TEMPEST

It should be possible to jam your own emissions, but that same noise might cause be picked up by your own equipment as well and cause errors. Also remember that most of these signals are detected by correlation, which detects a signal even with a very high S/N ratio. So it's not obvious just how to jam. My first guess is to phase-lock onto your own emmissions and then broadcast random bits at higher strength. With E/M monitoring, just like cryptography, you don't really know how to make defenses unless you know how to attack. Eric