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Automatic Magnetic Loop Antenna Controller

 - Under Construction -
1.  Introduction

    a) Initial Considerations

    This controller has been designed to automate the control of magnetic loop antennas.  The reader is encouraged to read through the information on this website to determine if he or she undertakes this project.  There are several concerns you need to evaluate:  
    b) Magnetic Loop Antenna Experiments
    For several years now I have been experimenting with Magnetic Loop Antennas at this QTH.  In the beginning, I tried octagonal 3/4" copper loops with homebrew ‘trombone’ capacitors wrapped in Teflon tape and driven by a surplus screwdriver motor.  While they worked OK, they were a real nuisance to peak after QSY’ing.  

    After this, I built a similar loop that used an MFJ butterfly capacitor, driven by the very same motor that MFJ uses in their magloops.  The motor was controlled by a microprocessor and an SWR bridge designed to automatically sense resonance.  This semi-automated approach worked better, but experienced difficulty hitting the ‘sweet spot’ as the analog motor – controlled by a M/P generated PWM  signal – would often drift past resonance, and the controller would keep hunting back and forth over it.  Operation was not always consistent.

    Finally, at the suggestion of Byron – WA8LCZ – I used a stepper motor in conjunction with the MFJ butterfly capacitor and programmed the controller to automatically hunt for resonance.  The microprocessor driven controller – when activated – will perform the following steps:

    If desired, the capacitor can be rotated manually, at either slow or fast speed.  Resonance will be noted by the marked increase in background noise.

    The stepper motor approach has proven itself to be much more accurate than the analog motor, for the following reasons:

    The current design works best with butterfly capacitors as it’s very difficult for the controller to determine in which direction to rotate the capacitor unless the detected SWR happens to fall within the capture range of the Directional Coupler (described later).  If the SWR is 4:1 or less, then the controller will move the stepper in one direction while noting any increase or decrease in SWR, adjusting accordingly.  However, if the SWR is higher, then the controller will keep moving in the initial direction while ‘watching’ for a decrease in SWR.  Once found, it will ‘zero in’. 

    With a butterfly capacitor (rotatable over 360 degrees), this is no problem.  I’m still trying to develop an efficient remote phase detector to ‘tell’ the controller in which direction to move the capacitor. The best I can offer for those wishing to use a vacuum variable, for example, is a ‘limit switch’ type of sensor at the capacitor itself which, when activated, will alert the controller to change direction.  The stepper motor design is more complex than conventional analog motors (as used in the MFJ Magnetic Loop design and in homebrew loops) where the end of tuning travel interrupts the current feeding the motor via the limit switch.  This current ‘interruption’ followed by a polarity reversal will not work with a stepper motor as the controller needs to reverse the stepping sequence.

2. Purpose

    This website will not go into any exhaustive details about how to build magnetic loop antennas as there are many excellent sites written by authors much more experienced than me.  For anyone who is interested, however, I will briefly describe the magnetic loop I'm currently using with the intelligent controller should anyone care to replicate this design.

3. How The Controller Works - View the Schematic Diagram

    The controller itself is driven by a Motorola MC68HC908QY4 flash programmable microprocessor (M/P).  The controller has been programmed to move the UNIPOLAR stepper motor in either the forward or reverse directions using the half-step mode.  The simplicity of this approach is that no additional stepper motor interface chips (aside from the 4 TIP-122's needed to drive the motor itself) are required.  All the "stepping' code is within the M/P.  

    However, one potential drawback is that the microstepping offered by the other more complex driving arrangements (with special purpose chips and software overhead) is not available in this design.  Rather, the builder may have to use an external reduction gear at the capacitor itself to achieve the degree of resolution to automatically tune the loop close to 1:1 SWR.  I used a reduction gear in my loop.

    There are just 3 switches on the controller, and 2 (normally open) push buttons.  
    On power up, the M/P initializes and goes to the WAIT mode.  When in the AUTO mode and when either of the UP or DOWN Tune buttoms are pushed, the M/P will apply ALC voltage, activate the SSB tuning tone and then key the transmitter.  If, after 250 ms, no RF is detected, the M/P will release everything.

    If RF is detected, the M/P will examine the SWR provided by the external Directional Coupler or SWR Bridge.  If the SWR is 4:1 or less, the M/P will step the motor along for a couple of steps and then re-examine the SWR.  If found to be less, the M/P will keep moving the motor in the same direction, hunting for resonance.  If the SWR has increased, the M/P will reverse the direction and keep hunting.  A front panel LED shows motor direction.

