Remote magloop capacitor tuning - a simple approach.

2018 for me has been the year of getting many little, unfinished projects completed at long last.

One of the longest-running campaigns has been to get a remote-controlled, motorised capacitor tuning for my various magloop experiments.

There have been moments when I felt like simply buying a commercial loop.   The very high cost of these loops (about £1200) could be offset with the mindset that you only ever buy a good magloop once.  But the problem for me is the likelihood for the proprietary loop tuning unit electronics failing, and proving to be difficult and/or expensive to repair.  I am not an electronics expert, at all!

It would therefore be silly to spend that much money simply to get a tuning unit that will inevitably fail at some point.  A magloop itself, despite the rubbish people write about them, are always a very easy thing to build to high quality, costing maybe £25 in new copper tube.

I've approached the problem in my usual way: what readily-available materials can be used to achieve cheap, effective and easily replaced or repaired tuning? 

The final protoype version appears below.  Of course the wiring hasn't been tidied up yet, but that is just a cosmetic thing.

Cheap tuning...

In brief, you take a 12V DC input into a DPDT switch that acts as the forward/reverse control.  Then the feed from this goes into (in my case) a discarded potentiometer, providing speed control.  One line from the output of that goes through a momentary push switch, which provides fine 'pulse' control as best match is approached.

The motor is a 3rpm unit from China, which is connected to a Delrin isolating rod via a semi-flexible reducing coupler (the motor shaft and planetary drive socket is 5mm, everything else is 6mm).  The rod is attached via another semi-flexible coupler to a 6:1 planetary unit. You can find all these things on E-bay.

The capacitor is attached to a plastic base, which is fixed with one screw at the top. This allows it to move a little, to line up properly with the motor couplings, although the alignment is actually already very good. 

I tested this in the garden last night.  Using an analyser I could see the simple system works really well, allowing the very fine control needed to tune these extremely high-Q loops without difficulty.  Moreover, it only cost, if you buy everything new, around £20 (couplers are the most expensive things, at about £5 each!)

BETTER SOLUTION!

I later tried a PWM motor controller for the amazing sum of £3 from China.  This is the best thing I've found in a long time!  It does much the same thing as my own idea, but the pot is much better ranged, and the two momentary buttons give instant, rather than switched direction changes.  As the father of a son with colour blindness, I'm also bound to say it's a shame that control makers continue to make red and green coloured buttons.

The PWM circuit is much better than my own version, because it provides much, much finer control of the motor position, and is as cheap or cheaper than it.

DC motor PWM controller.  Great solution for £3, delivered!

I have now tidied this up into a plastic box hand controller.  Battery input as banana socket at top, PWM output as BNC to right.