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What a great thread this has developed into!? Several good options for the "MFJ-1786 Upgrade".? Here is some "stream of consciousness" thinking for my situation, which might help others ...

CONTROLLER:
1) Jack/Al's "Double-Double" - Well published (tnx for extra article Jack), good field experience, mine has been half-built for a while;
2) Loftur's Controller -- Successfully used here with 10'-dia 2" heliax 80/40 loop, can readily unplug-and-try with MFJ;
3) Greg's Pico Controller -- Evolution of Double-Double on the fun Pico processor, already started here.

"TRANSPORT":
1) Dedicated Cables / CAT5 (Ethernet) -- As Greg says, straightforward to do 'natively' as extra cable;?
2) Wireless -- Bluetooth/WiFi eminently doable within reasonable distance (I look out my lab to backyard antenna garden) but requires hefty battery for driving stepper, will need recharging but not so bad with externally-plugged battery, and needs wireless receiver/driver module at antenna for driving steppers ... available? new design?

All Things Considered ... I will finish off the Double-Double and the Pico controllers, posthaste.? I don't want to take the Loftur controller (80/40) out of service and there's time with the mods needed for the MFJ loop.

My MFJ loop also is open (disassembled) out on the woodshop workbench and I see that exchanging a NEMA 17 stepper for the existing DC motor should be quite straightforward, even without special brackets.? Merely needs a larger hole for the drive axel and new mounting holes for the flange.? Note that the other components on that phenolic pcb holding the motor comprise the "bias-T" decoupling that allows the MFJ controller to send + and - DC on the coax to turn the motor accordingly.? (Also note that this is the reason that most guys burn out the controller by *not* using the requisite isolated 12V DC AC adapter.)?

// Related Aside:? Cool reference article on the techniques being discussed here ... QEX for Nov/Dec 2015, "Using an Arduino to Automatically Tune an MFJ-1788 Magnetic Loop Antenna and Elecraft KX3 Transceiver."? And BTW, it works pretty well!

Random Thoughts ...?

- No 'tension spring' will be necessary on the MFJ capacitor once a stepper is being used.? It will readily stay in place when the system is not being tuned.

- As someone said, "NEMA 17" prescribes the size of the stepper, although most available to us hobbyists have more than adequate torque for turning a butterfly cap (light load) or a vacuum variable cap (VVC, heavier load).? Just need to be prepared to have the DC drive source supply 3-5 amps for reliable drive.

- BT/Wifi receiver/drivers probably do exist for stepper control.? Will be interesting to research this and experiment.

- IMHO, graphical representation of the system's SWR while tuning (e.g., via the Double-Double) is essential.? For years I've coupled a little SARK-110 into my makeshift antenna systems for this purpose and it's worked quite well.? So much better than watching an SWR number go up and down; or worse, just letting the system take care of it.

- With my MFJ loop open, I noticed that the internal coupling loop was a different shape/orientation from what Greg (and others) had pictured with theirs.? While this loop is normally okay being small, I've found that it takes experimentation to optimally place it inside the main loop there at the feed point.? Perhaps MFJ does this during assy (naaah), but we can remember to tune this 'primary' loop shape and location for optimum performance when deployed.

- The metal joint/connections inside the MFJ loop were welded very well.? I ran some millivolt tests and determined each to be excellent, especially and importantly along the butterfly capacitor's stators. What I'm curious about, and may be incorrect, is that I thought that all the stacked blades+spacers along the rotor also needed to be welded for optimum 'zero-ohm' efficiency, as it is their combined capacitance that determines overall capacitance.? My rotor components are not welded, but I may have done overkill welding in past projects.

So that's my little thesis.? Hope I didn't bore you all with detail but it's been cathartic getting my thoughts and approach on paper.

73, George N2APB? ?

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Dr. Bill and Greg,

Bill, your email prompted me to look at an MFJ antenna that was donated, and has been a couple years sitting in the old parts pile.?
I got it out today and took it to our weekly lunch with the Club Members at a local State Park. Internally the Circuit Board is marked 1784
And that corresponds to a 15-40 meter remote controlled antenna.?

Our club has developed an interest in making loop antenna's and controllers.?

And Greg, your mention of the Loftur Antenna Controller that uses a Teensy 4.1 to control it.?


I would be interested in replacing the motor with a NEMA Stepper motor.
And I would be interested in building a Loftur Loop antenna controller.?

I will have to start looking for another one just like it. I would like to put a pair of them 90 degrees offset and experiment with a pair of Hermes (HL2) in coherent mode.

..jpw J P Watters

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Hi Luc-

I think I understand now.? You are referring to Q5 on the Power Amplifier board?? That circuit needs to change.
We had discussions about this problem in the early days of the T41.? I can't remember exactly how I modified the circuit.
However, I would suggest changing R15 to a 0 ohm.? Remove capacitor C12.? Then you will need to add a shunt 10 kohm
resistor from the Q5 gate to ground.? I believe that was what I did, and I believe others made similar changes.
There are other switching circuits using the BS170 transistor.? Use those circuits as a guide.? This particular circuit with Q5
on the Power Amplifier was designed differently, and it doesn't work well.? Thus the need to make small changes to the circuit.

--
73 Greg KF5N