Minimum loss (at which copper loss and dielectric loss cross) comes at
about 67 ohms. There are cables at that impedance, but I've never seen
one.
Copper loss dominates at frequencies below about 1 GHz for most
standard cables.
For a given outer diameter Foam cable has lower loss because the lower
dielectric constant allows the cable to have a larger center conductor
therefore reducing the copper loss. It is not because the dielectric
losses are lower. This is a common misconception.
Dielectric losses are not an issue for most cables below microwave
frequencies.
http://www.epanorama.net/documents/w...impedance.html
Mark