Two radios.
One firmware.

Simulators

Test the interfaces

Both of the QMX's user interfaces, running in your browser on one shared radio state. Change a setting in the terminal and the LCD menu has already changed; send a CAT command and the front panel jumps.

Every control is timed the way the real firmware times it. A press held past the double-click window (500 ms by default, and you can change it in Display/controls → Dbl. click) is a long press; a second press inside that window is a double-click. So yes — you really do have to hold the VOL knob to switch it on.

Every control is timed the way the real firmware times it. A press held past the double-click window (500 ms by default, adjustable in Display/controls → Dbl. click) is a long press; a second press inside that window is a double-click. So yes — you really do have to hold the VOL knob to switch it on.

Front panel

Four controls, twelve gestures

The laser etching on the case is the whole manual, if you can read the notation. A leading . means one click. .. means a double-click. A leading – means hold it down.

VOL knob (left encoder)

TUNE knob (right encoder)

Rotating a knob while its button is down commits you to the turn: the hold action is abandoned, and so is the click that would otherwise land when you let go. Otherwise reaching for a 1 MHz tuning step would dump you into the message list every time.

Select button — . Keyer   .. RIT   – Menu

Exit button — . VFO A/B   .. Preset   – A<->B

Inside the menu system the rules collapse to something simpler: Select goes in, Exit backs out. Select enters a submenu or starts editing a value; Exit saves and steps back up. If you are ever lost, Exit repeatedly returns you to the main operating screen.

The same gestures on a QMX+

Every gesture above is identical on the QMX+ — it runs the same firmware, so the firmware cannot tell the difference. Only the arrangement changes, because the controls moved from the lid to a desktop front panel:

The simulator above models the QMX's top-panel layout. Rotate the 3D model on the overview page to see how the QMX+ front panel differs.

The fastest way to type

While editing a number or a text parameter, the CW decoder is live. Key the digits or the characters on your paddle at roughly your configured keyer speed and they go straight into the field. For entering a callsign into a stored message, or a frequency into a preset, this is far quicker than winding a knob through the alphabet. It is controlled by CW → Decoder → Enable edit.

Front panel

Reading the display

Thirty-two characters have to carry everything. Here is what each region is doing.

Fifteen pixel columns, three jobs

Three characters at five pixels each gives fifteen columns, and every meter in the radio is scaled to land exactly on that grid: signal strength while listening, power and SWR while transmitting.

Configuration

The menu, mapped

Fifteen top-level groups, all of them reachable from either interface, all stored in EEPROM. Changes generally take effect when you leave the menu, not as you make them — the exceptions are the CW filter settings, which apply immediately so you can hear what you are choosing.

Both simulators below walk the same menu tree, with the real parameter names, types, defaults and value lists from the 1_04_004 manual. Editing a value in one shows up immediately in the other.

Over the wire

The terminal applications

Connect a terminal emulator to the QMX virtual COM port at 80×24 and press Enter. The radio stops interpreting CAT commands and hands you a menu. Arrow keys move, Enter selects, Ctrl-Q backs out one level.

The simulator below is faithful to the real screens: the cascading box windows, the green labels, the yellow status line at the bottom, the ASCII sweep plots with blue gridlines. Click it, then use your keyboard — or the buttons underneath if you are on a phone.

What is in there

Configuration, a band table, five kinds of filter sweep, a live hardware diagnostics page, a CAT tester, a log and a GPS sky map.

The Linux gotcha

When a virtual COM port appears, ModemManager assumes it is a modem and starts sending it Hayes AT commands. The carriage return in those commands puts the QMX into terminal mode, which disables CAT — so WSJT-X silently fails.

sudo systemctl stop ModemManager
sudo systemctl disable ModemManager
sudo systemctl mask ModemManager

The brltty braille service does the same thing on some distributions and needs the same treatment.

