Cbus on coax

My home is wired with cbus. I want to extend cbus to my outdoor shed which is about 100ft away. I already have a coax cable run to the shed that is not being used. Is it possible to utilize coax for cbus communications?

 

They reckon c-bus could run on two bits of wet string. Would you do it though is another question with the answer most likely being no. With coax it may well work, but certainly not something c-bus was designed for and no one in Clipsal will say do it.

Best bet is to run some proper cable out there, maybe even use the coax as a draw wire.

 

C-Bus is a balanced system; twisted pair is balanced - each of the two conductors has similar electrical properties with respect to any external earth reference.
Coaxial cable is unbalanced.
To connect a balanced system to unbalanced, you usually need to use BALUNs (balanced to unbalanced transformers) at each end of the coaxial cable. A suitable BALUN for C-Bus might be a hefty common-mode choke, as you might find in the power supply of a PC or plasma TV. A common-mode choke is basically a 1:1 transformer wired with primary in series with one conductor and secondary in series with the other. This would then allow use of an unbalanced cable with reduced likelihood of interference. A common-mode choke with 100 milli-Henrys of inductance (or more) and rated for the line current should be suitable. You need to be mindful of the voltage drop in the choke and in the cable; some 75 Ohm coax cables have very small centre conductors.
Having said that, C-Bus would probably work fine over coax without any special treatment, but if you do this, you might hear a 500 Hertz 'buzzing' in audio equipment nearby, as the C-Bus communication signal can couple capacitively from an unbalanced cable when connected without a balun.