Thoughts on IEDs
Off-topic, but always hoping somebody in DARPA or suchlike would pick up an idea I've had.
From what I hear, most IEDs are detonated by cell phones or walkie-talkies. Thus if you had a detector for these, you would have a passable IED detector.
Radio receivers actually radiate a bit. Processing the signal requires moving it up and down in frequency (some functions are best handled at high frequency, some at low). You have crystals generating a radio signal and mix it with the incoming signal. This means that the receiver also transmits. I seem to remember that shortwaves gave off 455 h or maybe khz. This is, btw, why they make you turn off cells at takeoff. A cellphone should thus give off a characteristic frequency, as would a walkie-talkie (dunno if they'd be the same). I also believe a cell gives off some signal so that the stations can keep track of just where you are in case there's an incoming call. That signal would be fairly powerful. There are also very broad spectrum receivers that search for ANY radio emissions nearby at any frequency. They're used to search for bugs and are cheap (since they have no tuning section).
Simplest proposal: detector on a pole, like a long metal detector. Use it to probe suspicious packages. If the guy detonates it, at least you're 10-20 ft away rather than opening it.
More elaborate: a directional Yagi antenna can focus reception and improve range by a factor of ten to twenty. At high frequencies like this, it'd be quite small. You might be able to spot a cell phone from a good distance, and know its direction (just sweep back and forth, listening for the strongest signal). Might even have enough range to spot the phone of the guy waiting to set it off. A continuous sweep could spot anything in the road ahead. Might be able to get ranges of hundreds of yards, if cell phones emit signals to tell the system where you are. Cost of antenna is under a hundred bucks, mfrg cost probably five bucks.
Or mount the detector on a toy remote controlled car, and run it up to the package.It'd be close enough to spot radiation from a walkie-talkie in that event.
UPDATE in light of comment: you could probably reduce false positives. The local detector on a pole or remote control toy car could be set to only alert if a cell is within five or ten feet. Sensitivity of one connected to an antenna could be set to alert only if a cell is in the direction the antenna points and within, say, fifty yards. If the antenna is pointed at a person when it alerts, it probably is a false positive. If it's pointed at empty road or a trash bag by the side of it... bingo. The rig could be built for a lot cheaper than a cell phone jammer. (I note the Warlock systems jammers look like they cost $15,000 each. This could be made for around a hundred. And wouldn't just jam the system while a convoy is near it, but would alert you to its location).
Further nasty thought: for the cell to tell its tower where it is, the signal it sends must identify itself. If you could intercept that signal, with help from the wireless phone co. you could determine its number. Post someone to watch it. When someone comes to retrieve it for relocation, dial the number.
FURTHER update: I found a British company that sells a cellphone detector for about $200. Says its good to 40 ft., and you could extend that greatly with the right directional antenna. Here's another, said to have a range up to 30 meters. With regard walkie-talkies ... I think they have a limited number of frequencies. Rig a transmitter that will sweep across these, link to a directional antenna, and you might be able to detonate the bombs ahead of you. At least until they start putting in countermeasures, requiring that the detonating signal convey some information, etc.
While I'm at it, a tip from Vietnam days. Chain link fence does great against RPGs, but they ought to invent something to defeat them. All it takes is a small dent in the nose cone. RPGs detonate when a piezoelectric crystal in the nose is crushed on impact and sends out an electrical signal. The signal passes via the outer casing to the detonator, then back via the inner shell. Dent the outer shell a bit and it shorts out. Stop it without anything hitting the nose and it doesn't function. I've wondered if an armored vehicle couldn't be protected by narrow metal slats pointing outward, maybe a couple of inches apart. If the RPG hits a slat square, it'll still detonate, but if it hits between them it might short. I've tried to figure out how to send a radio/magnetic pulse that would detonate one (now wouldn't that be fun--set off any RPG round in the launcher) but don't have the expertise to figure a way around the fact that it's basically two coaxial conductors, which is pretty resistant to a pulse.