The lowest loss cable I have here is 75 Ohm 1" General Cable Fused Disc;
its under a differnt name these days. Mostly air with poly discs and used
for the 200' runs for 10M, 2M, and 222 MHz.
For the 160/80 inverted vee it is 450' of regular foamed 3/4" 75 Ohm CATV
hardline with a RG-11 jumper and plenty of ferrite to the feed point. Ive
been using ferrite sleeve baluns since the mid 70's; I was introduced to
them by the company I worked for who was building equipment for the joint
CIA/DOD Tempest program.
The lowest loss cables have large, smooth conductors that are the maximum
possible size for the cable impedance. Dielectric is largely meaningless,
except as it might affect conductor size.
We can argue this point endlessly, but it will always come back to the
conductors. The exception would be some horrible dielectric or operation way
up above normal VHF/UHF with marginal dielectrics.
It is the way it is. The confusion probably occurs because dielectrics with
more air allow a larger conductor to be used for a given cable diameter and
impedance, it is not because the dielectric has less loss.
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