I haven't played d20 Modern, but if I were to take a blind stab at it, I'd say something like this:
Chaff Grenades: This device explodes one round after the pin is pulled, emitting large amounts of electromagnetic radiation and spraying a 10-foot radius around the point of the explosion with charged bits of iron and similar electronically-disruptive things. The explosion deals no damage to any form of life, but wreaks havoc on certain electronic systems, causing them to be inoperable for 1d4+1 rounds. After that period has elapsed, the affected systems recover and function normally. While most modern military-grade equipment is shielded against interference to the point that chaff grenades will not affect it, recording, surveillance, and targeting systems (video cameras, laser tripwire systems, computer scopes, etc) are (by necessity) more sensitive and are affected by chaff grenades. Consumer-grade appliances are also affected by chaff grenades. Also, consumer-grade recording equipment (such as a common digital video camera) must make a Fortitude save (DC 12) or be rendered permanently unusable.