By Dave Jensen, W7DGJ

(Preface by W7DGJ)

If you've been reading Trials and Errors since we first began publication on QRZ in the fall of 2022, you know that our column has included a mix of reviews, editorials and short clips of new ham products. I'm pleased to announce that T&E has now entered another phase of its development with the introduction of quality Amateur Radio publications from Guest Authors. We're presently assembling an advisory board of well-known hams who will help select and deliver great features to you on a regular basis from outside the material that I write as your editor. 

My columns will continue. In addition to that regular content on QRZ, I write a new, shorter monthly column for Ham Radio Club newsletters worldwide called GEAR. This is another new development (available free to clubs) and you'll no doubt see GEAR in your region in the near future.

Today's column is one that is exciting because our guest author, Dr. Daniel Marks, brings us the description of both a new radio (kit) as well as a new digital mode which has a great deal of future potential, not only as another mode for hobbyists, but as a practical tool for emergency communications. Alpha-numerical messages can be exchanged on this tiny radio digitally without the need for a computer. There are many cool aspects of communication with the RFBitBanger and its new mode, SCAMP, which Dan discusses here. Plus, it does CW and SSB to boot!

I'd like to note that Dan Marks is giving a presentation on this radio and his new digital mode on Saturday, September 9th at the QSO Today Academy. Dan's talk is a highlight of the event this year and if you are fascinated by the enclosed description of this new technology, please sign up and register for the QSO Today Academy. As the name "Academy" implies, his year's event is focused exclusively on material that will be educational in nature. My past participation in the QSO Today event has been fantastic for me as a ham radio operator. I've learned so much about antennas, modes of operation, propagation and so on . . . Even for the old timers, this is one $20 investment that is hard to pass up.    73 for now,  Dave Jensen W7DGJ

Daniel Marks Bio 

Daniel Marks, KW4TI, is an electrical engineer located in the Chicago area. Dan received his PhD in Electrical Engineering from the University of Illinois at Urbana-Champaign in 2001.  He works in the areas of computational imaging, mathematical inference, electromagnetic design, radar imaging, and optical engineering and has published over 100 research papers. 

Dan's amateur radio interests include antenna design, open source hardware and software, and minimalistic and stealth designs.   He has created a number of open source hardware projects which can be viewed at his github page http://www.github.com/profdc9.

The RFBitBanger: An Off-the-Grid Emergency Communications HF Radio

by Daniel Marks, KW4TI

The past few years have seen extraordinary global events that have affected almost everyone.  The effects of the confluence of a global pandemic, disruptions of global trade, and chronic supply chain shortages have been mitigated only by unprecedented levels of intervention by governments worldwide.  Perhaps those involved in emergency communications should reevaluate if the current responses of the amateur radio community to these kinds of events are adequate.  The amateur radio community should be planning now for how basic communications could be maintained during an even more severe pandemic or a more protracted interruption of manufacturing, travel, or trade. 

My own thinking on the matter was stimulated by a project called Collapse OS (http://www.collapseos.org).  Virgil Dupras, the creator of Collapse OS, has been considering how basic computational capability might be maintained if only scavenged electronics are available.  Collapse OS targets first-generation microcomputer hardware that is much easier to maintain and repair than the latest specialized hardware.  I have similarly considered how a simple HF radio transceiver might be constructed from common parts.  While transceivers manufactured by ICOM or Yaesu, for example, are extremely capable, these are difficult to repair without access to specialized parts and tools.  On the other hand, the typical parts possessed by an amateur radio hobbyist might include Arduino microcontrollers, alphanumeric displays, oscillator ICs, and various generic “jellybean” parts such as transistors, resistors, capacitors, and MOSFETs. 

I created the RFBitBanger transceiver so that it may be constructed from such parts using simple tools with the goal of providing basic, reliable communications capability. The RFBitBanger minimizes the number of necessary parts, as any required part can become faulty or not be obtainable.  The main processor, on which all of the signal processing is performed, is the ATMEGA328P used in the Arduino Uno/Nano which is somewhat dated but still ubiquitous.  A SI5351A/MS5351 PLL is used for radio frequency synthesis.  A 1602 alphanumeric LCD is used as the display.  All other parts are generic parts, for example, the 2N7000 MOSFET transistors used for RF amplification.  All of the parts are available in through-hole or surface mount versions depending on preference.  These parts are inexpensive and the amateur radio hobbyist is likely to keep these components as spare parts for other projects. 

Despite its low cost and simplicity, the RFBitBanger is a capable transceiver.  The radio can communicate over the HF band between 1.8 and 30 MHz.  It will both send and receive CW transmissions, with a built-in CW decoder and CW sending support for straight or iambic keys. 

With an added push-to-talk button and microphone, one can transmit SSB phone.  Furthermore, it can be controlled by a computer through its sound card for FSK digital modes, for example, FT8 using the WSJT-X software. 

There is also a new weak signal digital mode for this transceiver called SCAMP (Simple Conversational Amateur Message Protocol) that is implemented on the transceiver itself that does not require a computer.  The radio may be operated using five buttons on the transceiver or an external PS/2 keyboard, including entering messages.  The radio has a dual sideband (DSB) receiver based on a diode ring mixer, as it is one of the easiest receivers to construct from simple parts, with the primary disadvantage that the received upper and lower sideband signals overlap.  This is a compromise, as the ease of construction of the mixer is prioritized over the possibility of signal interference on a congested band.

