Andrew Cornwall
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Updated December 31, 2021

The automatic tuner for small magnetic loop antennas supported here is the topic of a major article published by TAPR in its on-line magazine Packet Status Register , Winter 2020, No 144: ( ).


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 (or 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.


Programming the Picaxe 18M2+ microcontroller requires a Picaxe development program such as the PICAXE Editor 6 for Windows (development programs for other OS are avaiable) downloadable for free from, and an AXE027, Picaxe USB programming/data cable obtainable to purchase from Amazon and many robotics store websites.

The programs may be examined and edited with the Picaxe development program or a text editor.

* * * NOTE * * *

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').

Programming Your Autotuner is Easy

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.

File will downlaod with 'txt' extension; rename with 'bas' extension upon saving
'Autotuner Circuit Test' runs through all of the functions of the autotuner circuit: buzzer, LEDs, measuring sample antenna signal* (optional relay off and on), and turning the stepper motor half through sixteenth partial steps. There is also a 10 second time when the motor is on but not stepped for measuring current to the circuit (mostly going tp the stepper motor). As the program runs the results of the program can be seen in the Picaxe development program monitor screen; set the baud rate to 4800. Make certain that a stepper motor is connected to the circuit. A good time to run the test program is before the circuit is mounted in the autotuner box, and circuit fixes are easier.

* 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.



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

'SMLA Autotuner' is the small magnetic loop antenna automatic tuning program. It is explained in detail in the TAPR Packet Status Register , Winter 2020, No 144, article referred to above, and in the draft article downloadable below.

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.

The Autotuner circuit is not complex yet does its job very well. Resistors can be as low as 1/8 Watts, and capacitors as low as 25 Volts. If a printed circuit is NOT used the layout of the circuit in the diagram is a good guide for component placement.

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

VE1COR_Autotuner_46a.pdf This is the draft article submitted to TAPR for publication in the PSR and expertly composed there. The draft may show some images and tables in more detail.
Autotuner_Parts_List_8.pdf 'Autotuner Parts List' is a collection of autotuner construction information - including a parts list. This is essential, comprehensive information for building an autotuner. Read this information before deciding to make an autotuner.
Estimating_Full_Transmit_Power.pdf 'Estimating_Full_Transmit_Power.pdf' explains how to estimate the full transmitting power the autotuner can withstand before endagering the autotuner reciever diodes and/or optocoupler LED. Full transmitting power is not the small power used when autotuning. Rather it is the highest power when transmitting, for example, making a contact. Transmitting power is picked up by the sampling antenna even when not autotuning. In my experience a fairly high voltage variable capacitor would arc before a dangerous level of transmitting power occurs.

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).

Autotuner_Report_Feb_11_2020.pdf When a computer is attached to the autotuner by means of a Picaxe programming cable the autotuner can report data on tuning progress.(In manual mode another type of report is produced.) During tuning the autotuner sends data as text to the Monitor function of the Picaxe Development Program. "Autotuner_Report_Feb_11_2020.pdf" shows the reports of three sequential autotunings, one in the 20 meter band and two in 40 meters (same frequency but the sensitivity switch is different). These reports are typical of tuning results.

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.

WSPRNet_Experiment_06.pdf This was a fun exercise. In the summer of 2018 I tested an earlier version of an autotuned small magnetic loop antenna using the facilities of and the thousands of WSPR signal monitoring stations worldwide. The article describes the challanges and results of WSPR transmitting in a field at the back of my property, with neither AC power nor Internet availability. The test transmitter was TAPR's $29 module, a QRP TX Shield for WSPR on 20 Meters', controlled by a Raspberry Pi. TAPR published the article in the No. 139, Fall 2018, edition of the Packet Status Register. Subsiquently, the Radio Amateurs of Canada also published the article in the January-February, 2019 edition of TCA, 'The Canadian Amateur'.

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.