    When resonance has been found, the M/P will release.  The user has an additional 2 seconds to use either the UP or DOWN buttons to fine tune the loop if the absolute 1:1 point has not been found.  Sometimes this happens.

    In the MAN mode, the M/P will work in either the FAST or SLOW modes.  The FAST mode is useful in finding near resonance by listening to the receiver noise / or actual signal.  In the FAST mode, the transmitter is not keyed.

    The SLOW mode is useful for fine tuning the transmitter as the M/P will first apply ALC, key the transmitter (etc) for as long as either the UP or DOWN buttons are depressed.  So, for example, an amateur could use just the MAN mode to fine tune a mag loop antenna without resorting to the AUTO approach.

4. Construction of the Intelligent Controller - Parts Inventory

    The ideal construction method is a printed circuit board (PCB).  Thus far, I have designed a PCB (about 3.5 x 4 ''), but have yet to submit it to the PCB manufacturing facility, mainly because:
Please Note: The breadboarded design picture shown on this website is more complex than necessary as the board mounted switches and the extra components are needed for the development phase and to program the M/P from a personal computer.
    If the amateur community expresses any interest in P/C boards, then I'll undertake its manufacturing so long as all of the per board manufacturing and shipping costs are covered.  I've had boards made before for other projects and the per board cost usually came out in the $18 / $22 range.

a) Fully Automatic Operation - Sensor Circuit Required

    For fully automatic operation, the SWR sensing is required.  The primary benefit of remoting an SWR sensor from the controller itself is that, in addition to removing RF from the M/P, the interconnection coaxial cables (from the transmitter to the sensing device and from the sensing device to the loop itself can be placed on the floor, away from the equipment desk and connected to the controller with a shielded 2 conductor cable.  The controller itself - while small - can be placed in a convenient space on the desk with the UP / DOWN push buttons mounted in a small plastic enclosure anywhere on the desk. Taking this one step further, the AUTO / NORM and FAST / SLOW switches could also be mounted in the same enclosure.

    Two types of remote SWR sensors will work with the loop controller if automatic tuning is desired.  The first is a directional coupler, and the second is a type of SWR Bridge.  I have personally used both.  The directional coupler was purchased from W8DIZ and modified with higher wattage 51 ohm resistors.  It comes complete with a P/C board, requires no adjstments and is easily built / installed.  

    The SWR Bridge type uses a single bi-filar wound toroid, a variable capacitor and a handhul of parts, is constructed on a small piece of (Radio Shack) perf board and requires a single (null tune) adjustment.

b) Manual Operation

    If only manual operation is required, then the remote directional coupler is not required.  Furthermore, other features may be omitted, like the SSB transmit Tone, ALC provision, etc, etc by not  installing the components shown on the functional schematic and in the 'Chinese menu' parts listing.

c) Breadboarding the Loop Controller Circuit

    Any builder experienced in breadboarding construction techniques could whip up a suitable controller using the parts shown on the attachment (some of which may readily be found in his / her junkbox).  I can provide a programmed M/P to complete the package.

5. Choice of a Suitable Stepper Motor

a) UNIPOLAR with a Small Step Size and (Possibly) a Reduction Gear

    This is perhaps the most challenging part of the process.  Ideally, you should look for a UNIPOLAR motor (BI-POLAR will NOT work in this design) with a small step size - say .9 degree.  This means that the motor will have 360/.9 = 400 steps per revolution if used in the full step mode.  If used in the half step mode, then each step will be .45 degree, and the motor will have 360/.45 = 800 steps per revolution.  

    When fine tuning a mag loop, you need the finest resolution possible to come close to a 1:1 SWR.  While 800 steps per revolution might be enough for the tuning of a vacuum variable capacitor, I didn't think that it would be fine enough for a butterfly capacitor which goes from min to max capacity in 90 degrees, or in 200 steps in the half-step mode.  So, when mounting the stepper motor on the capacitor, I interposed a 10:1 Jackson Brothers reduction drive and it works like a charm with my 20 foot, octagonal, .5 inch copper loop.  I'll try to provide an XCEL spreadsheet to better describe these requirements.

b) Low Current Drive
   
    When looking for stepper motors, I 'lucked out'  when searching the All Electronics website.  I found a NEMA 17 motor with a .9 degree full step that required just 450 ma per phase (you use both phases when half-stepping) - nicely satisfied by a 12 VDC supply.  So, I use a 1 amp Radio Shack "wall wart" supply to power both the controller and the stepper motors together.  In normal operation, neither the supply nor the TIP-122's get warm and no heat sinking is required.  Later on you'll find some additional reading / research material on stepper motors.   

