by Matt (W6CSN)

The Radio

The QMX by QRP Labs is a five band, multi-mode radio introduced by Hans Summers G0UPL at FDIM in 2023. The QMX is the next logical step in the radio development journey at QRP Labs, bringing together the innovative approach to FSK modes like FT8 from the QDX and the CW performance of the QCX series. This is all done in same enclosure as the QCX-Mini which is not much larger than a deck of playing cards.

The keen observer will notice that the QMX also sports a built-in microphone which, along with associated circuitry, supports future introduction of voice modes by way of firmware update. Other notable features include SWR metering with protection as well as solid-state “PIN” diode T/R switching, plus the option for “high band” coverage of 20 through 10 meters in addition to the original 80m-20m version.

Most hams have, at one point or other, forgotten to throw the antenna switch, adjust the tuner, or even connect an antenna before transmitting. The SWR meter and protection really sets the QMX apart from earlier QRP Labs radios. There are plenty of sad tales on the QRP-Labs forums from QCX or QDX users that “smoked” the BS170 mosfet finals in a moment of operating into a badly mismatched load. It’s remarkably easy to do, ask me how I know!

The Build

Based on experience with the QCX-Mini and having put together several QDX’s, I admit to being hesitant to starting assembly of this radio. The level of integration in the QMX as a multi-band, multi-mode unit is probably the highest yet to come out of QRP Labs. The components that were merely small in earlier radios are absolutely tiny in the QMX, specifically the LPF toroidal cores. The main board, internal switching power supplies, display, and controls board are all sandwiched together in a tight fit for the custom enclosure.

The build started with a slow and methodical approach of doing a little bit each day and working in the morning when my mind was fresh and there was good light on my workbench, a.k.a. the dining room table.

The first “disaster” happened when performing the factory recommended modification to Revision 2 boards shipped in 2023. The mod calls for a protection diode to be installed across a SMD mosfet. Several leaded 1N4148 diodes were available in my “junque” box so I attempted to carefully fit one of these in the right position on the board. While soldering the diode in place I managed to lift C508, a microscopic .1uF SMD capacitor, clean off the board!

Given the tools at my disposal, there was no way I was going to be able to get that capacitor back into place. Therefore, I grabbed a standard through hole .1uF cap and painstakingly got it connected in the right place, verified by lots of continuity testing.

Once finished with all the electronic components on the main board, I was feeling pretty cocky and also the completion of the project was more clearly in view. The cautious and methodical approach gave way to a faster build pace, which directly lead to the second “disaster.” While installing the headers that connect the main board to the display board, I failed to CAREFULLY READ THE ASSEMBLY MANUAL and soldered the male pin headers where I should have installed the female sockets!

This misstep might have ended the project right then and there if I had not at some point in the last year bought a proper desoldering tool. This is not an expensive automatic vacuum pump powered solder re-work stations, but rather a heating tip and manually actuated solder sucker built into one tool. Twenty-two unsoldered connections later, we were back on track.

The remainder of the assembly was pretty much drama-free. I did manage to break a couple of the leads on the rotary encoders (easily fixed by replacing the lead with a component cutoff) and it is kind of tricky to get the controls board properly through the hole in the display board and get the whole thing buttoned up in the enclosure. I’m still not convinced I got the fit 100% right.

With the build complete, and the radio connected to a dummy load, I applied a reduced voltage DC supply of 9v, and good news! No smoke!

I pressed the power switch and the QMX showed up on my computers as a USB flash drive. The firmware installs easily per the instructions in the manual after which the radio power cycles and the display comes to life.

First off, I performed audio and rf filter sweeps on all bands using the in-built terminal application and recorded screenshots to use as a baseline if I ever decide to try to tweak any of those response curves. In all honesty there is not a lot of physical adjustment to make other than compressing or spreading the turns on the toroids. And given they are pretty hard to reach, I likely will not make adjustments unless field testing proves that something is seriously misaligned.

