so ive been messing around with this idea for a few months now and finally starting to put it together. basically i want the arduino to read SWR from my MFJ meter via its analog output and then drive a stepper motor to tune my screwdriver antenna automatically. like a proper auto-tuner but one i built myself because i learn better that way and also the commercial ones for mobile HF are kinda expensive for what they are.

i have an uno right now but honestly thinking about switching to a nano for the final build since space in the car is tight. the SWR sensing part seems straightforward enough, im using a directional coupler and just feeding the forward and reflected voltages into A0 and A1, dividing them down with a voltage divider so i dont fry the ADC pins. the stepper control is where it gets a bit messy because i need to know absolute position and the motor doesnt have feedback so im thinking a rotary encoder on the antenna shaft itself.

has anyone done anything like this or ran into problems with noise from the RF getting into the arduino while transmitting? thats the part that worries me the most honestly. my shack is a mess of grounds and i have a feeling its gonna be a nightmare to debug.

yeah RF getting into the arduino is absolutely a real problem, been there. i built a similar rig for a remotely tuned loop and first time i keyed up on 40m the thing just reset constantly. what helped me most was ferrite cores on every wire going into the arduino, like every single one, and also keeping the sense lines as short as possible. i also put the whole arduino in a small metal enclosure and tied that to chassis ground, made a huge difference.

the encoder idea for position feedback is solid. i used a cheap 600ppr optical encoder off ebay and it worked fine once i debounced it in software. one thing to watch though — if your stepper skips steps under load the encoder wont save you, youll lose sync. might be worth oversizing the motor a bit or at least testing it under the actual load the antenna mechanism puts on it before you commit to the design.

also on the nano vs uno question, nano is fine, ive used both. just annoying sometimes that the nano clone boards need the old bootloader driver on windows but thats a one time thing.

RF noise into microcontrollers is no joke. i tried something similar with a raspberry pi zero for a different project and the thing would just hang randomly when i was running more than like 20 watts on 20m. ended up having to put so many ferrites on it that it looked ridiculous but it worked. the pi was probably overkill for what i needed anyway.

for your SWR sensing have you looked at the AD8307 log amp chip? lot of people use that for directional power meters and it gives you a nice clean log-scaled output that maps really well to dB which makes the math on the arduino side a lot easier than trying to calculate SWR from raw forward and reflected voltages. just a thought, might simplify your firmware a bit.