then tested my circuit in TX. It was great to see home brew SSB signals on my 'scope!

The final stages to be added were the RX/TX switching stage, low pass filter and TX amplifier
stage. Against advice, I chose to build the TX power amplifier stage on the same board as the rest of the circuit. At first, I thought all was fine, the rig seemed to work perfectly, with nice sounding transmitted audio (received on FT817 in another room, whilst TXing into dummy load) and just over 5w showing on the meter. I was high on my success and decided to take a break for a while. I came back to the circuit with a view to getting it ready for an on air test and I thought I'd give the circuit a thorough check over. I soon discovered that no matter how much I tried to null the residual carrier by using the variable resistor and capacitor on the balanced modulator, the lowest I could get it to was 400mW. I thought about the problem for a day or so and then decided that it must be some of the output leaking back into the input of the TX stage at the join to the shared band pass filter. Bill N2CQR had also had this problem with his circuit. To test my theory, I simply disconnected the coax connecting the LPF and BPF, sure enough, I could null out the TX carrier to a very low level. The solution was to add another relay in order to disconnect the RX line, both at the LPF end and at the BPF end. Luckily, there was just enough room on the board to add the third relay. By sheer luck, at each stage of the build, I just seemed to have enough board space for the circuit - I think this project was just meant to be!

Three relays to switch between RX and TX may seem excessive, but I had them to hand. One relay switches 12v to either RX circuits or TX circuits and grounds the line not being used. Another relay disconnects the RX path from the LPF and grounds the TX sense pin on the Arduino. The last relay isolates the RX path before the BPF and has one spare switching section, I thought this might be useful in future to possibly control a linear amplifier.

Once I'd solved to residual carrier issue, I was ready to try for my first QSO by the end of January 2015. Bill N2CQR encouraged me to try the circuit as just a bare board in keeping with ham tradition. I don't have a permanent antenna so I had to wait for a dry day and I put up a temporary 20m dipole in the garden. On 1st February 2015, I set up a table in the garden and took the BITX outside as a circuit board. I heard a German station calling CQ with a very strong signal, so I decided to give him a call. I was amazed that he picked me up with the first call. (I later found out that OM Edwin had also had a BITX QSO with the same station!)

It really was a magical moment having a QSO with a home made radio, something that took me about two years to achieve from first having the idea to having the QSO.

With the successful completion of the circuit, it was time to put the rig in a case robust enough for SOTA use. A case was made from half hard aluminium sheet, to fit the circuit board nicely. To compliment the blue LCD, I used blue push switches for the menu functions and also as finishing touch, I added a dual colour LED to indicate RX and TX; blue for RX and red for TX.

The rig was ready for it's first SOTA activation. I hadn't actually tried the rig into an antenna since the first test in the garden as bare circuit board. I do SOTA during the winter as you get bonus points between December and March. I looked at the weather forecast for my chosen weekend to activate and it was due to be very windy. I had to change my plans to a morning activation on Saturday 28th February. It just so happens that the long path propagation works well to VK at this time of year at around 0800utc.

I activated Sharp Haw (SOTA G/NP-029) on 28/02/2015, getting on air by around 0725utc with the BITX20 into an inverted vee dipole suspended by a 7m fishing pole. I called CQ a couple of times before being answered by Vlado OM1AX. I was relieved that my rig was still working well. I worked another five European stations before I heard VK1DI calling me. at first I thought I was dreaming or something, but sure enough, VK1DI was in the log at 0737utc. By 0755, my log included contacts with four VK stations! What an amazing experience! Picture at the top of this article shows my BITX SOTA activation.

I still have to achieve my goal of a home brew transatlantic QSO, but I'm fairly confident that I will be able to achieve this in the coming months.

Thanks expressed to Bill, N2CQR, and Pete, N6QW, for technical advice and encouragement, Paul M0XPD for Arduino code resources, Edwin Groot, PA1ED, for his excellent home brew BITX articles, and finally Graham, G3MFJ for supplying parts.