The printed circuit board and installed parts are shown in "Autotuner_Parts_List_8.pdf" accesable below. With the board I supply perimeter connectors including a 1/8" stereo jack, IC sockets for the 18M2+ and 4N35, plus headers for the A4988 module. The cost is not much, but changes with postage. Please email me at acornwallns@gmail.com (or ve1cor@rac.ca) if you wish a quote .

I have wired autotuners using a solderable perfboard board, about 4"x5", with interconnecting holes, and used the female end of a 1/8" stereo earphone extension cable to make a pigtail type serial data connector. I've generally used some type of screw-down connector for the four stepper motor wires so the top of the case with the electronics is easily removed by unscrewing the wires. Otherwise off-board connecting wires are soldered directly to the board. Having a PCB is not necessary for making an autotuner; it is just a lot easier.

The circuit diagram downloadable below, or in the TAPR article, is a pretty good guide for component placement. Wiring style is not critical although short connections between the end of the sampling antenna transmission line and reciever diodes, where there is RF, is a good idea. Beyond this there is no RF. I recommend sockets for the 18M2+ and 4N35, and headers for the A4988, making these components easily replaceable. Before starting you should study the information in the parts list, below.

Whether you use a PCB or handwired circuit I will be pleased to assist you.

UPDATE FALL 2024: There is an added instruction to the 'Autotuner_Parts_List' section, below, involving two "Through Hole Vias" in the Autotuner PCB. Each via hole needs a through wire soldered on both sides of the board to connect specific traces on the top and bottom of the board.

Some Internet servers and computer anti-virus programs object to passing an executable program with a '.bas' extension. Therefore, the two download programs below have name extensions of 'txt' (eventhough the link says '.bas'). To make them useable the extensions should be renamed to 'bas'.

Apparently the older HTML version I am using in this website may not support the normal 'download and save' option for saving the two autotuner programs with some browsers. Instead, when the program link is clicked the text of the program is listed in a new tab. In this case to retrieve and save the program: mark the entire text in the tab (ctrl A), copy it (ctrl C), paste it into a text editor, such as Notepad, and use the 'save function of the text editor to save the file with the correct name (including extension '.bas').

You need a Picaxe Editor program and an AXE027 USB data/programming cable.

- Plug in the USB end of the AXE027 cable into your computer, and the 1/8" stereo end into the jack on the autotuner circuit.

- Run the Picaxe Editor program.

- Load the Picaxe program file that you want to save to the autotuner (Top line, left corner: File>Open File), such as one of the two below.

- In Editor Settings (Left side, pull down menu) select the 18M2+ microcontroller.

- OPTIONAL. Check program for syntax and size errors (Second row from the top: click on 'ABC' icon). If OK there will be a succesful compile message.

- Turn ON 12V power to the autotuner circuit (stepper motor wires must be attached). If the 18M2+ is already programmed the circuit starts running - no problem.

- Start programming the autotuner with the 'new' Picaxe program by Clicking the 1/8" stereo plug 'Program Icon' (Second row from the top).

- At the bottom right of the Editor screen two progress bars appear: 'Program Loading' Status then 'Data Loading' Status.

- When programming is fully complete the circuit will start running with the newly loaded program.

- TURN OFF power to the autotuner circuit and disconnect the AXE027 cable from your computer and autotuner circuit.

- Anytime you want to run the newly loaded program in the autotuner circuit TURN ON 12V power.

Picaxe microcontrollers are rated for programming more than 100,000 times.

IMPORTANT: The four stepper motor wires (two coils, two wires per coil) should be attached to the circuit when running the programs in a powered circuit to avoid damage to the A4988 stepper motor controller module. Note, the autotuner needs to be powered to be programmed, and it will start running as soon as the program is loaded.

* Current is applied to the circuit antenna terminals. If too much current is used the receiver diodes and/or optocoupler LED can be destroyed. As a current source I use a 9 volt battery in series with a 2K ohm, or higher, resistance. Changing the resistance alters the current. The article on this page 'Estimating_Full_Transmit_Power.pdf' explains further how to apply current.

SMLA_Autotuner_05.bas

Files will downlaod with 'txt' extension; rename with 'bas' extension upon saving

If computer tracking of tuning progress or Manual mode is desired, set the terminal baud rate in the PICAXE development program to 38400.

When programming the Picaxe make sure that the stepper motor is connected to the circuit.

There are two versions, 03 and 05, of the SMLA Autotuner program available for download. They both autotune the same way. 03 is original on this site. 05 fixes a small problem in 03 where power to the stepper motor is not turned off properly when autotuning ends with an error (e.g. too high transmitter power when tuning causing saturation - probable saturation is indicated by a blinking red LED). The operator would usually turn off the autotuner anyway. Further, 05 has a 'tweaking' routine at the end of autotuning that attempts to improve loop antenna performance by a small amount.

At the time of its development the cost for the circuit electronic parts and stepper motor was less than $60.

Information about the PCB through hole vias is an addendum as of the Fall of 2024.

Note, 'Estimating_Full_Transmitting_Power.pdf' was written a couple of years ago and pertains specifically to earlier versions of the autotuner program and circuit. The article is still relevant (the 'green' LED referred to is now yellow).

At the start of tuning the stepper motor rotates the variable capacitor one complete turn recording the antenna's highest signal level. The variable capacitor encounters equal capacitances twice during a turn, thus providing the autotuner with two opportunities to discover a maximum signal value. The maximum becomes the goal for the susequent active tuning sweep (i.e. turn).

The report shows step positions and sample signal values for signal levels greater than 10 (10 or lower are not significant); absolute highest signal is about 1,000. There are two stepper motor partial step sizes during the tuning process: half-step when very little signal is measured, and one-sixteenth step as tuning slows and becomes more presice either: in the vicinity of a maximum signal level in the initial turn, or when approaching the tuning goal during the tuning sweep.

In the report all steps are counted as sixteenth partial step equivalents. A sixteenth step turns the variable capacitor 0.05625 deg. At the end of the report there are 10 signal readings over a few seconds to indicate tuning stability. A small 'wiggle' in the tail is not important.

Usually a computer is not connected to an autotuner. Indeed, tuning is automatic whether or not a computer is connected, and an autotuner's LEDs and a buzzer provide tuning status information. A computer,however, adds interesting autotuner diagnostic capability.

Note, at the end of each tuning report I have inserted information about tuning conditions and results. This is not part of the tuning report.

The amazing part of the test is the QRP TX Shield transmits a mere 100 mW (one tenth of a watt), which the autotuner was able to tune with a long sampling antenna. The small magnetic loop antenna sent a signal that was received by many WSPR monitoring stations far and wide.

Thank you WSPR monitors and WSPRnet. There is more information about WSPR elsewhere on this website, including how to set up an inexpensive and effective monitoring station.