1. My receiver is a Sangean portable (I think the 800a), with an odd
(RCA-style) external antenna jack. I understand that the antenna
impedience should match the receiver's. Will 300 ohm TV wire soldered
to an RCA jack work? Should I be looking at 75 or 50 ohm cable? (The
300 ohm TV cable has the advantage of being already in the wall).


You may find that too much of an external antenna will overload the
receiver's front end, causing images all across the SW bands. Some portables
are notorious for this, though I'm not familar with the 800A.

Perhaps try with a long wire first, before doing all the work required to
install a T2FD.