As the frequency of a signal is changed, what happens to signal loss in a transmission line?

Transmission line losses are primarily caused by the 'skin effect' and dielectric absorption. As the frequency of an alternating current increases, the skin effect causes the current to flow mostly along the outer surface of the conductor, effectively increasing its resistance. Additionally, higher frequencies cause the dielectric material separating the conductors in a coaxial cable to absorb more radio frequency energy, turning it into heat. Therefore, higher frequencies will always experience greater attenuation over the same length of cable compared to lower frequencies.