Ca Ca.

How can you tell, by the test described, that you have found a "dead spot", if your radio is 400 ft away and you have the model? There would be no commands transmitted to the servos except the neutral carring wave ("don't touch it"), which don't necessarily move anyway when a signal is lost unless you have PCM settings. Nothing was moving before you enter this mystery spot, and nothing moved while in it, so there is no proof.

The signal strength meter test maybe - so don't fly at 4 ft off the ground at 400 feet and you'll be OK. Iron is a very common element in the soil and may interfere.

Luckily, I keep my planes moving when I fly so maybe this is Earth's way of limiting 3-D fliers. Nature is amazing.