You didn't say if the Ethernet was 10MHz, 100MHz, or Gigabit. It really doesn't matter because you probably won't find anything useful here since it cannot be done simply or cheaply in a place with limted parts availability by someone with limited knowledge and experience. I've been doing this for a long time and I'm not certain I could do it.
You'd need to start with a pair of RF transformers. Unless you have the time and the test equipment (Vector Network Analyzer), you're not going to be winding your own. Then you have decide how you are going to pass signals in BOTH directions. You could always put in a pair of controllers and a CPU so you could make a store and forward unit, but now you're talking big bucks for emulators and software development tools.
If ya need one, buy it. If you don't have the money, and you have no realistic prospect of saving the money in reasonable time then it seems to me you have bigger problems than putting repeaters on you network. You need find a cheaper hobby like growing food and raising livestock.