The Ultimate3S QRSS/WSPR Transmitter Kit is the new edition of the third version in the "Ultimate" QRSS/WSPR kit trilogy. It can produce QRSS, Hell, WSPR, Opera and PI4 slow-signal modes anywhere from 2200m to 2m and even 222MHz bands. Plug-in LPF filters are available for all 16 HF/MF/LF/VHF bands from 2200m to 222MHz.

Shop order

The U3S kit was launched in January 2015. It is the new edition of the earlier U3 kit produced from November 2013 to December 2014. The U3S uses an Si5351A frequency synthesiser kit rather than the pre-built AD9850 DDS kit used in the earlier U3 kit. The AD9850 DDS kit prices are rising and they are becoming less easily available. The Si5351A frequency synthesiser kit was developed to ensure continued low cost of the Ultimate QRSS/WSPR kit series.

Please also see the original version Ultimate QRSS kit and Ultimate2 QRSS kit pages, and the earlier U3 kit page to appreciate the heritage. Much of the information in these earlier pages is still relevant and informative.


The Ultimate3S QRSS/WSPR transmitter kit includes an Si5351A synthesiser module kit and plug-in low pass filter modules which are also available separately for bands from 2200m to 6m. The kit can transmit on any frequency from the amateur 2200m (137kHz) to 2m (145MHz) bands and even the 222MHz band. Power output on 2m is lower than HF - 17mW was measured (with 5V PA and single BS170). Changing bands is a matter of plugging in the appropriate low pass filter kit to attenuate unwanted harmonic output. The relay-switched LPF kit may be used to automatically switch between up to 6 different bands.

A GPS receiver module such as the QLG1 kit can be used with the U3S kit. It is not strictly necessary. You can do everything manually. But the GPS receiver sets the time and maintains precise timekeeping, sets location (and Maidenhead locator), calibrates the output frequency, and corrects temperature-induced frequency drift. It's such a nice luxury, and for such a low price we really recommend it! The QLG1 kit is powered by 5V, includes proper logic level conversion to the 5V logic used in the U3S, and has onboard LEDs for visual status indication. It is designed specifically for the QRP Labs kits. It has a relatively large PCB ground plane that gives it excellent sensitivity!

There's an aluminium box available. It's a pre-drilled, printed anodised extruded aluminium case custom-made for the U3S. It includes an accessories kit: two buttons, two toggle switches, 9-pin D connector, power socket and matching plug, BNC connector, four self-adhesive "feet", and mounting hardware. 

The DELUXE U3S set makes ordering easy - the deluxe set contains U3S, relay-switched LPF kit, six LPF kits for the most popular bands (10, 15, 20, 30, 40, 80m), QLG1 GPS kit, and Enclosure kit. Also included are two extra BS170 transistors which you can fit in the U3S for increased output power (or keep as spares). 


The kit supports the following modes:

- QRSS mode (plain on/off keyed slow CW)
- FSK/CW mode (frequency shift keyed slow CW)
- DFCW mode (dual frequency CW)
- WSPR mode (Weak Signal Propagation Reporter)
- WSPR-15 experimental WSPR mode with 15-minute frames
- Opera modes (8 speeds)
- PI4 beacon mode
- JT9 modes (5 speeds)
- Slow-Hellschreiber (frequency shifted slow-Hell)
- Full-speed Hellshreiber
- Half-speed ("DX") Hellshreiber
- CW (plain CW)
- FSK (0-999Hz shift, fast-speed FSK CW)
- Customisable FSK patterns
- manually-keyed CW/FSK transmitter
- JT65 modes
- ISCAT-A and B

Other features:

- Si5351A synthesised output frequency (no SMD soldering!)
- Plug-in low pass filter boards (available for 10 HF and 2 LF bands)
- 2-row by 16-character blue back-lit LCD + two-button user interface
- User-programmable settings stored in EEPROM
- GPS interface, for frequency lock, timing and location information
- On-chip generation of WSPR encoded message (no PC required)
- WSPR maidenhead locator from GPS-derived latitude/longitude
- Selectable “frame” size, for stacked QRSS reception
- Plain CW callsign identifier at selectable interval
- Produces 250mW RF output on 30m (lower output on higher bands)
- Higher power with more PA transistors and/or higher PA supply voltage

Instructions - current kit version

Printed instructions are not supplied with the kit! You need to download them here. You need FOUR manuals, and to choose the version appropriate to your PCB and firmware version: the Assembly manual, the Operating manual, the Si5351A Synth kit manual and the Low Pass Filter kit manual. Depending on the kits you purchased, you may also need the relay-switched LPF kit manual, and the QLG1 GPS Receiver manual.

