I'm a fresh I&C engineer, i want to know if there's a way to send 4-20mA signal to the substation which is 80km away from the field. I know there's wireless way and optical fiber way, anyone could explain which is better and cheaper?

 

i think fiber optic is better.

 

Considering the construction cost. If there's only several cables, is that really necessary?

or better to use wireless but which is not stable i think

 

Regarding to use Wireless technique, I suggest you to check the reliability of the system and the interference which might be happened.
Fiber optic would be a good choice, is there Network capability available over there ?

 

depends on the application.

Generally I would consider gsm/gprs for wireless or a dial-up/leased line if available. You can have both also for redundancy if you have availability considerations.

 

Unless you already have some installed wire or fiber Spread spectrum radio is the best way to go. You will probably need at least one repeater or more if you are in hilly terrain.

Another possibility is cellular if you have good coverage.

Roy