Friday, March 22, 2024

A few thoughts on the QMX transceiver


A recent get-together of Central Florida POTA ops, led, of course, by Bill N4NYM, allowed me the opportunity of meeting a dozen or so kindred spirits at Lake Louisa State Park where I very much enjoyed seeing their set-ups and eating their food.

One of those in attendance was a newbie CW op, Glenn KO4NTA, and somehow Glenn and I got to talking about QRP. He had several rigs with him but the radio he most wanted to show me was his new Army-green (tr)uSDX.

"It costs about $100, covers 5 bands and is super tiny!"

That very statement - since it applies equally - got me to thinking about my inbound QMX, and I asked Glenn if he'd heard of it.

To my surprise, he had not heard of QRP Labs at all.

At that point, I think I may have unwittingly become a bit of a salesman. Glenn had not yet been on the air with his (tr)uSDX and I had not yet been on the air with a QMX. I do have a (tr)uSDX though, and have made a number of hard-won contacts with it on both CW and SSB.

I know the shortcomings of the (tr)uSDX and, now that I've made almost 100 contacts (and two POTA activations) with my new QMX, I have a pretty good idea of where it stands in the $100-Rig Department.

Most $100 radios are "novelties". Nothing wrong with that, and I knew it as I placed my order for the (tr)uSDX. The novelty descriptor also applies to the many eBay radios promising communicative abilities on 5 (or even 8) bands for the same amount of money. Sometimes it's fun to see what you can do with minimalism.

So comparisons will be made between the (tr)uSDX and the QMX, based on their similar size, price, output power and bands covered. You and I know that these comparisons are not valid, but for those who don't - These radios aren't even in the same league with each other:

  • The QMX receiver handles multiple signals well. The (tr)uSDX falls apart in that scenario.
  • Menu accessibility of the features I need during a POTA activation (RIT, keyer speed, CW message memories) are easily accessible on the QMX. Not so with the (tr)uSDX.
  • Audio fidelity of received signals is great with the QMX at any level of AF gain. The AF section of the  (tr)uSDX goes into self-oscillation unless kept to a very low level.
  • I can read the display of the QMX outdoors on a Florida afternoon. With the (tr)uSDX I have to create a "hand shadow", lean forward and squint.
  • The QMX does not (yet) have SSB capability; the (tr)uSDX does, sort of.

A very popular all-band transceiver is advertised as being "Optimized for digital" (it is not). The QMX though really is optimized for digital - it was dead-easy to configure with WSJT-X, taking all of about 90 seconds, even for me, and man, that's saying something. Signals filled the screen and I was off and running.  

A fix is currently being worked on by Hans G0UPL to correct the CW thumping issue, primarily on the lower bands. I am lucky in that only 80m seems seriously affected on my QMX - a band I never use anymore (sigh).

The QMX can be built (or ordered pre-built) to accept a DC input of either 9 or 12 VDC - and 12V does not mean 13.8V. There is a very narrow range of acceptable input voltages for the QMX and it is best measured in fractions of a volt, not volts. This makes portable (battery) operation a bit more difficult since so many of us have migrated to LiFePO4 batteries that have an unloaded voltage close to 14V.

The ideal battery for a 12V QMX is a fused 3S (11.1V) LiPo. They are cheap but will not have any ham radio type of connector on them. I have a few of these from my R/C plane

hobby that work perfectly, now that I've put Anderson power pole connectors on them.

If you want to use a LiFePO4, it might be recommended that you put a few high-current diodes in series with the supply to drop the voltage. Here's a similar implementation of that idea. I personally wouldn't do this though because the voltage drop provided by the diodes will change as current through them changes. So, for our application, the voltage drop during receive will be different than the voltage drop during transmit.

The QMX has a configurable SWR protection circuit built in. I've tested this circuit intentionally and accidentally! It works. It exists because accidental transmission into a high SWR load can, and often will, destroy the finals.

The three-part combination of SWR protection, SWR sensitivity and the inability to easily lower RF output power means that automatic antenna tuners are probably best avoided when using the QMX since they intermittently present a high SWR to the rig while looking for a good match. The SWR protection circuit will (and did in my case) shut the transmitter down before a match was found. If you disable the protection circuit, you risk the finals.

A work-around would be to power the radio with a 2S LiPo (7.4V) while the ATU is doing its thing, then switch to the 3S LiPo for operation. Or use a manual tuner, tuning first for maximum noise during receive before doing final touch-ups while transmitting.

Or use a manual tuner with an antenna analyzer - then substitute the radio for the analyzer after getting a good match.

The QMX has an SWR sweep function and allows supply voltage to be decreased internally while this function is in operation. It would be great if a future fw revision allowed this ability with a Tune Mode selection.

I like Split implementation on the QMX. I chased, but failed to work, 7P8EI a couple nights ago. They were on 20m CW, listening 2 to 4 kHz up. It was easy and intuitive to switch back and forth with the two VFO's to find out where he was listening at any given moment. Maybe I'll get them yet...

I'm looking forward to seeing what develops with this radio in regard to firmware updates, especially if SSB becomes a reality. Incremental updates are, for me, one of the radio's main attractions.




  1. John I have had a lo-band QMX since about March 5 and have had an opportunity to make many contacts, including many POTA. I had to install FW 017 to get latest fixes. Major remaining problem, for me, is severe audible key click effects. Some reduction has been accomplished with AGC parameter changes, but clicks persist. Today, I experimented with "semi QSK" setting. Or lack thereof.
    QSK as described in the Operating Manual does not exist. In fact, something is piped through the audio during key press. A search of found an entry by Hans stating that QSK didn't work and th as t it was high on his list to fix. I believe that any QSK fix will also solve the key click problem.
    In any event, I'm having a ball hunting POTA activations. I'm amazed at how this QRP rig get "through".
    You won't be disappointed,
    73, Rick K8BMA

    1. Hi Rick,

      I've tried various AGC setting recommended by others but nothing seems to help on 80m. The AGC settings seem nearly infinite and I've returned them to their default values. I haven't experimented with the QSK settings yet - I'll do that tomorrow just to see what the effect is. Perfectly adequate for POTA'ing and I'll do me next one with this rig Sunday.


  2. Good Afternoon! The QMX is a powerful little radio. Under the hardware tests menu there is a configurable tune function. It also has R.I.T. for split operation. Hope this makes your operation more enjoyable!

    1. Hello Pete - there is an SWR measurement function but this only measures the SWR at the center of each band, not the selected frequency of operation. I am enjoying the radio quite a bit and did another POTA op with it this morning.


  3. Hey John,
    I just finished my QMX kit build - boy that was a toughie! Anyhow, it's done and I'm looking forward to taking it outdoors to play. Can't wait for the SSB firmware update!

    Take care,
    Tom, M7MCQ.

    1. Well done on another successful build - you don't waste any time!