Getting started

Everything else you need

The radio is the easy part. What actually decides whether you enjoy this is the antenna, a power source that will not cook the transmitter, and something to key or talk into. None of it has to be expensive.

Power

Feeding it without cooking it

The QMX takes a centre-positive 2.1 mm barrel at 6.0 to 12.0 V — and, since firmware 1_03, up to 14 V provided you cap the transmitter first.

The setting that does it is Protection → Max. PA voltage. It limits what the amplitude modulator will ask for regardless of what arrives at the barrel, so the output transistors never see more than you allow. It defaults to 11.5 V, which means a 13.8 V shack supply is fine out of the box. QRP Labs swept the supply from 6.5 V to 14.5 V with the cap at 10.5 and found output flat above 10.5 V, with a full minute of continuous key-down at 14 V leaving the modulator transistor barely warm on 80 m.

What still bites is running above 12 V with the cap left wide open. That stresses the BS170 output transistors, worst of all on digital modes where the duty cycle is high, or into an antenna whose SWR you are not sure about. Check the cap before you change supply.

Two ways to make a high supply safe

The first is the one to reach for; the second is belt-and-braces.

While you are in that menu, set Supply voltage to Prevent TX and give it a sensible min and max. The radio will then refuse to transmit out of range and flash the battery icon at you, which is a much nicer failure than a dead PA.

Antennas

This is where your results come from

A hundred-dollar radio on a good antenna beats a two-thousand-dollar radio on a bad one, and at 5 W the difference is starker still. Spend your effort here.

One constraint shapes every choice: the QMX has no antenna tuner. The built-in SWR protection will simply refuse to transmit above your configured threshold. So you want either a resonant antenna, or an external tuner between radio and feedline.

If you would rather buy something

Or build a dipole, which is better and nearly free

Two lengths of wire, a centre insulator, and coax to the middle. It is the reference antenna that everything else is compared against, it needs no transformer or tuner, and it costs the price of the wire. Start from a half-wavelength total, and cut each leg to:

Trim it with the radio itself. This is the neat part. Open the terminal's SWR sweep, or the SWR measurement tool on the LCD, and you get a plot of SWR across the whole band at reduced power. Cut long, sweep, and snip a few centimetres off each end until the dip sits where you want it. You do not need an antenna analyser — you already bought one, it just happens to also be a transceiver.

The things that actually matter once it is built

• Height beats everything. Getting a wire up 8 metres will do more for you than any amount of radio. A throw line and a small weight is the best five dollars in the hobby.
• Get the ends up too. An inverted-V with the apex high and the ends at head height is a fine compromise; ends touching wet grass is not.
• Verticals need radials. A whip without a counterpoise is half an antenna. More wires, laid on the ground, always help.
• Keep the feedline short and thin. RG-58 is fine for QRP at HF; RG-316 is lighter for carrying. Do not coil the excess tightly against the antenna.
• Insulation counts on portable wire. Silicone-jacketed 26 AWG stranded is light, tough and does not kink. Bare wire in a tree will eventually short on something.

Controls

Keys, paddles and microphones

For CW

The QMX will work with a traditional up-and-down straight key, or a modern two-lever paddle driven by the keyer in firmware, in Iambic A, Iambic B or Ultimatic. If you are learning, a paddle is the kinder choice: the keyer enforces correct dit and dah lengths and inter-symbol spacing for you, so your timing is right from the first evening. A straight key teaches you rhythm the hard way and is worth owning eventually.

Learn to send without transmitting. Turn on CW → Keyer → Practice mode and the radio runs the keyer, the sidetone and the decoder with the transmitter disabled. You send, and the decoder tells you on the bottom row whether a machine could read you. It is a ruthless and extremely effective practice partner, and nothing goes on the air.

For SSB

The microphone input is deliberately primitive, in a good way: an electret element and a push-button is the entire bill of materials. An internal 2.2 k resistor to 3.3 V powers the element, and no coupling capacitor is needed.

