I have a Sony MHC-GX450 compact mini stereo system. The connector for the FM antenna is coaxial, and is labeled 75 Ω. I'm wondering if somehow I can use speaker wire for the FM antenna.
I have seen wire antennas for the FM band before. Such an antenna usually has two wires that are connected to twin-lead feed line, which then connects to a device called a matching transformer or a balun that looks like this:
So you could connect two wires to a device like that, and plug that into your receiver. The best way to arrange the wires is in a straight vertical line, with the matching transformer in the middle. The question is, how long should the wires be? I'd suggest trying about 1.5 m (59") total length for both wires. You could try trimming the ends of the wire a little at a time (don't trim more than about 10% total) to see if the reception improves, but it probably won't make much of a difference. The thickness of the wire doesn't matter much, the main thing is that it should be copper.
Or you could just go to a local store that sells electronics, or an online seller, and ask for an FM radio antenna with a 75 Ω connector, and hopefully they will have one for you.