Distortion Fields
These are mountable on mechs but again atleast 3 are needed more if you wants. The pyramidal nature of the field would also allow for aircraft to hide in the field, but the field would get smaller the higher they flew. Within the field practically no radar information is available. This would allow large or small teams to disguise there numbers. Obviosly you would not be sneaking up on a tgt. but teams could launch mutiple assult groups only one of which was hiding the main battle force the others would be diversions, but the only way for the other team to know what was coming would be through visual confirmation. Again the sourse points of the field would become very large and easy tgts to hit. because every one would know where they are, but 0 or 50 mechs could be hiding in the field.
While I understand the principle behind this concept I feel that it should be an item that requires multiple "dedicated" Mechs, for each level, to power it and the technology should exist to be able to "see" through it. This would be too powerful an item otherwise and it should still be a strategic choice, not a catch all or end all, with a chance that the "enemy" has planned for its use.
The reason that I am making this comment is that I feel this type of item is merely a lazy man's way to make up for poor strategy. Any technology/ability/magic that can accomplish this type of effect has to have serious weaknesses OR other technology/ability/magic to overcome it. It can not be considered to be an absolute.
Even stealth planes can be detected, you just have to know what to look for.