I would agree with you except for one thing: This is what we strongly believe to be a lightbridge signal. And what we know about lightbridge is it travels over amazing distances. 80 meters is not a particularly long distance.
Also, since you have modded your controller with the argtek, when you disconnect that antenna, the cable that leads from your transmitter inside the controller to the coupling which you installed - that cable acts as an antenna. So to claim there is no antenna isn't quite accurate. What you really mean is you have a poor antenna which is the cable.
I suggest, if you're interested, flying out a mile or so, then disconnect each antenna and see what happens.
Thank you for your efforts...
Shielded cable is weak, very weak antenna actor. According to that and very low power output, 80 meters should be more than enough to see significant signal drop.