so i've been messing around with an arduino mega for a few months now and i finally got to the point where i want to do something actually useful with it for the shack. the idea is pretty simple in my head — read the SWR from my antenna analyzer output, feed that into the arduino, and have it drive a couple of stepper motors to rotate the capacitor and inductor on my homebrew L-network tuner. sounds simple right

the problem im running into is the analog reads are noisy as heck. im using a directional coupler i wound myself on a FT-37-43 core and the voltage divider output feeding into the A0 pin, but even with a 100uf cap across the input and a small RC filter i built i still get like 30-40 points of jitter on the analogRead. tried averaging 50 samples and it helped some but when youre trying to detect a minimum in the SWR curve even small variations mess up the algorithm

anyone dealt with ADC noise on arduino specifically in an RF environment? im wondering if i need to shield the whole thing in a metal box or if theres something in the code side i can do. also considered switching to a Pi Zero with an external ADC over SPI but that feels like overkill for what is basically a PID loop

yeah the arduino ADC is honestly pretty mediocre, it gets worse when theres any RF around at all. what i did on a similar project was use the AREF pin with an external 3.3v reference instead of the internal one, made a noticeable difference. also worth looking at the ADC prescaler — the default is way too fast for accurate conversions, slowing it down to like 128 prescale and throwing away the first read after switching channels cleaned things up a lot for me.

the metal box thing is worth doing regardless tbh, even if it doesnt fully solve the noise issue. i run all my arduino stuff in old altoids tins or hammond boxes and it helps. your coupler placement relative to the arduino matters too, keep those leads as short as you can.

if you do end up going the external ADC route the ADS1115 is only a couple bucks and its 16 bit over I2C, way better than anything the mega's built in ADC can do. wouldnt call it overkill, its like $2 from aliexpress

i did something similar a while back, not exactly the same but close enough. one thing that bit me was ground loops between the coupler circuit and the arduino — had a separate supply for the RF detector side and forgot to tie the grounds together properly and the readings were all over the place. probably not your issue but worth checking.

the Pi Zero idea isnt bad but honestly for a control loop that needs to be fairly snappy the latency from linux scheduling can get annoying. had a Pi 3 running a rotor controller once and it would just hiccup randomly. arduino is dumb and simple and thats kind of the point for this stuff