so i spent most of saturday running coax and getting my IC-7300 set up in the cab of my F-150 and overall it went pretty well but now when i transmit on 2m (using a separate radio for that, a Yaesu FTM-400) i get this weird buzzing in the 7300's audio. not sure if its RF getting into the power leads or what. both radios are running off the same fused leads direct to the battery.

the coax for the 2m antenna is a mag mount on the roof right now just temporary until i figure out where to put a permanent mount. ive read about common mode choke stuff but never actually built one. anyone dealt with this before? its really only noticeable when the 400 is transmitting on high power so i might just run it at 25w for now but id like to actually fix it properly

yeah this is pretty classic common mode current issue, happens all the time with mag mounts especially. the shield of the coax is carrying RF back into the cab and its coupling into everything. a few ferrite cores snapped around the coax right where it enters the cab makes a huge difference, i used mix 31 toroids on mine and it cleared up almost immediately. also worth checking that your ground braid between the body and the frame is good, sometimes trucks have lousy continuity there from the factory and it makes RF problems way worse in a mobile install.

the fact that its only bad on high power kind of confirms its RF getting in rather than some kind of audio ground loop. try the ferrites first before you go crazy chasing anything else

i had something similar in my jeep and it turned out to be the power leads running too close to the audio cables inside the dash. rerouting them helped a lot. also are the two radios sharing a common ground point at the battery or are they going to separate spots? that can matter more than people think