6. Building the Magnetic Loop Antenna

    Since every ham has his or her own construction methods, I'll show you 'mine' if you promise not to laugh (hihi).

        a) Copper Loop Construction

    First, I constructed an octagonal magnetic loop antenna using 20 feet of copper tubing and 8 - 45 degree elbows purchased at Home Depot.  I cut the tubing  into equal lengths, cleaned the joints and soldered it up.  Nothing really special here, right??

    Next, I mounted the stepper motor onto a piece of spare aluminum that I had from a small Radio Shack project box.  I used 3 of these sheets, one to mount the motor itself, one to hold the reduction gear, and the last one to bolt the whole thing to the MFJ variable capacitor.

    Here's how the capacitor assembly looks all bolted together.

    Next, the 'whole thing' is placed within a 4" PVC drainpipe which will remder it waterproof when the top is placed on it, as shown here.  

Note: There is a 4 inch cut in the top member of the octagon to allow connection to the variable capacitor.

    That's Home Depot 5 conductor thermostat cable hooked to the stepper motor.  Finally, the capacitor assembly is mounted on the top of a PVC mast, and the top copper member is opened up so that each end of the loop may be connected to the top of the capacitor with heavy gauge wire.  Not very pretty, but it is waterproof and it does work well.

        b) Feeding The Loop - A Toroid Seems Better

    For feeding the loop, I've used both wire and Faraday Loops.  By adjusting the coupling loop, it's possible to get a very low SWR on several ham bands (e.g. 40, 30 and 20 meters with a 20 foot loop).  Recently, I replaced the wire coupling loops with an FT 240-43 toroidal core.  The toroid is placed one the lower leg of the loop, shimmed and centered in place with a short length of PVC pipe.  The coax from the transmitter terminates in a 3 turn link (of #14 enameled wire) wound thru the toroid (see picture).

     In my experience, this feeding method is preferable as the loop seems somewhat quieter in the receive mode and presents a very near 1:1 match without any manipulation (as was common with the wire-type coupling loops).

7. Testing It Out - Verify the Stepper Motor Operation First

    a) Identifying the Stepper Motor Leads

    If you decide to 'encapsulate' your capacitor / reduction gear / stepper motor assembly in a waterproof container,  verify its operation with your controller before enclosing it.  Accordingly, connect it up to your controller, referring to the manufacturer's data (if any).

    Your surplus UNIPOLAR stepper motor may have 5 leads, but will generally have 6 or even 8 wires.  The wires for the 6 and 8 lead varieties exit the motor in either 3 or 4 lead 'bundles', respectively.  UNIPOLAR steppers have 2 coils per stator pole.  
    Please check out the following information to determine how to connect the power, and the A, B, C and D leads to your magnetic loop controller.  As long as you have correctly identified the power lead(s), and the respective stator pairs, the worst that can happen when they are connected to the controller is for the motor to 'quiver' when either the UP or Down buttons are depressed in the Manual mode.

    Once you have the motor 'poled up' correctly, ensure that there is no binding of either the capacitor or the reduction gear (if used) when operating in the FAST mode.

    b) Interfacing with the Stepper Motor

    Connect the coax directly to your magnetic loop antenna.  Do not connect the KEY / PTT, ALC or SSB tone leads at this point.  Then, in any amateur segment for which your loop is designed to operate, peak up the received signal in the MANual mode, FAST speed by pushing either the UP or DOWN buttons.  If  you can, the motor is working properly.  Verify that the REV LED changes state when the direction of rotation is changed.

    Place the radio in the CW mode, reduce the RF power as low as it will go, switch the controller to the SLOW mode, depress your key and watch the SWR bridge as you hit either the UP or DOWN button to reduce the SWR to its lowest point.  If you see a dip, but not close to 1:1 and if you're using a wire loop / Faraday Shield, remove the RF and adjust the position of the loop.  Then, try it again.  If your wire coupling loop is 1/5th the size of the main loop, you should be able to get very close to 1:1 by adjusting the coupling loop shape (i.e. 'squashing' it a bit from a circle to somewhat of an oval).

    You may now consider your loop adjusted.  If desired, you can use the controller as it is, in the purely manual mode - it works pretty well.  You may also decide to activate the ALC and PTT / KEY circuits to save you the time and effort of changing modes / reducing power when tuning up the loop.  But, if you want totally automatic operation, then you'll have to build a suitable RF sensing arrangement, as shown below.

    c) Testing the CW Keying Interface

    Connect the KEY leads from the controller to your radio.  If you are using an external keyer, you may have to 'tap in' on the output side of the keyer. 

