Automatic Feedback-based Magloop Tuner: For cheap!

Discussion in 'Homebrew and Kit Projects' started by VE3CEN, Jan 15, 2020.

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  1. VE3CEN

    VE3CEN Ham Member QRZ Page

    Hello everyone,

    I hope that this is the correct forum for this topic. I wanted to share this idea with you in case anyone else found it useful. I have (hopefully) attached a .PDF outlining the process of how I built this tuner.

    In short, this is an automatic tuner controller for a transmitting magloop antenna that is tuned using a single capacitor. Many people have these antennas, and many home-made tuners have a stepper motor to control the capacitor, but are manually tuned. My idea was to use an Arduino Uno to get rid of the manual step. If you have a magnetic loop antenna with a variable tuning capacitor, then this project may be of interest to you.

    This system uses a small sense antenna (located at the transmitting loop, in my case right in the centre) to optimise tuning of your antenna at the push of a button (approx 3 second tune time). Pick your operating frequency, put out a low-powered carrier (ID First!) and push the button, and this tuner will find the optimal capacitance for operation. You can save multiple "band-settings" that are used to limit the seek range of the tuner, decreasing tune time significantly.

    It's very simple, it has no directional couplers or anything to actually measure SWR; it simply optimises the near-field E-field strength for a given frequency. If the frequency you are trying to use is in fact tunable for your loop, then this will correspond to the minimum SWR.

    I hope this is interesting to someone, happy to answer questions about it! I have to imagine this is not an original idea, but I didn't see anything online like this when I did an initial search, so I did it myself. Based on an Arduino Uno, stepper motor + motor controller, and a few very inexpensive components (switches, an indicator LED...), the overall cost of this is very low.

    Kind regards,
    VE1CEN (previously VE3CEN)

    Attached Files:

    Last edited: Jan 15, 2020
    KW4TI likes this.
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    Nice. I'll give it a try within a few weeks.

    VE3CEN likes this.
  3. KW4TI

    KW4TI Ham Member QRZ Page

    Is your Figure 1 consistent with your Figure 2? Figure 2 shows a twisted pair going back to the shack. Figure 1 shows only one wire leading from the antenna.

    Could you replace the diode chain with two back-to-back zener diodes, for example 5.1 volt zener diodes?

    What kind of motor control driver do you use? Perhaps something like two L298N chips would be standard, cheap, and widely available, and controllable by an Arduino.

    It is a well described and simple project and should be useful, though placing a tandem match in the shack would avoid the sense antenna altogether so that only the power wires to the stepper would be required. However, the sense antenna avoids having to mess with placing something in-line with the high power RF signal and the isolation that would be required for that. I have dealt with isolating RF from microcontrollers and it can be a pain, with the microcontroller going bonkers when you key up at full power.


    VE3CEN likes this.
  4. VE3CEN

    VE3CEN Ham Member QRZ Page

    Good catch, you're right, I forgot to add the reference connection to figure 1. Figure 2 is correct; it's a twisted pair. I chucked a few ferrites on there as well; this is a rectified, DC voltage on the twisted pair.

    You could absolutely do this. I used a diode stack because I have been told that it's better at dealing with quick transients when compared to a zener diode; my first intuition was just to use a zener. I am sure it would work fine, and it would sure be simpler. I had a bunch of these diodes laying around so I used them.

    I actually bought a very cheap motor driver that is designed to connect to the Arduino. I will have to check for the exact model number later; two L298N chips would certainly work. The stepper is not typically under much load, so as long as your driver can supply sufficient voltage to account for drop across the wires running to your antenna, it should work just fine..

    As to your last comment about placing a tandem match in the shack: One reason I designed it this was was that I wanted to come up with a super cheap, antenna-agnostic tuner that didn't require any direct coupling to the RF chain. As you said, this way I don't need to put anything in-line with my RF path. One disadvantage of this design is that I am not actually measuring SWR at all. In general, this design works great *if your antenna CAN tune at the frequency you're trying to tune at*. If, however, you try to tune it to a frequency that the capacitor cannot get a match for, then this tuner will not really tell you. It will find the "optimal capacitance" within it's range, but this tuner won't know that it hasn't achieved an acceptable match. That's the major compromise here. If you had a tandem match/directional coupler, you could actually measure the SWR and that information is very useful.

    Thanks for reading, both of you! I'll make good use of your input.
    KW4TI likes this.
  5. KW4TI

    KW4TI Ham Member QRZ Page

    For some other ideas about making RF circuitry compatible with microcontrollers, you can check out my ModularTuner automatic antenna tuner project:

    You might want to consider a github repository for your project, so that you could have revision control, as well as put documentation, schematics, and other useful materials there.

    I was considering making a stepper/servo motor controller with RF isolation using the L298N full-bridge ICs for the ModularTuner so that I could use it to tune a capacitor for a small transmitting loop. If you want help on how to isolate your RF from the microcontroller let me know. It isn't difficult once you know how but I had to make all the mistakes before I got it right. :) Hopefully what I have done can serve as a working example to those trying to do it in the future.


    DL3VB and VE3CEN like this.
  6. DL3VB

    DL3VB Ham Member QRZ Page

    Nice project, i own an AMA41 and also think about to build a automatic tuner for this antenna.
    If i found time i give it a try

    73 DL3VB
    VE3CEN likes this.

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