Tuning a Mag loop antenna


I have done the introductory investigation with the TIG control software and am getting ready to launch into my application.

I want to turn a 20-turn vacuum variable capacitor with a NEMA 17 stepper motor to tune a magnetic loop antenna. The antenna will operate on 40, 30 and 20 meters. My objectives are two: fast tune between bands , which will require about 8 or so turns in about 15 seconds. And slow tune within a band which will require less than 1 turn and should take about a minute. Is there one operational mode of the TIC 36V4 that will do this well ? It would be nice if I could use analog control with a potentiometer for the fine tune and the TIC USB software for the band change.

The assumptions of the TIC 36V4 regarding power-up position would seem to be relevant. The sensitivity of a single-turn potentiometer would also be a concern. perhaps a multi-turn pot with a radial turns indicator would be appropriate.

Your suggestions and observations are solicited and appreciated.

Paul Hamilton, KE7UAE

Hello, Paul.

Any speed limitations will come from the stepper motor itself and not the Tic; although th speeds you are mentioning are very slow (32 RPM), and I generally expect almost any stepper motor to be able to achieve that.

If I am understanding your application well enough, it sounds like you want to be able to quickly switch between a few different regions of the range of motion and fine tune the position once you are there. For that kind of application, I would probably recommend a separate microcontroller such as an Arduino or one of our Arduino-compatible A-Star controllers that you can program to monitor a few buttons (one for each of your bands) and your potentiometer (for fine tune control) and communicate with the Tic through I2C or TTL serial commands to move the motor appropriately. By the way, depending on how you want the controls to work, a rotary encoder might be a better option than a potentiometer in this setup, since it does not have physical end-stops. So, for example, each button would be assigned a certain position to move the motor to, then turning the rotary encoder would be free to increment or decrement the target position accordingly, whereas with the potentiometer, the buttons would essentially be changing the range of target position values the potentiometer is mapped to. The downside of a rotary encoder is that they are a bit more complicated to handle in the programming and require more overhead to keep track of than a simple potentiometer.

On power-up the Tic will not know the position of the stepper motor. The Tic does support homing with a limit switch, so if you can add a limit switch to one end of your range of motion, that would be one potential solution. You can find more information about this in the “Homing” section of the Tic user’s guide.Alternatively, you could make sure the stepper motor starts in some specified position each time and send the Tic a “Halt and set position” command right after it powers up to give it a starting reference.



I will keep your suggestions in mind. I am presently getting set up with a 10-turn potentiometer (on order) and will bench test using it and your TIC control software over USB as needed.


Paul, KE7UAE


One other thing. Your suggestions seem to assume that this tuning system is open-loop and feed-forward. But it is not.

I have a broadband RF noise generator that adds a small signal going to the antenna. At the resonant frequency, where the antenna is matched, I can observe a dip in the signal level on the spectrum display of my Icom 7300. This enables controlled and observed location of the capacitor rotation as a function of the resonant frequency. The tuned mag loop antenna is a LC resonator and at the matching frequency it reflects back less of the broadband noise.

So a simple solution of a multi-turn resistor should work. The control feedback is in my radio and my brain :grinning:.

Best regards,


Is there an ideal size (ohms) for a control variable resistor?

Thanks ,

Paul, KE7UAE

Anything between 1kΩ and 100kΩ should generally work fine.