is it possible to cut the herelink antenna cable to reduce its length or it will change the impedance of the whole antenna?
I would not, with such small coax it’s always going to be a negative unless it’s done 100% perfectly.
It should no affect antenna performed if done correctly.
Another solution is to buy mmcx to sma cable and use any 2.4g antenna
Every connector you add reduces signal strength significantly…
Hacking the antenna is only for the highly skilled or brave… changing the length is not bad… but it’s easy to mess up the connection when soldering to the Antenna PCB.
Remember that the antenna is tuned for the case, so if you open the case, replace it again