Minimalist Amateur Xcvr: 3-Band

Pacificon 2023.pdf

Pacificon 2023

Quick introduction to the MAX-3B, and how the REST API enables integration with SOTAmat

Complete MAX-3B Station

iPhone 12 mini for scale. USB-C power bank, battery powered speaker, N6ARA tiny paddle, EFHW with internal 49:1 unun.


The MAX-3B (Minimalist Amateur Xcvr, 3-Band) is a QRP HF radio designed with the Summits On The Air (SOTA) in mind. There are a few goals for this project:

Full technical details are available at:

The process for turning a board into a full radio is available here.

The latest version of REST API documentation is linked in the github project readme.





Digital & Connectivity

Ongoing Firmware Development

The following features are not implemented, but should be possible with existing hardware:


Does this only work with iPhones?

Any web browser, including a computer, can act as the front panel for the MAX-3B. Multiple devices can connect to radio.local simultaneously. The radio's API could work with a mobile app.

How well does it radio work?

The receiver is based on the SA612 mixer, which has relatively poor large signal performance. (See: Robust band-pass filters are included to avoid interaction with out-of-band signals, and otherwise performance is generally acceptable in remote operating locations (such as a SOTA peak). This is a similar receiver architecture to the LNR Mountain Topper series, which is widely regarded as suitably high performance for SOTA. Subjective RX quality was similar to a MTR-2B in a field test.

The IF crystal filter is wider than most CW only radios in order to support SSB reception and minimize ringing from the crystal filter. An additional CW analog filter provides ~500Hz bandwidth when CW reception is selected.

FCC Part 15

This project should not be considered a "product." The board is a useful subcomponent of a larger radio system that a builder can design, and once fully integrated can be one of the builder's home-built devices allowed under section 15.23: 

The WiFi section of the board is not implemented with a FCC-certified module, so operation of this device should be in accordance with ham radio best practices and the ARRL band plan. No encryption should be used, identification should be done via a callsign in the SSID, and operation should not cause interference for other users of the spectrum.

Sparkfun electronics has a brief intro to topics like this:

Can I modify this project? Can we collaborate?

Collaboration is appreciated on this project. I'm not a software engineer or web developer, so the project could use some expertise adding features and improving the quality of the code. Please contact me if you would like to work together on this!

This radio is meant to be a high quality platform for amateurs to develop solutions that serve their needs and preferences. The software is licensed under GPLv3 to encourage collaboration and tinkering on derivative hardware. The hardware is licensed under the TAPR Open Hardware License to encourage disclosure of design materials in any derivative work.

I found a bug. What can I do?

Please file a bug on GitHub or send an email to

How can I get one?

Currently, the MAX-3B is not sold anywhere as a finished product, and I am not able to commit to a schedule for production runs of new hardware. If there's sufficient interest, I will coordinate assembly runs and kits of parts so builders can make their own. I have a small number of initial boards set aside for people who commit significant time to field testing or software development. Please send an email to if you are interested in a board. Please note that this is a project for the advanced builder, and I may not be able to help debug issues.


This documentation and any related physical hardware are provided in the spirit of amateur radio experimentation. Any use of this documentation or hardware is at the operator's risk, and should comply with all safety best practices and applicable laws.