You really have to compute antenna length. It's related to the wave length of the received signal. Just adding wire will likely not improve performance unless you happen to stumble on an electrically correct length. I am curious as to why you would want to add any length at all because the typical 72 Mhz receiver with a standard length of approximately 39" (1 meter) will receive a signal beyond the range of visible control of the aircraft. In other words, if your plane is beyond the range of the sight needed to fly it, what's the point of extending the signal reception?