In message , Jim Lux
writes
Antonio Vernucci wrote:
since most of the loss in practical coax cables is due to I^2R loss
(compared to V^2G)
A quick question. If most of the the cable loss is due to I^2R, how
can one explain that the foam versions of common coaxial cables show a
much lower loss than versions having solid PE insulation?
For instance RG-213 is rated at 8.5dB loss for 100 meters at 144
MHz, while RG-213 foam at only 4.5 dB. If G is relatively unimportant
with regard to loss, how can one explain that a change of insulation
material yields such a tremendous change in loss?
Thanks and 73
Tiony I0JX

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.

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)

Indeed. If you compare a frequency response of a reel of coax with a
plated centre conductor (say, copper on steel) with one with a solid
inner conductor, the former often has a noticeable kink at around 40 or
50MHz. A long time ago, we found this out at work when trying to find
drop cables and miniature cables which we could be used as simulations
of large-diameter, low-loss CATV trunk cables (for use in the design
lab).

At least one major CATV supplier had lots of very large reels of various
lengths the 'real thing' (no pun intended) in a massive trailer parked
immediately on the other side of lab wall. These were patched through
the wall to the test benches. The engineer could then test and adjust
the flatness of the frequency response of wideband amplifiers against
the length of appropriate cable. Thankfully, this technology has largely
been superseded by optical fibre/fiber equipment!
--
Ian