I saw this apparent discrepancy many years ago, and wondered the same
thing. I've come to believe that the extra loss is due to the braided
shield, not some unknown additional dielectric loss. There's very little
quantitative information about that loss mechanism, but from time to
time I've come across comments that it can be substantial.

Roy Lewallen, W7EL

Owen wrote:
I am trying to reconcile the following in respect of for practical low
loss RF transmission lines:

In the RLGC model for Zo and gamma, it is generally accepted a good
approximation is that R=c1*f**0.5, G=c2*f, and L and C are constant.

If the term (G+j*2*pi*f*C) can be rearranged as
(2*pi*f*C(G/(2*pi*f*C)+j)), and substituting c2*f for G, written as
(2*pi*f*C(c2/(2*pi*C)+j)).

If we regard G to be principally the loss in the dielectric , then
c2/(2*pi*C) should give us the dielectric loss factor, D, 1/Q,
tan(delta), dissipation factor, power factor, whatever you want to
call it.

alpha= 0.5*R/NomZo+0.5*G.NomZo
It also seems generally accepted that Matched Line Loss (MLL) can be
modeled well by the expression MLL=k1*f**0.5+k2*f.

(Remember that alpha= 0.5*R/NomZo+0.5*G.NomZo)

It follows then that c2=k2/(10*log(e)*Ro), and that (G+j*2*pi*f*C)=
2*pi*f*C(k2/(10*log(e)*Ro)/(2*pi*C)+j) which implies that D is
k2/(10*log(e)*Ro)/(2*pi*C).

Problem is, that whilst PE has D somewhere about 2e-5 up to 1GHz, the
loss model for RG58CU (PE dielectric) indicates D of 2e-3, much much
worse than would be expected from D of the PE dielectric alone.

Any thoughts. Is there an inconsistency between the explanation that G
is principally due to D of the dielectric material, or I have I messed
the maths up?

Owen
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