2 devices, or perhaps just one that can be set to different modes, capable of sending signals and receiving them. The signals are pulses of EU, which lose strength over distance (I was thinking strength drops as the inverse square of the distance in meters). The receiver, set to a particular frequency (strength of incoming EU pulse) activates an attached object when it encounters a pulse of the right frequency.
So, an emitter sends out a 2000 EU pulse, and a receiver set to frequency 5, and 20 meters away, picks up the signal and toggles a connected device, say the transfer/store toggle of a partially charged MFE.
Multiple receivers within proper range to accept a particular frequency could be triggered by the same pulse. Three receivers 23, 24 and 25 meters from an emitter sending out 2000 EU pulses, all set to a frequency of 3 would all toggle, while the receivers that were closer or farther on the same frequency would not, as the pulse would have a higher or lower value at that distance.
Would be neat for radio controlled doors to prisons, towns, nuclear reactors and the like, or as a means of operating a sprawling facility from a central location. A string of such towers could be daisy-chained for making a fully automated setup (the last receiver toggles an emitter which pulses back to a receiver at the beginning of the line, starting the process over).
Complications could be added, such as height being a factor in the distance, or a signal traveling through other blocks loses power faster.
, and you shut that section of the city off 