A gutted PC headset works. So does a hand microphone from a scrapped rig, or an element and a momentary switch in a 3D-printed shell. Once it is connected, open the Mic test screen and raise Gain until your speech peaks just tip into the red — then leave it alone, because compression, AGC and CESSB downstream will do the rest.

On the built-in element. Both boards carry a microphone element on the panel — the manual's control diagram labels it, and the QMX+ feature list calls it an internal microphone. But the documented SSB audio sources are Ext. mic, USB, Two-tone and Auto, all of which take the external jack or the sound card. Plan on an external microphone, and treat the internal element as a bonus if a future firmware exposes it.

Listening

The audio output drives 32 ohm earphones and is not a speaker output. Any small in-ear set works. For desk use, a cheap powered computer speaker on the 3.5 mm jack is fine. If you are chasing the last of the battery life, earphones draw less than an amplifier does.

First light

Your first hour, in order

This sequence is deliberate: each step confirms something before the next step can damage anything.

1. Dummy load first, antenna later. Screw a 50 Ω load onto the BNC before you ever key up. It protects the PA while you are still finding out whether you soldered everything correctly.
2. Run the diagnostics. Connect a terminal and open the diagnostics screen. Supply voltage sensible, all three regulators green, every button and both encoder directions registering, both paddle contacts responding.
3. Sweep everything. RF filter sweep on each band — the peak should sit near band centre. Audio sweep — the unwanted sideband around 60 dB down. LPF sweep. Then transmit into the load and confirm you get power on every band. A dead band almost always means enamel left on a toroid lead.
4. Configure the protections. Band version, Band limits for your region, Max. PA voltage for your supply, SWR protection enabled with a threshold of 3, and Supply voltage set to Prevent TX.
5. Now the antenna. Sweep its SWR from the radio and trim until the dip lands in the part of the band you want.
6. Listen for a while. Turn the AGC on, find the CW segment at the bottom of the band and the SSB activity higher up. Get a feel for what the bands do at different times of day before you transmit at all.
7. Prove the station with WSPR or FT8. One USB cable, DiGi mode, WSJT-X. Within an evening you will see your own callsign being received on other continents by people you never spoke to. It is the fastest possible confirmation that everything — radio, antenna, feedline, you — works.
8. Then make a contact. CW at whatever speed you can manage, or SSB if you have a microphone. Answer someone calling CQ rather than calling yourself; it is easier.

If you bought the QMX+ instead

Everything above applies unchanged — same supply range and same PA cap, same antenna advice, same keys and microphones, same first hour. Three small additions:

Transmitting requires a licence in essentially every country. Receiving generally does not, so there is nothing stopping you building the radio and listening while you study. The exam is not hard, and the QMX will still be the right radio when you pass.

Building one

A kit you can finish

The QMX is sold as a kit, but not the kind that defeats people. Every surface-mount component — which is to say every fine-pitch, static-sensitive, hard-to-place part — is already fitted at the factory on 6-layer through-hole-plated boards. What arrives for you to do is through-hole soldering and, unavoidably, winding toroids.

It teaches you while you debug it

This is what makes the QMX unusual as a first build. Most kits hand you a circuit and wish you luck. This one contains its own test equipment — a signal generator, an SWR bridge and a suite of measurement tools — and will show you its own insides on a computer screen. If something is wrong, the radio generally tells you where.

The hardware keeps gaining features

The strongest argument for buying one is what happened to the people who already had one. The QMX shipped in August 2023 as a CW and digital-modes transceiver. In May 2025, firmware 1_02_000 added SSB — transmit and receive, with compression, a parametric equaliser and CESSB — to hardware that was already sitting on desks, at no cost. Since then AM reception, an accessibility menu, a fourth and fifth CW keying envelope, and a full emulation of the Ultimate3S beacon transmitter have arrived the same way.

Updates are free, always, and the update procedure is dragging a file onto a virtual USB stick. There is no bootloader utility, no driver, no programmer, and it works identically on Windows, macOS and Linux.

