CY0S Worked With Simple QRP Rig

I have been trying to find ways to stay active in ham radio during challenging times. Lately I have been experimenting with simple homebrew 40 meter CW rigs. My fist try was a Pixie kit. The Pixie is a true minimalist transceiver with very few parts. I had one years ago and thought it worked pretty well, but this latest one I got didn’t seem so great. Perhaps the circuit has changed or I have! It suffered terrible BCI (broadcast interference) from strong stations in the medium wave band. This was mostly cured by adding a 40 meter band pass filter. I tried to add a VXO (variable crystal oscillator) to get some frequency agility as I had done with my old Pixie 20 years ago but after weeks of playing around I gave up. This one did not want to move more than a kilohertz or so (the old one had no problem going 7 kHz), and the receive-transmit frequency offset changed as the VXO was tuned through its limited range.

The kit built Pixie with band pass filter

I soon gave up on the kit built Pixie and tried making one from scratch. I had all the parts on hand so why not? I experimented with adding audio filter stages and learned quite a bit in the process, but the pure homebrew version also didn’t want to move very far with a VXO and had the same problem with receive-transmit offset. I soon abandoned it too.

Homebrew Pixie with audio filter stage

My friend Tom, KN4RRQ had built a two transistor transmittter following a design by VK3YE but was having problems getting good power out of it. It was supposed to do about five watts on 12 volts and 20 watts on 30 volts, but it wasn’t. It uses a BD139 transistor oscillator followed by a IRF510 or IRF530 amplifier. Tom sent me a few BD139s since I didn’t have any and I threw one together to see what I could get it to do. I had the same problem with very low output (about half a watt) until I added a bias circuit which brought it close to five watts. Harmonic levels were too high for my liking with the original low pass filter so I made the second stage of that filter a 20 meter trap which got that under control. Modifying the filter must have changed the load seen by the FET. Output was up to 8 watts operating on 12.5 volts! I put it on the air, using my FT-2000 to receive and my CQ was answered by Wolf, IN3TWX. I knew him from my 2 meter EME days so that was fun! Wolf gave me a 578 report with this little rig. It does have a bit of chirp.

The 8W20W transmitter as set up for the IN3TWX QSO

I was still looking for a QRP transceiver with frequency agility. I was able to save a few pennies and buy a DC40B transceiver kit and audio filter kit from Pacific Antenna. It put out a little over two watts when operated at 12.5 volts on a battery. I built a synthesized VFO using an Arduino nano and Si5351A synthesizer module. I started with a design by W3PM from QEX July/August 2015 and made a few minor changes. I added a low pass filter and post-amp to bring the level up enough to drive the DC40B. I had a few trials getting the VFO to properly drive the rig but eventually got it working well. Describing the rig in detail is not the focus of this post.

On the morning of July 29 I was calling CQ with the DC40BV (my nomenclature for the DC40B with VFO) on 7021.5 kHz when I heard the CY0S Sable Island DXpedition begin calling CQ on 7023 listening up (working split frequency). I was able to hear them because the filtering on this little rig is OK but surly not great. I wondered if I could manage to work them with this little rig? The receiver is direct conversion. I have it configured for a positive transmit offset since I normally prefer to listen on the low side of zero beat but of course being a D-C receiver it works equally well on the upper side of zero beat. If I tuned up to 7024 or so I would be able to hear CY0S and be transmitting one kilohertz or so above them, in about the right place for their split since they had just started up and there was no pileup. I hastily dialed up, actually going a bit higher than that but I could still hear them due to the somewhat wide audio filter. My first call went unanswered and they called CQ again, but my second call got them! I worked CY0S with a 2 watt kit/homebrew transceiver with direct conversion receiver! If there had been a pileup the QRM would have killed my ability to hear CY0S since I would be listening right in the middle of it, but I was in the right place at the right time when they had just started up and had very few callers. A 400 mile QSO is no great feat but it was fun working a semi-rare DXCC entity with such a simple and unlikely setup!

The DC40BV immediately after my CY0S QSO
Inside view of the DC40BV rig. I am still making a few minor changes and have some more tweaking of the VFO code and capabilities to do.

I was already thinking about having the ability to toggle the receive-transmit offset to plus or minus 500 Hz (my preferred pitch for listening to CW) because with a direct conversion receiver listening on the opposite side of zero beat can be helpful in avoiding QRM. But this QSO got me thinking. Perhaps I will try to make the offset completely variable, so I can go either side of zero beat and even have fully configurable split frequency capability. That will test my limited Arduino coding capabilities, but I think I can get it done with some trial and error.

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