IEDs could be triggered by a walkie talkie, cellphone, ham radio or ANY thing that transmits a radio signal provided the frequency of the IED's receptor is the same as the transmitter.
It would be possible for an approaching vehicle to broadcast a "sweep" accross "all" frequencies. This would trigger the "dumb" IED's. Of course, then, the terrorists would have to develop a sequence of signals (a code) that the IED would recognize. Both us doing this and them doing this makes it more expensive.
Terrs started using remote controlled (RC) switches to initiate improvised explosice devices in the '80's. Since then the bidness of countering these devices has been keping pace - understandibly under deep wraps for op sec reasons.
Now, with their widespread use in Afghanistan & Iraq these developments are coming more and more out in the open, commercially-wise.
Coalition forces are now investing heavily in technologies designed to detect and detonate RCIED's prior to their contact with troops.
These devices are in two primary classes:
Low-power devices - low frequency, low power devices such as garage door openers, key fobs and door-bell activator switches. Jamming/activator devices used to counter these use relatively low-power RF frequency energy to jam their freq's and prevent their functioning at a distance thereby enabling adequate protection. These jammers are intentionally low-power to prevent interference betweenan RF emitter (the jamer) and our own SINCGARS (VHF) radios and Blue Force (satellite) communications.
The second class is:
High-Power microwave systems - These devices cause greater interference and require more power to operate. This increase in the power amplifier of the jammer also draws more power from vehicle generators, detracting ferom other vehicle systems.
This is a newly created class of anti-IED devices that ranges from man-portable to vehicle mounted to truck based microwave emitter devices.
The technology is keeping pace with the threat. But there really is no secure protection to a terr who will do the suicide/homicide trip on Coalition forces.
With attributes to Janes IDR, Vol. 39, Jan 2006 - RCIED Countermeasures.