so ive been sitting on this idea for a while now and finally started putting something together. basically i have 4 antennas — a 40m dipole, a vertical for 20/15/10, a 2m yagi, and a homebrew magnetic loop for indoor use — and im sick of physically going behind the radio to swap coax or flip the manual relay box i built like 3 years ago. it works fine but its just annoying especially during contests or when im doing some quick digital work and switching bands constantly.

started sketching out an arduino mega setup with a bank of relay modules, some basic SWR sensing from a directional coupler i pulled off an old Kenwood tuner, and the idea is to have a little 16x2 LCD showing which antenna is selected plus forward/reflected. nothing fancy. the arduino would talk to the PC over serial and i wrote a small python script that listens to the CAT interface on my IC-7300 and automatically suggests the right antenna based on the operating frequency but i still want manual override on the physical box.

the part im stuck on is the relay isolation. using those cheap 8-relay boards from amazon and the RF bleedthrough between channels is probably gonna be a problem at HF even with the relays open. thinking i need to either add some grounding on the unused ports or swap to actual RF relays but those get expensive fast. anyone dealt with this? or is it not as bad as im making it out to be?

the bleedthrough thing is real but honestly at HF with 100w or less its probably not gonna kill you unless youre running a really critical setup. i built something similar with an UNO and one of those 4-relay boards and ran it for about 8 months without any issues i could notice on-air. the bigger problem i had was the relay coils generating noise that got into the audio — had to add some flyback diodes and a small cap across the coil supply. the cheap boards dont always have adequate snubbing.

for the RF side if you're really worried just add a coax stub or a series choke on the unused ports, 50 ohm termination on the open ports helps too. but if youre just doing casual HF operation id just test it and see before spending money on proper RF relays. those surplus RF relays are great but finding ones rated for your power level at a decent price is a whole thing.

the CAT integration sounds really clean by the way, been meaning to do something like that with my 7300 too. what library are you using for the serial communication on the python side?

ran into the exact same relay noise issue last winter. ended up just buying like 6 of the TE connectivity RF relays off ebay, think they were PE4259 or something similar, not cheap but not crazy either and they're actually rated for RF. the arduino side of it is easy, the RF plumbing is where you spend all your time.

also if youre using a mega you might as well throw a small ethernet shield on there and ditch the serial/python thing entirely — just serve a basic webpage from the arduino and hit it from the shack PC or your phone. i did that and its way more flexible, can pull up the antenna selector from anywhere on the network. took maybe an afternoon to get it working with the wiznet library.