The default control range of the Syma X5C drone is only 30 meter, adding a true 2.4ghz antenna you can double this range.
Step 1: Find a 3db - 5db 2.4 ghz WIFI antenna
You can buy one, or remove from an old WIFI device.
Lucky for me, I found a bricked WIFI access point.
I removed the antenna from it.
Step 2: Open the Syma transmiiter
This is a very easy step, don't forget to unsolder the power wires.
Remove the original antenna (that 2cm wire).
Solder the new WIFI antenna in the Syma X5C transmitter.
Step 3: Reasamble the transmitter
After you checked everything, you can solder back the power wires and reassemble the transmitter.
The new transmitter looks good with the new antenna, and also has a better control range.
You can read more about this instractables here: Syma X5C Hack