I'm not sure there is a specific answer to the question, Dave. It depends so much on the antenna placement and the orientation of the copter. As you know, I use Bluebeams and I've certainly gone up to 400m line of sight, but when I yawed the copter around, on occasion, I'd get a loss of signal beep from the MX20 -- but if I yawed back, the signal would return. I suspect that there are other factors too -- like the amount of radio frequency interference in the 2.4Ghz band. For example, do you have a low-pass filter installed on the 1.2Ghz video downlink -- assuming you're using one? I've been meaning to put a Cinestar up on a wooden picnic table and then walk away from it testing my ability to control it from the MX-20 (doing something benign like switching the FPV input) and getting some idea of what the actual range was -- but even that's not a very realistic test as the Cinestar's so close to the ground -- and in-flight radio transmissions go much further -- but it would establish a lower bound to the distance. I could reasonably expect that the airborne range would be more than the table-top-based range. Andy.
No need for that. Only for 1.2Ghz to allow that frequency through, but attenuate the first harmonic which is 2.4 Ghz. Andy.