In years past, you could always count on a QRP-ARCI Four Days in May (FDIM) talk by Rev. George Dobbs, G3RJV. You might never know what the subject would be, however. As Bill, N8ET, put it, “He always submitted a paper for the FDIM Proceedings, but would often talk about something completely different.”

G3RJV became a Silent Key in 2019, but Jack Purdum W8TEE, Ashhar Farhan VU2ESE, and Hans Summers G0UPL have all stepped in and picked up the slack. All three have become FDIM regulars, and each presented interesting talks at this year’s FDIM. And, to round out the lineup, we were treated to talks by N8ET, WA5BDU, AI6XG, NT7S, and VK1UN as well.

Click on the video link next to each title to watch the recorded video.

The T41EP: Seven Years Later (video)

Jack W8TEE and Al AC8GY presented a short history of the T41, their software-defined transceiver (SDT) that they’ve been working on for the past seven years. Jack recalled that the T41 had its birth on the drive home from a Field Day trip in 2018. During that trip, Jack and Al hashed out some of the basic requirements, including:

  • It would be a software-defined transceiver (SDT).
  • It would have a spectrum/waterfall display capable of displaying up to 192 kHz of bandwidth.
  • It would be self-contained and not need a PC or laptop.
  • It would support CW and SSB with up to 20 W output power.
  • It would operate on the 80-meter, 40-meter, 20-meter, 15-meter, and 10-meter bands.
  • Both the hardware and software would be open-source.
  • It would be affordable for someone whose income was mowing lawns over the summer.

They explained how they made their design decisions, including how the hardware and software has evolved over the years. They also discussed some of the benefits of sticking with a project over several years. I think that this approach has certainly paid off. There are more than 1,000 T41 units currently in the field, and if this talk is any indication, there will be more in the future.

A QRP Messaging System Using FT8 Technology (video)

In this talk, Farhan, VU2ESE, described his software for passing short messages using FT8 technology. He has currently implemented this software on the zBitx transceiver, but it is still in beta mode.

VU2ESE’s paper is not, unfortunately in the proceedings, but if you’re interested in obtaining a copy, you can contact Farhan directly. Look for Farhan’s specification for his messaging software in the next issue of QRP Quarterly.

An Introduction to LTspice (video)

Nick, WA5BDU, then introduced the crowd to the wonders of LTspice, a free software package that allows you to simulate analog circuits. In addition to the analog simulator, the package includes a graphical interface that allows you to input circuit schematics and view waveforms. This software is being developed and maintained by Analog Devices and is available for Windows and Mac OS.

In his talk, Nick quickly ran through the features of LTspice using a simple transistor amplifier as an example. He also showed how to analyze crystal filters using the software.

In his paper, the author asks the question, “Is it (LTspice) hard to learn?” His answer is, “You can learn to draw a schematic and run a simulation in 30 minutes or less. You can also use it off and on for several decades and still have a lot more to learn. Sort of like electronics itself.” As this talk demonstrated, this is certainly the case.

A Project for the Decades: A SOTA CW Tube Transmitter (video)

This talk was a lot of fun. This project had its genesis in an article found in the March 1967 issue of Electronics Illustrated titled, “A 1 Bottle Xmitter for 40.” Dan Koellen, AI6XG, took that circuit and adapted it for portable operations, namely Summits on the Air (SOTA).

One of the challenges Dan faced was powering the transmitter with a 12 V battery so that it could be operated portable. The original project plugged into an AC outlet and included a power transformer to supply the 6.3 V filament voltage and the high voltage on the plate. Providing the correct filament voltage was easy—Dan used a 12AQ5 instead of a 6AQ5.

Supplying the high voltage was a bit trickier. Fortunately, he found a DC-DC converter designed to power Nixie tubes, which require a voltage of approximately 180 V. Not only did he find one—the Omnixie NCH6300HV—that could output up to 230 V, they were electrically quiet as well.

To implement the transceiver functionality, Dan used a QRP Labs QCX as the receiver and keyer, along with a T/R switch and bandpass filters that he designed for this project, Dan successfully activated W6-NC-402. He made seven 40-meter CW contacts, including two summit-to-summit QSOs. According to Dan, “The fun factor was a Spinal Tap 11 out of 10.”

