Three are three ways power can be lost from coax:
- Resistive
- Dielectric losses
- Radiation losses
Resistive losses are simply due to the electrical resistance of the conductors. Because of skin effects, the total mass of the conductor is less important than its surface area. The centre conductor in high power coax can be made of aluminum with a thin coating of copper.
Dielectric losses are losses in the coax's dielectric. The coax is like a long series of inuctances and capacitances. Every time the capacitance is charged and discharged there are losses. Different insulators have different losses. Larger diameter coax often uses air as the dielectric. Air is less lossy than many other dielectrics.
Lastly, imperfection in the shielding can allow the coax to act like an antenna. Better shielding (for example solid metal rather than braid) and keeping the centre conductor in the centre help to minimise this.
Other sources of loss are impedance mismatches which cause some of the signal to be reflected.