Assembly instructions PCB QCU Rev 3 or 4 click here! (A4 size)
PCB QCU Rev 3 or 4 FRENCH translation, click here
PCB QCU Rev 3 or 4 click here! (Letter size for US)
Operating manual Download from firmware page (Japanese version also available)!
Si5351A manual Si5351 Assembly: download here!
Low pass filter kit LPF Assembly: download from page here!
Relay-switched LPF Relay-switched LPF kit assembly, if applicable: download here!
QLG1 GPS Receiver QLG1 GPS Receiver kit assembly, if applicable: download here!


Many thanks Gilles F1BFU for the French translation (see link above). 

Version upgrades can be purchased at QRP Labs or you can upgrade your own firmware using the .hex and .eep files from the forum.

Firmware Version History - please CLICK HERE

Application Notes 

These documents provide application ideas for your Ultimate3S, solutions to common problems, further technical information, etc. Please click "Letter format" for US Letter size (US, Canada) or "A4 format" (rest of the world).

AN Title    
AN001 Frequency drift reduction in the Ultimate3S transmitter A4 format Letter format
AN002 My QTH is many km wrong in the WSPR map – how to fix it! A4 format Letter format
AN003 Controlling additional relays using the Ultimate3S “Aux” A4 format Letter format
AN004 HF PA kit with Ultimate3S transmitter kit A4 format Letter format


Instructions: translations and previous PCB versions

Carefully check your PCB revision!

Many thanks Toru JG1EIQ for the Japanese translation. Toru also translated the v3.07 operating manual available from the firmware page. Many thanks to Andrey R1CAD for the Russian translation. Andrey also translated the v3.08 operating manual available from the firmware page. Many thanks also to John F5VLB for the French translation.

Assembly instructions PCB QCU Rev 1 click here!
PCB U3S Rev 2 click here!
PCB U3S Rev 1 click here!
Assembly RUSSIAN Russian translation click here! (PCB Rev 2)
Assembly FRENCH French translation click here! (PCB Rev 2)
Assembly JAPANESE Japanese translation click here! (PCB Rev 2)
Japanese translation click here! (PCB Rev 1)


Kit contents

The kit contains the PCB, back-lit blue LCD, programmed microcontroller and all board-mounted components. It also includes the separate Si5351A synth kit and a LPF kit for one band of your choice. Download the instruction manuals using the link above. The PCB is quite compact and care is required in the construction. A rich array of functionality is provided by the design; please read the instruction manuals to understand the facilities provided. Builder modifications will be shown on this page as they are developed!

Dimensions: the LCD module is 80mm x 36mm. The main board is 80 x 37mm.

Kit photos (click for larger versions):

More photos...

These photos show a working U3, complete with attached relay-switched Low Pass Filter board, available for GBP 9. The relay-switched LPF board can have up to five additional LPF's plugged in, permitting the U3 to sequence between 6 bands (including the LPF on the main U3 board). Note that my photos show NO additional LPF's plugged in (as I only have one built LPF here right now).

Ultimate3S on 4m, 2m and 222MHz bands

The LPF kit page has designs for a 4m and 2m LPF. Rick NM3G wrote this article on a 222MHz LPF design. These designs are also available as kits in the shop. Note that the 222MHz is supplied as "2m", the inductors are just wound a little shorter for 222MHz. 

PCB tracks

These diagrams show the PCB track layout for the latest "QCU Rev 4" PCB. 

RED = Top Layer
BLUE = Bottom Layer
GREEN = Pads and vias

The left diagram includes the ground-plane (top side of PCB); the right diagram shows only the traces top and bottom, without the ground-plane.