Boba Fett and Captain Rex both have these strange flag-like things sticking up from their helmets.
Are these things radio antennae? It would be strange if they were, because the Clone Troopers communicate with their gauntlet radios. Are these flag/antenna things ever shown to do anything or are they just decoration?
The "flag" houses a holographic scanner and a range-finding sensor which interface with a HUD inside the helmet. The entire whole unit is typically referred to as a 'rangefinder'
Targeting rangefinder (retracted) The helmet tracking sensors sent information directly to the helmet displays. It gave Fett precise readings on the speed of, and distance to any given target.
and
Holographic targeting display
When flipped down, the targeting display interfaces with the visor to give tracking information on up to 30 targets at a time.
And, as you can see from these image from The Empire Strikes Back, The Clone Wars, the flag folds down so that it makes an eyepiece, as can be seen in Attack of the Clones when Jango fires his bolo at Obi-Wan.
The little flag is actually a targeting system (known as a Rangefinder) for the missile on the back of the Z-6 Jetpack You can see this in the Mandalorian episode entitled "The Marshal (Chapter 9)." The sheriff (Cobb Vanth) lowers the targeting mechanism and fires the missile from his jetpack. From his perspective, you can see a targeting display. Once the target is acquired, he fires the missile. Hope this helps.
