Jim Lux wrote:

144 MHz isn't HF, which is where the original statement is valid. At
frequencies above around 50 MHz, depending on the dielectric, the
dielectric loss starts to be more significant.


The analyses I've done using published and realistic values for
dielectric (PE and PTFE) and conductor loss indicate that dielectric
loss doesn't become significant until the 1 - 10 GHz region, well above VHF.

Another trap for the unwary, when comparing coax losses, has to do with
skin effect and the thickness of the copper or silver cladding on the
center conductor. You could have an air insulated coax with silver
plated over stainless steel where the loss is actually greater at low
frequencies than higher, because the skin depth is greater at low
frequencies and the current is flowing mostly in the SS, rather than the
copper. (such coax is used in cryogenic applications, lest one think
it's overly contrived as an example)

It's unusual to find a plating thickness that's less than several skin
depths thick except at MF and below, or perhaps the low end of HF. It
does happen, though. I have some 0.1 inch diameter 75 ohm cable which
has a very small center conductor which is made of several strands of
even smaller Copperweld wire (thick copper over steel). Even though the
copper is probably a sizable fraction of the total wire diameter, the
wire is so small in diameter that the copper isn't several skin depths
thick at lower HF. It has very noticeably excessive loss at 7 MHz,
something I discovered the hard way one Field Day.

Roy Lewallen, W7EL