for PC that would be a very valuable tool

And scanning interceptors? An equipment slot mod that, when active, will detect any active scanners pointed in your direction, thus revealing them in the same way they reveal you.

Make it jam comms too. It'd be great for setting up ambushes.

Rather than a deployable equipment, it should be an equipment that is part of the suit. It should also work like tank modules where once you activate it, the jammer runs for a certain amount of time and jams enemies in a certain radius, and then enters a cooldown phase.

And rather than just affecting the results of an active scanner, it should flood the enemy's radar with a bunch of false positives (like that one booster in Halo 3 (?))

I don't think adding a device to specifically hard counter is the way to go here. I think if anything is to be added it should be multipurpose. I propose an equipment or grenade that has a limited number of uses, but would both be useful in general and provide a counter to radar and sighting in general, but over a very localized area for a very limited time.

I propose a chaff/em blast that shuts down all HUD functions for those hit for a limited period of time. I'll say base 10 seconds pending a proper analysis. I'm also not just talking radar here, but all of your HUD. Health, radar, cross hairs, health bars of the people you're shooting at, and friend or foe chevrons all turned off. The hows of the delivery system can be worked out later, but a single burst that flags players as having no HUD for the duration would likely be simpler to program in then localizing the effect to an area.

This lets people counter scanners up close because if the people you're fighting are on top of you it doesn't matter if you're scanned or not because they can't use their radar, but if they're at range then they can still figure out your position. This lets the user fight the scanner without making the scanner useless because you would still be scanned, but the people your fighting couldn't make good use of that information.

Most importantly when adding any new weapon or equipment it adds new tactics to the battle on its own merits. Even if no one is using the scanner a weapon like this would be great for opening an ambush. With no HUD getting the drop on them would be easier because they wouldn't be getting direction markers letting them know which direction its coming from. In PC a well timed use could have a group of targets firing into each other because they can't tell friend from foe. In FW or PC it can cause the undisciplined to hesitate in a fight for fear of wasting ammo or harming friends.

Finally when it comes to answering the question of what can counter this we already have an answer to that. Discipline among squad mates and fire discipline in general can greatly mitigate the offensive uses of such a device and that's a counter that relies on the character of the player rather then SP or suit cost.