If you are interested in getting maximum range from Ronja, you should first know the limitations of nature that limit the range in Ronja. When a bad weather comes, the signal drops exponentially. That means that every kilometer eats out a fraction of the signal. If say a kilometer eats out half of the signal, then after 2 kilometers you have only 1/4 of the signal and after 8 kilometers you have already only 1/256 of the signal
This unfavourable mathematical behaviour is called exponential decay and it's what limits the range of Ronja. Even if the signal were made much stronger, the range would be increased in adverse conditions only by few kilometers.
The nominal range of red Ronja is 1.4km and the infrared one 1.25km. If you want to increase, you can't do much. You can increase range by building 2 or 3 transmitter heads, hooking them up to the same cable and pointing the beam into the same spot. But not much, you can get to 2.0km with this. Here is a detailed discussion of the range with exact calculations and graphs.