Construction report QRP-Labs.com QMX

In 2019, I decided to build the QCX monoband-TRX to have a lightweight TRX that serves a radio band that should be okay to use throughout the year, 40 m. I was still building up a basic ham radio gear “portfolio” and didn’t own HF antennas that could specifically be used for SOTA. I could be using an end-fed for SOTA, I thought, but didn’t own a compact telescopic mast to be able to lift the wire up on a summit. I do own the MFJ-1979 telescopic vertical, though, that I sometimes use when operating HF from the car. But, it is only resonant on 20 m and up. For 40 m, I’d have to add the extra coil. The MFJ-1979 alone is already quite heavy, though, even more together with the coil. So, I got the idea of buying the QCX-mini for 20 m, which would reduce some weight for both TRX and ANT, and 20 m would raise chances for DX during the summer season when I intended to roam summits. I’d still need to carry a small tripod, though, for the vertical.

Then came the QMX, the same size as the QCX-mini, but a multi-bander for 80/60/40/30/20 m! This would be the solution, and I purchased one kit at the QRP-Labs booth at the HAM RADIO fair in Friedrichshafen last month, right after walking through the doors.

Assembly promised to be not so adventurous as that of the QCX anymore, as all resistors, several capacitors and all ICs were already SMD-soldered to the PCB. I still had to wind the filters and transformers, though, and add the other components. A funny thing to read in the assembly manual was, that Hans somehow got aware that left-handers wind mirrored toroids. I mentioned this in my QCX construction report, the toroids end up a bit skewed on the PCB. Since the QMX PCB is much more compact, this would be an important thing to keep in mind. To achieve the same result, I also wound the toroids counter-clockwise, left-handed, but starting at the 12 o’clock position, not at 6 o’clock, like the right-handers do.

This was already my assembly report! I’ll only mention some stumbling blocks I experienced.

First, it is obvious that the assembly manual is in an early stage. While there aren’t any real errors in it, it can be a bit confusing due to sections of the QDX assembly manual having been pasted in, obviously. So, I got the first shock when turning on the TRX for the first time, it staid dully dark. But, reading a bit closer into different sections, I read that the QMX initially starts in firmware update mode, that is, it was simply just a USB thumb drive at that point, nothing more! The picture in the manual showed a happily glowing display with a frequency reading, though. I connected the QMX to the notebook, pushed the power button once more, and voilà, a new drive appeared! I could successfully copy the firmware file to it.

When I measured power output into a dummy load, I could read about 4 watts when feeding the QMX about 13 volts from a LiFePO₄ battery. I didn’t do any alignment, because there isn’t a description of this procedure yet. A thing that disturbed me was that extremely loud noise in the headphones on key release. I inspected the PCB closely, but didn’t find anything suspicious. So, I went to the QRP-Labs group. There, I learnt that the QMX offers a USB terminal console with a configuration menu! That’s not too surprising, after all, as the QMX is an SDR-based TRX. In the config menu, I went to Band Configuration and set the “TX/RX param 2” value from “0” to “1”, and a reboot had this issue resolved!

Later, there was a topic discussed in the group about the QMX being quite deaf at 20 m. Hans proposed some solutions, and I went with his recommended solution #2. That means: It is enough to wind the 20 m (19 windings) and 30 m (11 windings) sections onto L401. Have the windings spread out such that the 20 m tap goes one place further into the 30 m hole, and the 30 m tap to the 40 m hole. Then patch the 40 m to the 80 m hole, and do the configuration changes for the BPF. (This is already adapted in version 1.00f of the manual.) The BPF frequency sweep diagram looks promising; this is another awesome feature of the terminal console.

When using the optional enclosure, it is important that the control PCB can pass through the outlet in the display PCB easily. Therefore, you should take care that all protruding elements are honed away, just as mentioned in the manual. You can carefully wiggle on the controls to help it pass through. And that reminds me of another stumbling block: One following day, the TRX suddenly didn’t turn on anymore. I removed the enclosure, put apart the PCBs and connected them again, then the TRX turned on again. I suspect that the left control’s outer hull was touching the DC socket’s back. Therefore, be sure to put some insulating tape over the control’s left side and/or the DC socket’s back. Also, when soldering the controls themselves, don’t use too much solder. Initially, I made another short-circuit that disabled volume control.

Apropos volume control, that one was difficult to tame. Sidetone volume always was too loud, I had to turn the volume down before keying and up afterwards. Also, turning the left control didn’t really turn the RX volume down counter-clockwise or up clockwise. Rather, the volume was cycling between a few different levels when the knob was turned into one direction. I seemed to have tamed this behavior by either insulating the control from the DC socket (as described above) or by setting sidetone volume to 005 in the menu.

An interesting feature of the QMX is the possibility of new functions coming with later firmware updates. For now, the decoder doesn’t seem to be working (which I see as feature solely for non-telegraphist/non-ham spectators), and the built-in mic has no function yet (and I’ll never use it anyway).

I want to use the QMX to finally do my first SOTA activations later this month. At the HAM RADIO fair, I also stumbled upon another solution to save weight for ANT and tripod: The CleverSticks, which are hiking sticks that can be transformed into a tripod, for use with a camera or an ANT, namely the Elecraft AX-2 telescopic vertical. It is shorter and smaller than the MFJ-1979, but the raised location of a summit (plus doing CW) should be able to compensate for that. Since one would carry hiking sticks anyway, there is no extra weight of a tripod at all. The last piece of the puzzle finally was the key: I purchased the Palm Radio Single paddle at the fair. This is the gear I’ll be using, and I will find out if it serves me well.