Introduction: An Experimental Magnetic Loop Driven by Android and a Stepper Motor
So I am a big fan of Kevin Loughin's YT channel and also of Mag Loops having already bought an unwanted YouLoop on Facebook Market Place. Watching this video Ham Radio - Magnetic loop transmitting antenna overview and details inspired me to aquire some parts. The first issue for a Mag Loop is TX power, the YouLoop is rated for 250mW, not going to get me far, the secret to a good Mag Loop is a variable capacitor form an old valve radio. This will handle the sort of voltages involved upto maybe 25w. I am QRP at 5w so not big deal. Sure enough a silent-key (amateur operator who has died) son was advertising some assorted parts, one box contained a couple of air dielectric capacitors. I set off my my motorbike into deepest Staffordshire last Autumn and acquired them. One has a slight misalignment causing shorts (which I have since fixed), but the other was fine. I measured them with my multimeter and they are 100pf close each gang. This means I can set them to 50pf, 100pf or 200f depending on whether I use 1 or 2 gangs in series or in parallel.
I built a prototype using an Arduino Uno, but I wanted to have soldered connections, so I rebuilt in on a prototype board specifically designed for Arduino Nano (wider spacings in the centre) and installed a Arduino Nano Every that I had spare.
The full video is at https://youtu.be/pGmuCjrnVGE
Supplies
An Arduino Uno or Nano
Stepper motor with ULN2003 driver pcb
A motor coupling
Air dielectric variable capacitor from a valve radio
RG23 cable 5m
RG58 a few metres
multiple PL259 to fit both cables
multiple SO 239 sockets
a project box
Step 1:
So the intention is to build a Mag Loop development system where I can swap elements and do comparisons. So I will make everything pluggable or reconfigurable.
I also found through research that they YouLoop is too small to perform well on lower HF bands. SO I decided that my main loop should be started at 5m of RG213. The thicker RG213 should increase the bandwidth somewhat and I can always shorten it. This should give good results at 15m, 20m and hopefully be usable at 40m. I plugged some dimensions into a couple of online Mag Loop calculators
https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx
https://miguelvaca.github.io/vk3cpu/magloop.html
I am not done with these but I had ordered 5m of RG213 so that is my starting length, The Coupling loop should be around 1/10 to 1/5 of this length so again my starting point it 1m of RG58. For both I am initially going to use only the outer braid, but I may try other configurations, that is what I plan to test.
Step 2: Tuning
I do not intend to sit next to Mag Loop even transmitting a minimal 5 watts so I want to remote tune it. I have seen Kev Loughlin and others dabbing UP and DOWN buttons to tune the loop initially on receive, then fine tune on transmit. I hope to do better. They used a simple DC motor coupled to the capacitor shaft, this is ideal for simplicity. My concept from the beginning is to use a cheap stepper motor controlled by an Arduino (any Arduino but developed using an UNO) and instead of buttons use a Digital Encoder. This encoder usefully has a push switch. I will initially use this to regulate the step size so that we can do fast and slow tuning, with steps of say 1 and 10. But of course we can change every function in software. I would also like to have preset positions for each band and store them. This will require some non-volatile memory or an always-on PSU which I will look at later.
How to calibrate the tuning. Well, it is logical to fit limit microswitches to the tuning capacitor and use them as feedback. However my cheap ULN2003 5V motor does not object to being stalled then it is software to the rescue! We simply initialise by running the motor fully in one direction for 50% of one revolution, then run it 50% of a revolution backwards, it seems to work ok!
Step 3: Using
The initial config Initialises the capacitor position, then is ready for input from the decoder, the steps size will be set to 10 for RX tuning, then push the button and it is set to one for fine-tuning while SWR meter connected.
Cable 5m RG213 many sources, caution this is 10mm thick and needs special RG213 PL259 plugs.
3 x SO239 sockets.
An air dielectric variable capacitor, amateur rallies, Ebay, FaceBook MarketPlace. Mine is 100pf x 2.
Multicore cable
Project boxes.
Arduino Uno or Nano, other chips will work with code changes.
Digital Encoder, I got the 5 pin ones with built-in push button and knobs in a multi-pack.
Bracket (had to be drilled to suit my motor) https://shop.pimoroni.com/products/pololu-stamped-aluminum-l-bracket-for-nema-14-stepper-motors
5v Stepper motor with ULN2003 controller https://shop.pimoroni.com/products/5-vdc-stepper-motor-with-uln2003-driver-board
Shaft coupling 5-6mm https://www.ebay.co.uk/itm/144107765861?var=443409959839
