I have a feeling that would pick up noise
If the connections are solid it wouldn’t pick up any more noise than a 6.5mm to RCA adaptor would have. Any jiggle to the cable would result in popping or cracking though, depending on the configuration could be quite jarring.
I think it does act like an antenna a little, so some noise is to be expected. Might be easy to attenuate that band though.
The antenna looking piece of wire is the ground so it doesn’t matter if it picks up anything.
No more than a standard connection. Every cable inherently acts as an antenna, so that’s why we try to avoid running them parallel to power lines and other things that would give off audible interference. If you actually want to reject interference, you’d need a balanced signal. Regular RCA and 1/4” are both unbalanced, so they’ll both pick up interference regardless of how they’re tied together.
When dealing with unbalanced cable, the most important part is making sure your signal to noise ratio is good. If you can get a hot enough signal that your gain can be lowered, you may be able to reduce the interference completely below your noise floor. Of course there are arguments against this (like how running things that hot could potentially mean you’re clipping your outputs, which introduces a whole host of other issues) but as a general rule, you want your gain to be as low as possible, so you can reduce the amount of background interference and noise you’re picking up.
