If your transmitter and antenna are 15 metres apart, but are connected by 60 metres of RG-58 coaxial cable, what should be done to reduce transmission line loss?

Transmission line loss is directly proportional to the length of the cable. The longer the cable, the more RF energy is dissipated as heat in the conductor and dielectric materials. Therefore, having 60 metres of cable for a 15-metre run introduces 45 metres of unnecessary signal attenuation. To minimize this loss and deliver the maximum possible power to the antenna, you should cut the cable to the minimum length required to comfortably reach the antenna. Coiling the excess cable does not reduce the physical length the signal must travel, and thus will not reduce the line's fundamental insertion loss.