Sometimes an antenna gets damaged but you can often repair it and don’t have to buy a new receiver or replace the antenna.
The formula you need to remember is:
wavelength = 300 / frequency in gigahertz
The frequency in gigahertz is the band which your antenna is receiving, for a standard R/C radio in europe i 2.4 GHz but in USA/Canada it is a range between 2.400-2.485. This is important to know because you need to enter the correct value for your area, the center band. The center band is the band which is in the middle of the frequency range. In Europe the middle band is easy (2.4) but for e.g. USA/Canada it’s 2.4425Ghz (it’s safe to use 2.44GHz).
So the length of the antenna in europe needs to be close around:
300 / 2.4 = 125
So this is a full wavelength and the antenna should then be 125 mm, but we want a quarter of a wavelength so we need to divide this by 4
125 / 4 = 31,25
So our receiver antenna should be 31,25 mm long, that is how much we need to expose the inner wire. If the antenna is at least this long we should be able to repair it.
The antenna is only receiving with the core w/o the mesh (also called screen), the length of the antenna with the mesh/screen is not important for the receiver but makes it easier to get the receiving end to “clear sky” or components that might give interference.