You don't need coax. I do a LOT of my shortwave on ladder line, and that's for comms thousands of miles away.
No coax involved.
/johnny
The difference here that you are routing your signal with a bare wire into a Faraday Cage when you use a pressure cooker. That could make a difference - that is my reason for saying coax might be required. I once designed a 2 Ghz receiver on a circuit card that used a simple wire for its antenna. So I know that works just fine.
So I agree on the half wavelength antenna. What is the frequency here. How long is that wire? Pretty long if 45 MHz. Pretty short a 2 Ghz but that would be line of sight.