A History of FDIM (video)

Right after lunch, Bill, N8ET, ran through some of the history of Four Days in May, dating back to when it started in 1996 at the Days Inn Dayton South. The organizers planned for 50 attendees, and were quite surprised when more than 100 registered! Bill attributes this to the great slate of presenters, including L.B. Cebik W4RNL, Dick Pascoe G0BPS, George Dobbs G3RJV, Gary Breed K9AY, George Heron N2APB and Joe Everhart N2CX, and Rex Harper W1REX.

The proceedings has the tables of contents for most of the symposium proceedings, and I’d encourage you to get a copy if you weren’t fortunate enough to attend the event. It’s really quite impressive the number of quality talks and projects that FDIM has supported over the years.

Beyond the ‘612 (video)

Since its release in 1987, the SA612 Gilbert Cell mixer, and all its variants, have been used in countless QRP projects. This era is, however, coming to a close. No new parts have been manufactured since 2022, and soon, new parts will be unobtainium.

This situation prompted Jason Mildrum, NT7S, to search for an alternative. His search was made more difficult because so many radios now are software-defined radios (SDRs) and don’t require mixers the way that superheterodyne radios do. Fortunately, Jason discovered the Maxim MAX268x series. Although designed for microwave applications, Jason concluded that the parts could be used for HF QRP radio projects after a fairly comprehensive series of tests. Using these parts, he built a QRP transceiver, with which he has completed several successful POTA activations.

At the end of his talk, Jason hinted that a return of Etherkit, his kit company, might be in the cards. Stay tuned for that.

Bringing SSB to the QMX (video)

Hans, G0UPL, opened his talk by apologizing for taking nine whole months to implement SSB on his QMX transceivers. I don’t know about you, but I think it’s rather remarkable that he was able to do it in such a short time! It would have taken anyone else much longer to do it.

Hans then launched into an explanation of how he was able to achieve this feat. After a review of digital signal processing (DSP) basics, he jumped into an explanation of polar modulation, the technique used in the QMX to generate single sideband. Hans’ paper in the proceedings includes references to papers which really get into the details of how polar modulation works, if you’re so inclined to read them.

Hans also got into some detail on his development process. After some initial failures, he decided to simply try the code that runs then uSDX and was quite surprised that it actually worked on the QMX. That began a series of improvements that resulted in the software that QMX users can now download and run on their radios.

I can’t really say that I understand everything that’s going on, but I think that with some review of Hans’ paper and some of the others that he references, I could get a better handle on it. I do know that his talk has inspired to complete the QMX kit build that I started months ago.

Stealth Operations from Hotel Rooms and Other Unlikely QTHs (video)

Ross Valentine, VK1UN presented the last talk of the day. It described his operations using QRP radios and short wire antennas from a number of “unlikely” locations that he’s been in around the world.

As far as radios go, small and light are the ticket. He has a selection of QRP Labs’ QCX and QDX radios, and uses small lithium batteries to power them. To run digital modes, he uses a Raspberry Pi 400 with a small LED/LCD display.

For antennas, he favors wire-wrap wire, often wound on a fishing reel. He notes that the wire is light and very difficult to see, even in bright sunlight. He tunes the antennas with a variety of homebrew tuners, including simple LC tuners, Z-match tuners, and FRI-match tuners, some using homebrew variometers for the inductors.

With this simple gear, Ross’s results have been impressive. He showed plots of his WSPR signal being spotted all around the world.

If these talks don’t inspire, I don’t know what will. I mentioned earlier that Hans’ talk has inspired me to finally get my QMX working, but I’m now also motivated to play around with LTspice, and I have some ideas for tube circuits that I want to build as well. The trick is going to be completing these projects before next year’s FDIM, when I’m sure another all-star lineup of presenters will inspire me to tackle yet more projects.

To get a copy of this year’s proceedings, contact Bill, N8ET at toystore@qrparci.org  or   n8et@woh.rr.com.