The secret to sending analogue video signals over UTP is to balance them well. Video balun is a device made for this. I earlier write about Video over UTP and my own experiments related to it. Here is a look wat what is inside a commercially made inexpensive video balun, in this case LLT-201 from Dealextreme. That video balun works pretty well for carrying composite video over CAT5, CAT5e or CAT6 UTP cable (and also works on older telephone wiring on short distances).
Here is LLT-201 video balun tear-down:
Ad you can see inside the device there are just one special transformer, few resistors, one capacitor and one semiconductor that looks like diode (I quess that it some type of over-voltage protection component). The transformer is would on toroidal core and has three windings that look like to have same number of turns in then. To get idea of the transformer I also measured the inductance of one coil with my RLC meter and got reading 14 mH.
I looked at the circuit board and I draw a circuit diagram of the device based on that:
The two upper coils seem to form a current balun / common mode construction like what I described on my Video over UTP. The special thing seems to be on the third coil and the components connected to it.
I think that they could in some way tape part in some form of impedance conversion circuit (75 ohms to 100 ohms), but I am not completely sure of that. I tried to measure the impedance on the both sides of the circuit, but I did not see any significant impedance conversion: When there was 75 ohms on the BNC side, the twisted pair side was still closer to 75 ohms than 100 ohms. Anyway the balancing act seemed to work well.