A picture of the surface mount version of the RFBitBanger is shown (constructed by Paul Williamson, KB5MU, of the Open Research Institute) with the various components indicated.  Starting at the lower right, the antenna attaches to a BNC connector.  When changing the HF band, the corresponding band filter module is plugged into a socket at the lower right-hand part of the board.  There is a RF ammeter that may be placed in circuit with an LED indicator to determine if power is being delivered to the antenna, and to provide a way to trim an antenna for a resonant length.   There are potentiometers to adjust the RF gain and audio volume.  The input to the radio is through five push buttons: up, down, left, right, and enter.  There are three 3.5 mm stereo jacks.  An audio output jack may be used with headphones or a small speaker.  It also is used to connect to the line-in port of a computer under computer control.  There is a microphone jack that connects to an electret or dynamic microphone for audio input, or it may be used with the line-out port of a computer.  The remaining jack is for a push to talk button, a straight key, or paddles.  The straight key or “dit” paddle may be used as the push to talk button.  The display is a standard 2 by 16 character display.  The top row is used to select menu items, enter messages for transmission, and show signal strength.  The bottom row is used to display decoded text, for example CW, RTTY, or SCAMP messages.  Power is supplied through a 12-14 V and 1 A DC adapter, and a PS/2 keyboard may be plugged into a Mini-DIN 6 connector to control the radio and enter messages.  

A principal goal of the RFBitBanger is to be able to communicate alphanumeric messages under weak signal propagation conditions without requiring a personal computer and sound card.  To this end, I have created the SCAMP digital mode and the RFBitBanger transceiver to demonstrate its use.  Like other weak signal propagation modes such as the popular FT8, SCAMP is an orthogonal frequency shift keying mode with a low symbol rate, forward error correction, and synchronous signaling.  Unlike FT8 which has been designed to exchange signal reports, SCAMP must be able to communicate arbitrary messages that may be required in an emergency situation. 

SCAMP must operate without global clock synchronization which may not be available.  Furthermore, SCAMP is designed to be implementable on an 8-bit microcontroller.  In order to meet these difficult goals, I had to design SCAMP so that computationally intensive calculations could be efficiently approximated using simple formulas.  Because I learned to program on the 8-bit Apple II computer, I became adept at writing code to maximize the capability of this simple hardware.  SCAMP largely uses simple computations such as integer addition, subtraction, and Boolean bit operations, and it may use multiplication if available.  SCAMP fulfills an unaddressed need for a weak signal digital mode that can be implemented on a low cost microcontroller.

The RFBitBanger is designed so that two stations can easily establish communications using SCAMP.  Each SCAMP mode may be recognized by listening for its type of modulation (on-off keying or frequency shift keying) and its symbol rate.  The two stations select the same mode, for example SCAMPFSK, short for SCAMP with the frequency shift keying variant, 26 symbols per second.  This mode has a corresponding BFO frequency which is 633 Hz, so that the two stations set their VFOs to be offset from each other by 633 Hz.  This can be accomplished accurately if a station adjusts its frequency to maximize the received signal strength indicator in response to a transmitted SCAMP message.  Once the station has maximized its own signal strength indicator, a SCAMP message transmitted by the station is aligned to maximize the signal strength at the receiver of the station that transmitted the original message.  There are a number of options for transmitting a SCAMP message to better ensure a message is received such as repeating synchronization frames and message frames.  While the repetition may be inefficient it can help an urgent message get through marginal band conditions.

In order to promote the adoption of SCAMP, the source code to the RFBitBanger, which includes an implementation of SCAMP, is available under the zlib open source license which permits commercial sales and modification.  Furthermore, the RFBitBanger itself is licensed under the Creative Commons Attribution Share-Alike 4.0 license which permits commercial sales and modification.  Because of its potential to provide emergency communications with minimal, cheap hardware, I hope that SCAMP is adopted by the amateur radio community.  There is a detailed document describing the SCAMP digital mode and how it may be implemented using integer calculations.  To encourage use of the RFBitBanger apart from emergency communications so that operators can be practiced using it, the RFBitBanger incorporates many features of standard transceivers, such as CW and SSB modes, so that it may be useful for QRP and portable use. 

All of the hardware, software, and documentation are available at a github respository, https://github.com/profdc9/RFBitBanger , including gerber files for both through-hole and surface mount versions of the RFBitBanger, an Arduino project for the transceiver firmware, and a detailed instruction manual and documentation on the SCAMP digital mode.  This work has been developed in conjunction with the Open Research Institute ( https://www.openresearch.institute/ ) .  The ORI is offering the surface mount version of the RFBitBanger as a kit with prepopulated SMT components and I thank Paul Williamson KB5MU and Michelle Thompson W5NYV for their help in preparing the RFBitBanger as a kit.  The kit can be received for a donation to ORI with instructions available at the link https://www.openresearch.institute/2023/08/08/rfbitbanger-kit-defcon31-show-special/ .

[Editor's note: I am personally looking forward to hearing Dr. Daniel Marks at the QSO Today Academy presentation on Saturday, September 9th (presently scheduled for 1 PM Pacific). Please check the Academy schedule beforehand for any time changes, and I hope to see you in the audience! --  Dave Jensen, W7DGJ]

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