Antonio Vernucci wrote:

In reasonably well constructed coax cables, the main source of loss up
to about 1GHz is the I^2R loss in the centre conductor. The inside of
the shield carries an equal (and opposite) current, but the current
density is lower so the I^2R loss there is less important. Dielectric
loss is usually less important still.

Ian and others,

thanks for your clear explanation, but I still have a doubt that you may
kindly clarify.

The 300-ohm TV flat ribbon specifications show an attenuation generally
lower than that of plain RG-8, despite the conductors of the ribbon are
by far thinner than those of RG-8 (especially than the cable shield).

What am I missing now?

300 ohms vs 50 ohms.

Since IR losses dominate at these frequencies, reducing current reduces
loss. The higher Z means more voltage and less current for the same
power. Loss will be 1/36th, assuming all the conductor sizes are the same.