The reciever is tuned to the antenna length, the replaced antenna MUST be exactly the same lenght.
This is true for 72mhg and 2.4G or any other type of transmitter. Dont forget that the 2.4G units talk both ways meaning that teh RX is also a TX.
If you change to a different length and the fact that 2.4G radios talk both ways it will fry the reciever..
Hope this helps

Ron Sr