    Place the radio in the CW mode, reduce the RF power as low as it will go, switch the controller to the  MAN / SLOW modes, depress either the UP or DOWN button and verify that the radio keys and outputs a low RF level.  Release the button and verify that the radio stops transmitting,  By watching the SWR meter, 'toggle' either the UP or DOWN button (as required) to achieve a low SWR and then release the button.  

Note: If you use CW most often, if you don't mind adjusting the RF power, and if you don't need automatic operation, you're done!

    c) Testing the SSB PTT Interface

    Connect the KEY leads from the controller to your radio.  Using a shielded cable, connect the TONE lead (and ground) to the SSB mike input.  Most of the newer radios provide a rear connection for this purpose and make these leads available in a DIN jack.  If not, then you'll have to make the connection at the microphone plug.  Place the radio in the SSB mode, switch any compression off, and switch the controller into the MAN / SLOW modes.

    Turn down your radio's RF power to its lowest level. Next, switch the controller to the  MAN / SLOW modes, depress either the UP or DOWN button and verify that the radio keys the PTT function.  Adjust the SSB TONE LEVEL control in the controller to produce a few watts of RF output with your mike gain set in the normal operating position, and verify that you can tune the loop through resonance.  When done, release the button..

Note:  you don't mind adjusting the RF power when tuning up, and if you don't need automatic operation, you're done!

    d) Testing the ALC Interface

    Note: This is an important test as connecting an improper ALC voltate to your radio may damage it.  

    Don't connect the ALC lead to your radio yet.  Remove the magnetic loop antenna and connect a dummy in its place. Set the CW mode.  Set the controller to the MAN / SLOW modes, and connect your voltmeter to controller's ALC output lead and ground.  Set the voltmeter to read a negative voltage.  

    Hold down either the UP or DOWN button and check the negative voltage produced.  Vary the ALC LEVEL ADJ control (R9) to both extremes, noting the maximum and minimum voltage levels.  If the maximum negative ALC voltage produced exceeds what your radio can tolerate (check your owner's manual), then you'll have to change Zener diode D10 to a lesser value.........

     If the ALC voltage level does not exceed the limit defined in your operator's manual, then power down both the radio and the loop controller, and connect the ALC line and ground to your radio.  Power up the radio and then the loop controller. Be sure that the dummy load is still connected.

    Put the radio in the CW mode, operate the MAN / SLOW switches on the controller, and depress either the UP or DOWN button while noting the RF power delivered to the dummy load.  Turn up the power on the radio to max output and adjust R9 until the transmitter's power drops down to its lowest level, around 1 or 2 watts.  Then, release the button.

    If you do not see the need for automatic loop antenna tuning, and are content to tune the loop manually (in the FAST mode for maximum noise and then in the SLOW mode for fine SWR tuning), you are finished.

    If not, move on to the next step which is connecting and aligning the remote directional coupler or SWR bridge.

9. Connecting and Aligning the SWR Sensor

    Two types of RF sensing arrangements can be used, either a fixed-tuned Directional Coupler, or a more conventional SWR Bridge type of arrangement, which requires a simple 'null' adjustment.  These sensors will send low level DC (FWD and REV) signals to the loop controller board which, in turn, will compute the actual SWR and move the motor / capacitor to resonance.

a) Building, Adjusting and Using the Remote SWR Sensors
10. Using the Automatic Mag Loop Controller

    a) Automatic Tuning a Butterfly Capacitor

    If you are using the controller with a butterfly antenna that's free to rotate 360 degrees, tuning up is very easy.  Just push either the UP or the DOWN buttons with the controller in the AUTOmatic mode and the tuner will do its thing, provided, of course, your antenna has been proven to resonate in the band you are using.  When the controller stops tuning (and your radio returns to the receive mode), you have an additional 2 seconds to 'fine tune' your antenna with either the UP or the DOWN buttons.  If you wait longer than 2 seconds, the controller will go into another automatic cycle.

    b) Manual Tuning a Butterfly Capacitor

    You can tune your loop in the MANual mode using either the FAST or SLOW buttons.  Pressing the FAST button simply moves the stepper motor in the fast mode - without keying the transmitter.  This is a useful way to peak your received signals while tuning around the band, while SWL'ing, etc.

    The MANual SLOW mode will key the transmitter, apply ALC, generate an SSB tone, etc. and is useful for fine tuning your antenna to the point of the lowest SWR.

    c) Automatic Tuning a Vacuum Variable or Similar Capacitor

....more to follow...

11. Interesting Magnetic Loop Antenna Websites and Related Articles