I was using nimhs at 1.2v, so 4 of them was fine. But taking it account what you said about the linear voltage regulator, I'm now using 3 AA.

Thank you a lot for the schematics and all the explanations ! I followed it and it now works perfectly !
I did struggle a bit with the channels between the receiver and the transmitter not following the same order. But other than that, it all went very smoothly !

Thanks again Jeff ! I'll be in French Guiana in two weeks, I can't wait to test it in the field =)