This page documents QRP and portable radio builds from the WZ4JM shack — kits, experiments, and field-ready setups.
QRP Labs QDX Digital Transceiver — High Band (20/17/15/12/10m)
The QDX is a 5W digital-only transceiver kit from QRP Labs, designed by Hans Summers G0UPL. I built the high band version covering 20, 17, 15, 12, and 10 meters — purpose built for FT8, WSPR, JS8Call, and other single-tone FSK digital modes.
This is not a general purpose radio. It does one thing and does it exceptionally well — clean, efficient digital operation at QRP power levels.
The Kit
The QDX arrives as a partially assembled kit. All surface mount components are factory installed on the PCB, leaving the builder to handle through-hole components, connectors, and the output transformer winding. The transformer is the most fiddly part of the build — winding and fitting the toroid correctly is critical to getting clean output power and good harmonic suppression. QRP Labs provides detailed documentation and the build is well within reach of anyone comfortable with basic soldering.
Build time was a few hours spread across an evening. The PCB quality is excellent — double sided, through-hole plated, with clear silkscreen markings throughout. The optional black anodized aluminum enclosure fits perfectly and gives the finished radio a very professional look for its size. The enclosure measures just 89 x 63 x 25mm, small enough to fit in a shirt pocket.
What’s Inside
The QDX is built around an Si5351A synthesized VFO with a 25MHz TCXO reference for accurate and stable frequency output. The receiver uses an embedded SDR architecture with 60-70dB of unwanted sideband cancellation. On transmit, the QDX outputs a pure single signal — not SSB with a suppressed carrier, but a true single tone FSK signal with zero intermodulation distortion. At 5W output on the high bands the signal is exceptionally clean.
Everything connects to the PC via a single USB cable. The QDX presents itself to the operating system as three USB devices simultaneously — a 24-bit 48ksps USB sound card for audio, a virtual COM port for CAT control, and a USB mass storage device for firmware updates. No special drivers are needed on Windows 10/11 or Linux.
Connectors are minimal and sensible — a 2.1mm power barrel for 9-13V DC input, USB B for the PC connection, and a BNC for the antenna.
Software Setup
Getting the QDX running with WSJT-X is straightforward. In WSJT-X settings, select the QDX USB audio device as both input and output. For CAT control, select the QDX virtual COM port and set the rig type to Kenwood TS-440 or use the generic Hamlib entry — both work reliably. PTT is handled via CAT so no separate VOX or hardware PTT connection is needed.
JS8Call configuration is identical to WSJT-X. WSPR via WSJT-X works the same way. The QDX handles band and frequency changes entirely under CAT control from the software, including solid-state transmit/receive switching — there are no relays to click.
One thing worth noting: the QDX is not suitable for CW, PSK31, or Winlink Vara — it is designed specifically for single tone FSK modes. For the digital modes it does support it excels, but go in knowing it is a dedicated digital radio, not a general purpose HF rig.
On the Air
Running the QDX at 5W on 20m FT8 with the end-fed wire antenna produces results that are consistently competitive with the IC-718 at much higher power. FT8 is a weak signal mode and 5W is genuinely enough to work DX — Europe, South America, and the Pacific are all reachable on a good 20m opening.
WSPR at 5W is particularly effective. The combination of the QDX’s clean single-signal output and WSPR’s extreme sensitivity means spots come in from well beyond what the power level might suggest. Checking PSK Reporter after a WSPR session regularly shows spots from 8,000km or more.
JS8Call at 5W on 20m works well for regional contacts and is a great portable mode — low duty cycle stress on the finals and enough signal to maintain keyboard conversations under reasonable conditions.
Specifications
Kit price: $69 (assembled $114) from shop.qrp-labs.com
Bands: 20, 17, 15, 12, 10m (high band version)
Output power: 5W at 9V supply (approximately 3.5-4W on 10m)
Modes: FT8, WSPR, JS8Call, RTTY, Olivia, and other single-tone FSK modes
Receiver: Embedded SDR, 24-bit 48ksps USB sound card
VFO: Si5351A synthesized with 25MHz TCXO
CAT control: USB virtual COM port, Kenwood TS-440 compatible
Connections: USB B (audio + CAT), BNC (antenna), 2.1mm barrel (power)
Size: 89 x 63 x 25mm in optional enclosure
Current draw: 100mA receive, 1.0-1.1A transmit at 5W/9V
Verdict
The QDX is one of the best value kits available in amateur radio. For anyone who operates primarily digital modes it is hard to beat — clean signal, USB audio and CAT in one cable, dead simple PC integration, and a build experience that is rewarding without being overwhelming. The high band version covers the best DX bands for digital work and the compact size makes it ideal for portable operation, travel, or as a dedicated second radio for WSPR monitoring.
Highly recommended. More details and ordering at qrp-labs.com/qdx.html.