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?


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.

In low-loss cables that have the same outside diameter as the classic PE
cables they are replacing, the reduction in loss is almost entirely due
to a larger centre conductor. But that change cannot be made on its own.
In order to maintain a 50 ohm impedance and keep the same outside
diameter too, it is necessary to reduce the dielectric constant of the
insulation material.

In other words, they're using foam or semi-airspaced construction
because they *have* to. Replacing some of the solid PE with gas may make
a small contribution to the lower losses, but nowhere near so much as
the advertisers would have you believe. The main contributor is always
the reduced I^2R loss in a larger centre conductor.



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73 from Ian GM3SEK