On Fri, 01 Jul 2005 02:29:31 -0700, Roy Lewallen
wrote:

One other thing I did long ago was to check some semi-rigid lines with
solid shield and center conductor and PTFE dielectric, against
theoretical calculations. As I recall, the agreement was pretty good --
much better than the garden variety coax.


Ok, that is one of the reasons that I looked at how k2 changed with
size of the LDF series.

I have just had a look at LMR200 wrt LMR1700. (They have an aluminium
tape +braid outer.) I do not know the value of D for the closed cell
foam dielectric that they use. The k2 factor (and therefore G) of the
larger cable is about 50% of the smaller cable, though there is nearly
a 1100% increase in diameter. It seems that of the few cables that I
have looked at that ones with better shielding result in less
variation in k2 (and G) with change in diameter ( for Zo and
dielectric remaining constant). That suggests that shielding and loss
effectiveness of the outer conductor is to some extent reflected in G
(proportional to f).

Tom, the D figure I quoted for PE was from the ITT Radio Engineers
Handbook.

G seems not to be solely or principally dependendent on D of the
dielectric in a general sense. It may be with very good lines, but it
doesn't seem to be so with single braided PE insulated coax. It is
clear that using D (even at 2e-4) to calculate G in an RLGC model of
R58C/U will not give a very good fit to published attenuation
characteristics.

Regarding using the published specs, I am trying to glean as much as
is reasonable from the published specs. I note and agree with your
comment Roy about the quality or accuracy of some specs.

Thanks all for the thoughts on possible contributions to G.

Owen
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