Room to modify

It is not a modular radio in the plug-in-board sense — there are no expansion slots. What it does have is a lot of software-configurable hardware and a community that pushes on it.

Value

What you get for a hundred dollars

The kit is $102.47. It is worth listing what is inside that number, because several of these are things people normally buy as separate boxes:

Add the optional anodised aluminium enclosure and you have a finished instrument rather than a bare board. The QMX+ is $125 as a kit for 160 m through 6 m, a desktop case, a battery-backed clock and space for an internal GPS.

If you are new to the hobby

This is a reasonable first radio, and an unusually good first build. The case for it:

But if the appeal is a small, extremely well-engineered thing that you assembled yourself, that you can take up a hill, and that will keep getting better for free — the price-to-capability ratio here is genuinely difficult to beat.

Reference

Technical detail

The engineering, the specifications, every connector, the CAT command set and the firmware procedure. Use the Plain and Full detail switch in the header to set how deep the page goes.

Engineering

Where the cleverness is

Plenty of radios cost a hundred dollars. What makes this one worth understanding is that several of the things it does are not cheap versions of what expensive radios do — they are different approaches, and in a couple of cases better ones.

SSB without a linear amplifier

Transmitting speech normally needs a power amplifier that stays faithful right across its range, and faithful amplifiers are expensive. The QMX avoids needing one. It splits your voice into two separate questions — what pitch and how loud — and answers them by completely different means. Pitch is handled by retuning the radio's oscillator thousands of times a second. Loudness is handled by varying the voltage fed to the amplifier. Neither job needs an expensive linear amplifier, and the two recombine into clean SSB in the output stage. The technique is called polar modulation.

The radio calibrates itself against its own PA

Every amplifier distorts a little. This design's particular weakness is that its timing shifts depending on how loud the signal is, which smears the transmission. So the QMX measures its own distortion and corrects for it. Connect a dummy load, run the calibration tool, and it works out how far out it is at every power level on every band, then applies the opposite error from then on. You only ever do this once.

CESSB, which is close to free power

Speech is mostly quiet with occasional loud peaks, and a transmitter has to be set up for the peaks — so most of the time it is loafing. Controlled Envelope SSB reshapes the signal so the average sits much closer to the peak, without the peak getting any higher.

On speech that is worth about 4 to 5 dB: your 5 W arrives at the far end sounding like something over 12 W, for free, and it is switched on by default.

Digital modes with nothing to leak

When the QMX sends FT8 it does not mix an audio tone up to radio frequency the way a conventional radio would. It listens to the tone the computer is sending, works out its frequency, and simply tunes itself there. Nothing is mixed, so there is nothing to leak: no unwanted sideband, no leftover carrier, no distortion products.

Details that suggest someone cared

A handful of small decisions that a cheaper design would not have bothered with.

And it sips current

80 mA while listening is the number that turns this from a desk radio into a rucksack radio. A small USB power bank will run it for a full day. At 220 grams with its enclosure, the whole station minus the antenna fits in a jacket pocket — which is what hilltop and park activators are actually buying.

Numbers

Specifications

Two members of the family share a schematic and a firmware image. The QMX is the palm-sized one built for a rucksack; the QMX+ is the desktop one, with room inside for a GPS and a battery-backed clock.

Band options

A QMX is built for one of three band sets — the band-pass and low-pass filter toroids differ. You choose the set at first power-up, and it is shown read-only in System config → Band version. Getting it wrong means a factory reset and choosing again.

Protecting the PA

The plain supply range is 6.0 to 12.0 V, but that is no longer the whole story. The Max. PA voltage setting caps what the amplitude modulator will ask for, independently of what you feed the radio — so with it set below 12.0, supplies up to 14 V are usable. It defaults to 11.5 after a factory reset, and the protection menu's own upper range check defaults to 14.

