A 5W HF PA kit compatible with the Ultimate3S transmitter kit, and also for standalone use in homebrew projects. It has the unique feature of onboard raised cosine envelope shaping for perfect click-free CW. There is a discrete component power modulator with foldback current limiting, adjusted by an 8-bit DAC controlled by the microcontroller to generate the raised cosine envelope shape.

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This 5W HF PA kit features an IRF510 MOSFET as the amplifier. But it also includes a discrete component power modulator (like a voltage regulator), which is controlled by an 8-bit Digital to Analogue Converter (DAC). A microcontroller can load a 74HC595 shift register with a sequence of DAC values which closely replicate an ideal raised cosine RF envelope. This makes a very clean CW transmission, practically eliminating key clicks. 


  • Approx 16dB of gain at HF
  • IRF510 MOSFET PA stage providing 5W or more RF output from 200mW drive (varies by band)
  • Raised-cosine keying envelope generation
  • Option for standalone envelope shaping, without external microprocessor control
  • 80 x 37mm PCB, compatible with Ultimate3S transmitter kit
  • Discrete component power modulator with foldback current limiting
  • Designed for typically 12V or 13.8V operation
  • Large heatsink
  • Supports RD15HVF1 transistor substitution for VHF/UHF operation

The kit is on an 80 x 37mm PCB, the same size as the Ultimate3S transmitter kit and relay-switched filter kit. It can be used with the Ultimate3S transmitter kit. Firmware versions v3.12 and above are capable of generating the raised cosine keying envelope. The connections required for use with the Ultimate3S transmitter kit are described in App Note AN004. Requires Ultimate3S firmware v3.12 and above.

The kit can also be built in a standalone mode, with onboard raised cosine envelope keying generated by an optional ATtiny84 microcontroller. This is plugged into the IC socket on the PCB instead of the 74HC595 shift register chip. The ATtiny84 is programmed to produced the raised cosine RF envelope and also provides a key-out signal which can be used to key a driver stage. This application of the kit is perfect for use in your homebrew transmitter projects. It is described in App Note AN005, which also includes discussion of a 5W CW transmitter built using the QRP Labs VFO kit and this 5W HF PA kit. The programmed ATtiny84 microcontroller is available as an option when purchasing the PA kit

Finally, one or more circuit blocks may be bypassed. A simplified assembly manual details a simplified way to assemble the kit, without using the power modulator and raised cosine keying generator circuits. This may be suitable for people who wish to build a more simple PA that does not include the raised cosine keying facility.

This is a Class-C or D amplifier for CW operation or FSK modes. It is not a Linear for SSB purposes. It might be possible to bias it for Linear operation, this will require some experiment. 

Assembly instructions

Printed instructions are NOT supplied with the kit. You can download the kit-building instructions from the links below. Please click "Letter format" for US Letter size (US, Canada) or "A4 format" (rest of the world).

HF PA kit full assembly instructions A4 format Letter format
Simplified assembly instructions (no raised cosine key shaping) A4 format Letter format
Simplified assembly instructions (no key shaping), FRENCH TRANSLATION A4 format  

Many thanks to Gilles F1BFU for the French translation!

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    
AN004 HF PA kit with Ultimate3S transmitter kit A4 format Letter format
AN005 HF PA kit with built-in standalone raised cosine controller A4 format Letter format


Kit photographs