For plot reasons, I need to cut off a planet from all possible means of conventional electromagnetic communication with the rest of the universe. I would also like a impose a "knockback" on the local civilization's technological progress. The "problem" I've come up with is a Clarke's Law-abiding shell of "stuff" that hangs in the planet's upper troposphere, and generates a planet-wide effect very similar to an electromagnetic pulse but a) without the usual nuclear detonation, and b) the effect is constant, lasting unceasingly for at least a couple centuries.
My questions about this scenario are as follows:
Does the EMP effect being constant/recurring make a difference in terms of what devices it affects/how badly those devices are affected?
What is the absolute height of technology for unshielded electronic devices under such conditions?
Would this have any biological effects, for example interacting/interfering with normal bioelectrical fields?
Does this make the actual process of radio communication within the shell's EMP effect impossible, provided that hardened radio transmitters and receivers could be constructed? If it remains possible, does it reduce range/fidelity, and by how much? Is the effect similar for radar?
Thank you in advance!