To connect COAX directly or via a Balun to UTP is Impossible!
UTP uses Duplex channels in two pairs.
Four if the eight wires are used.
Coax uses only two wires (Strictly one, the outer layer is an shielding)
How for god sake connect COAX for any reason directly to UTP??
Buy a "Linkbuilder" or a HUB with BNC AND UTP inputs.
Then (Still only at 10Mbit/s) you can communicate between the UTP leg and the Coax leg of the network.