DIY Radio Tuna Tin "S" QRP Transmitter

DIY Radio Spring 2021 - Tuna Tin "S" CW Transmitter

The December 2020 issue of QST p. 30 featured Bob Fontana's do-it-yourself article entitled 'The Tuna Tin "S",' subtitled "A Bare-Bones Synthesized QRPp Transmitter". QRPp means the transmitted signal is less than 1 watt. Our "Elmers" will guide you in building this CW transmitter comprised of 2 circuits: an Arduino controlled digital readout Si5351 frequency synthesizer that outputs a square wave into an IRF510 RF amplifier with a band pass filter that transmits an FCC-clean CW signal. Choose one of 3 bands: 80, 40 or 20 meters. Plug in a Morse key and a battery. Rotate a single knob to choose the displayed frequency. Depress the key and it transmits. Bare-bones is the operative word so that any club member should be able to build a working rig with assistance of virtual meeting attendees. For less than the cost of a book, a ham can build a working transmitter that can be heard on an existing receiver.

An Arduino Nano microcontroller is the brains of the rig executing 4 pages of software that can be modified to add new features. Our group building project will be conducted with Zoom meetings (the next one is pending schedule). Phil Sittner and Bob Mix have developed a kit that is better than Phil's working prototype.  The kit includes all the parts.  You just provide a low wattage soldering iron with a fine tip, 60/40 rosin core solder, solder wick, needle nose pliers, diagonal cutters, wire stripper, basic soldering skill and a desire to have fun.  You will need a computer to load the software to the Arduino Nano, and a multimeter and 50 ohm dummy load to test the completed unit.  There is no assembly manual, but we can probably guarantee your rig will work.  The 50 ohm dummy load can be as simple as a 1/2 watt 50 ohm BNC terminator or two 1/2 watt or greater 100 ohm resistors (not wire wound) in parallel.

Tuna Tin S Arduino Sketch Repository

Project Updates

LTSpice Program Download Page

LTSpice model file for the 7 MHz RF section

While building the Tuna Tin "S" transmitter, we observed that the 3 volt peak output of the Si5351 synthesizer module can only drive the IRF510 MOSFET amplifier to 500 mW output power. The amplifier stage circuits below raise the drive power to the IRF510 sufficiently to achieve more efficient Class E operation and 4-5 watts of power output.

Complementary Symmetry Push Pull Driver Stage

This driver offers the stability and noise immunity of digital switching, requires no inductors or transformers, and should provide sufficient drive power to the IRF510 gate up to 14 MHz.  The potentiometer on the Q1's input adjusts the duty cycle, and the value of R2 affects the rise and fall times of the trapezoidal drive waveform.  The model file shows R3 as a fixed resistor as LTspice does not model variable resistors. LTspice Model File  Click here for more details.

2N3866 Driver Stage with Toroid transformer

This driver uses a single transistor and a toroid impedance transformer.  LTspice Model File

Modification adds 5 MHz to a 7 MHz Tuna Tin S Transmitter

This modification adds a 10.7 MHz trap to the 7 MHz Tuna Tin "S" and allows it to transmit on either 5 MHz or 7 MHz.  LTSpice model file

Modifications for the Tuna Tin "S" in September 2022 QST

Bob Fontana, AK3Y, published a Tuna Tin "S" project update on pages 32-33 of the September 2022 issue of QST.  His project improvements include: frequency extension; more tuning increments; precision calibration; EEPROM Recovery; and, an improved drive circuit.  Current ARRL members may download the complete project summary from the QST in Depth web page at http://www.arrl.org/qst-in-depth