How to repair broken antenna
I had a crash with my FPV quadcopter last week and broke the FrSky receiver antennas. I didn’t wanted to wait for a new receiver, so I decided to repair the antenna. The connector of the second antenna was totally destroyed and it couldn’t be repaired.
In general you have to possibilities: buy a new antenna and replace it by removing the old one from the receiver connector. Or you can cut the broken antenna, so its length is appropriate to the 2.4Ghz wave length.
The wavelength on 2.4Ghz is 0,125 meters (125mm). The antenna should be 1/4 of the wavelength. You need to cut the rest of the antenna to 31,25mm. By cutting I mean first to remove 31,25mm of the plastic/PVC. The next layer is the shielding. Remove it to the length of 31,25mm. So the last 31,25mm should be the dielectric and the wire in it. That is the “active” antenna. That’s all. Now the antenna is functional again.
Note: there is a discussion if only the last unshielded 31,25mm are important or the whole length of antenna. I didn’t find a clear answer on RCGroups.