Next, I tested the radio at the full 12v (still on the dummy load) and measured the following CW transmit power results:

20M – 3.6W
17M – 4.4W
15M – 5.9W
12M – 2.9W
10M – 3.6W

All are very acceptable power levels for CW and FT8. For some reason this particular QMX is an absolute blowtorch on 15 meters!

Field Testing

As of this writing, the QMX has been on two field test outings at my local POTA spot in the Presidio of San Francisco (K-7889).

The first on-air test of this “high band” QMX was with an MFJ-1979 17 foot telescoping whip antenna clamped to a short steel fence pipe next to San Francisco bay. This installation gives pretty close to a 1:1 SWR on 20 meters with the whip fully extended. By collapsing sections of the whip, a good match can found for the higher bands.

The test/activation started on 15 meters and the first two-way contact on the QMX was with W8NGA from Ohio on CW. Eleven more QSOs on 21 Mhz, including DX from Japan, proved that the radio worked well. Up to 12m for a several QSOs and then 28Mhz for a handful more. Working DX to Chile with CE3FZL showed that the QMX as built was sufficiently sensitive to be taken seriously as a QRP rig.

Next, I tuned the antenna for 17 meters but there seems to be some sidetone induced audio clicks that are so strong when using 18 Mhz that it was actually hard to send accurately. I gave up on 17 meters without a single contact before wrapping up the test on 20 meters.

At home, I next consulted the QRP-Labs forum on groups.io for the audio clicking issue and made some adjustments to the AGC and sidetone settings. Others have reported a similar problem and it only seems to effect the audio channel on the QMX, the transmitted CW signal is clean.

The next testing session came a few days later, but conditions were substantially different. Locally, the wind coming through the Golden Gate was strong enough to preclude the use of the MFJ 17 foot whip, the antenna is just too bendy in high winds for my comfort. In addition, solar activity made propagation a bit of a crap shoot. I operated from my vehicle with the Gabil GRA-7350T antenna on a rooftop mag-mount. The good news is that the AGC settings changes substantially improved the audio clicking issue, the bad news was that I was only able to muster a single QSO on 17 meters.

As yet, I have not tested the Digital mode features of the QMX.

Final Thoughts

With its quadrature sampling front end, custom SDR based receiver architecture, plus multi-band and multi-mode capabilities, the QMX is an extremely good value in my opinion. At less than US$200 for a factory assembled unit, this is price competitive with other field radios with less capability that use 20 year old transceiver architectures.

The other side of the coin is that currently you’ll need to either assemble the QMX yourself or accept a lengthy lead time for a factory assembled unit. In fact, the very reason I built the QMX kit was that I wanted to possibly have the radio available for an overseas trip before the QMX on order from last fall arrives. It’s a good performing multi-band radio that if it somehow got damaged, lost, or stolen while traveling then it would not be a huge financial loss. I’d much rather sacrifice a $200 QMX to the perils of world travel than a $1600 KX2.

Another reality is that the QMX is a bit bleeding edge. The firmware, which is a substantial portion of the radio, is under constant development and building and operating the transceiver to its full potential requires some effort. It’s not an “appliance operators” radio and you definitely need to read the manual and be willing to consult the forums when you need support.

Materials

With some affiliate links to support QRPer.com

• QMX kit from QRP Labs
• Pinecil smart mini portable soldering iron
• Premium soldering iron holder with brass mesh tip cleaner
• Jameco BenchPro Desoldering Pump
• Work mat
• Various Xcelite hand tools
• Talentcell 12v Li-Ion battery
• NM0S Power/SWR meter kit
• ABR RG-316 BNC cable with ferrite bead current choke (use coupon code ARB10QRPER to save 10% on your order)
• MFJ-1979 Whip antenna
• 3/8×24 “mirror” mount antenna clamp
• Gabil GRA-7350T Antenna
• Triple magnet roof mount