|I'm not sure exactly what they use but it would have to be some form of a media converter.|
I'm not sure why you think you want to use coax to be completely honest.
Any kind of cabling, be it coax, Cat5/6 or fiber, would have to be physically run between buildings. This means a lot of work and if you want it done right, digging trenches and buying conduit, etc.
A simpler solution would be wireless. To save yourself a lot of work and extra expense you could use point-to-point antennas between buildings. Or, you might be able to get away with a couple of well located omnidirectional antennas to service the whole property.
Deciding which would be the most effective means would require more in-depth knowledge of the size of the property and distance between buildings etc.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.