I have the antenna plugs but I am too lazy to go buy some coax cable. What would happen if I simply used some plain wire and soldered it on the plugs?
it might work poorly and it might work ok.
given a strong enough signal a paper clip will work as a tv antenna.
modern tv tuners are looking for a incoming impedance of 75 ohms which tv antenna coax has. when other values appear it doesn’t work as well. other wire types will have other values.
cat5/6 pairs have a Z0 near enough 120 ohms. If you parellel two of them (one signal, and one ground in each twisted pair) then you will have a 60 ohm transmission line that will probably work well enough for lots of things. The PVC dielectric will be significantly higher loss at V/UHF than the PE used in most coax though.