so ive been sitting on this idea for a while now and finally started messing around with it last weekend. basically i have a 4-port antenna switch that i have to manually flip whenever i want to go from my 40m dipole to the vertical or the 2m/70cm colinear. its fine most of the time but when im trying to do anything remotely automated like an unattended APRS gateway or scheduled beacon transmissions it gets annoying real fast.

started looking at using an arduino uno with a relay board to handle the switching, triggered over serial from the shack PC. the logic isnt complicated, just need to make sure i dont accidentally key the radio while the relays are mid-switch. thinking maybe a 50ms delay after relay actuation before i allow PTT but not sure if thats enough depending on the relay type. using some cheap SRD-05VDC-SL-C relays off amazon which i know arent ideal but its what i had.

has anyone actually built something like this and if so did you run into any RF getting back into the arduino and causing weirdness? thats my biggest worry right now since the coax runs pretty close to where the arduino would sit.

yeah i did almost exactly this about two years ago, except i used a mega because i wanted more IO for future expansion and honestly the uno would have been fine. the RF getting into the arduino is a real thing, especially if youre running any real power. what saved me was just being really religious about bypassing - 100nf caps on every power pin, ferrite bead on the USB cable coming from the PC, and i put the whole arduino in a small metal project box with a grounded lid. after that it was rock solid even running 100w on 20m with the coax like 6 inches away.

the 50ms delay is probably fine for most mechanical relays, the ones you mentioned should be well settled by then. i actually used closer to 100ms just to be safe and added a small LED that shows when the switch is in a safe state. the serial control is nice but i eventually moved to controlling mine through a raspberry pi running flrig so i could tie it into the whole station automation thing. if youre planning to go that route anyway might be worth thinking about whether the pi is a better brains for this from the start.

ferrite ferrite ferrite, cant stress it enough. i learned the hard way that arduino projects in the shack are basically RF noise sponges until you treat them right. slap a clamp-on ferrite on every cable coming off the thing and youll probably be fine. also make sure your ground braid is actually going somewhere and not just floating.

tangentially - if you havent looked at the hamlib library for controlling this stuff from a PC it might be worth a look, theres some hooks in there that people have used for antenna switching logic tied to band changes. not exactly plug and play but the pieces are there.