QRP Labs tested this: with Max. PA voltage at 10.5 the supply was swept from 6.5 V to 14.5 V, output levelled off above 10.5 V and stayed flat all the way up, and a full minute of continuous key-down at 14 V left the amplitude modulator transistor barely warm on 80 m with no failures or degradation.

Three protections are worth turning on before your first transmission: SWR protection with a threshold (an S appears beside the frequency and transmit is inhibited), supply voltage range checking set to Prevent TX, and band limits for your ITU region — which puts a B on the display rather than letting you key up out of band.

Hardware

What plugs in where

Six connectors, and one of them does three completely different jobs.

On a QMX+, two more

The QMX+ puts the earphone and mic/paddle jacks on the front panel and everything else on the rear, in this order left to right: RF, GPS, AUX, USB, PTT, DC. Two of those have no QMX equivalent.

Power, RF, USB and PTT behave identically on both models.

Over the wire

CAT control

The QMX implements a subset of the Kenwood TS-480 command set, with a few QRP Labs additions. Commands are semicolon-terminated and never contain a carriage return — which is exactly why a stray Enter switches the radio into terminal mode instead.

Recent hamlib builds carry a QRP Labs entry. In WSJT-X, Kenwood TS-440 works well; TS-480 is the fallback if your software does not list it. For ordinary operating that is the entire story — pick the rig, pick the port, and your logging or digital-modes software does the rest.

Operating

Digital modes in five settings

One USB-C cable, and the radio must be in the right mode: DiGi for pure FSK (FT8, WSPR, JS8Call, RTTY), SSB in USB for everything else (PSK31 and other phase-shift modes, multi-tone modes like VARA). Transmissions sent while the radio is in the wrong mode are simply ignored.

Set every volume to exactly 100%

Three of them: the application's own Pwr slider, the QMX output device level, and the system master volume. Not more, not less. The QMX measures the period of the incoming audio waveform to derive the transmit frequency, and the Rise threshold — 80% of full scale by default — is what decides that a tone has started. Below that, key-down never happens.

The radio tells you which of the three failure states you are in, using the top-left character under the VFO letter:

You cannot overdrive a QMX the way you can overdrive an SSB transceiver, and it has no way to transmit at reduced power on command. Leave the Pwr slider at maximum; if you want less power, lower the supply voltage or use Max. PA voltage.

Maintenance

Firmware updates are a drag-and-drop

QFU — the QRP Labs Firmware Update procedure — makes the radio pretend to be a 4 MB USB flash drive with a FAT16 filesystem and exactly two files on it: the firmware image, and EEPROM. Copy the new .QMX file in and the bootloader erases and reprograms itself. No drivers, no tools, no bootloader utility, any operating system.

Three ways in: the Update firmware item at the end of the LCD menu, the same item in the terminal's System menu, or — if you are truly stuck — a jumper across two pads near the top left corner of the main PCB under the LCD, which forces bootloader mode at power-up.

In bootloader mode the backlight is off and the display is blank; that is normal. Some units appear to power down instead of entering update mode — press the VOL button to switch them on. Firmware images are always 529 K and AES-256 encrypted, so only genuine QRP Labs images will load, and only onto a genuine QMX.

Copy the EEPROM file off before you experiment. It is your entire configuration, and dragging it back restores everything.

Reference

Sources

Everything on this page — the specifications, the menu tree, the display behaviour, the terminal screens and the control gestures — is taken from QRP Labs' own documentation for firmware 1_04_004, dated 23 July 2026.

• qrp-labs.com/qmx.html — the QMX product page
• qrp-labs.com/qmxp.html — the QMX+ product page
• Operating manual, firmware 1_04_004 — 137 pages, by Hans Summers
• groups.io/g/qrplabs — the support group

Assembly is covered in a separate document, and the Virtual U3S beacon has its own manual. This is an unofficial reference and simulator; it is not affiliated with QRP Labs.