Dealing with IEDs
Just in case any readers have connections with DARPA or suchlike, I'll post an email I sent to the Marine Corps lab--
From what I hear, many of the improvised explosive devices use cellphones as detonators.
Most electronic receivers put out a radio frequency field, due to the necessity to "mix" frequencies up and down (it's often better to discriminate between signals at one frequency range, then take it back down before amplifying). Thus even a receiver transmits, albeit faintly. That's how police are able to have "radar detector detectors" that spot when a driver goes by with a radar detector operational
in his car. The police detector picks up radiation emitted by the driver's radar detector. That's also why the airlines make passengers shut down cell phones, laptops and ordinary radios during takeoff.
I don't know if cellphones radiate a more powerful signal, so the network knows where you are when a call comes through to you. They might keep track of your location, or might query the entire network when a call is received.
Might it be possible to build a cellphone detector? The range might be improved with a yagi or other directional antenna, which would also let you judge which direction is the cellphone.
It could be mounted on a remote control vehicle (heck, even one of the fancier remote control toy cars) to probe ahead, or to approach a suspicious package, in which event the range need only be a few feet. It could relay back by radio its findings, or if designed on the cheap, just light up a light to show cell phone presence. I suspect these could be made in quantity for fifty dollars or so.