On Fri, 01 Jul 2005 20:56:25 -0700, Wes Stewart
wrote:


Without wading throught the ASCII math, a couple of thoughts.

I see someone else grizzling about the "ugly maths". Oh well...


1. As Tom said, most references give the D of poly as 0.0002,
although the "Handbook of Coaxial Microwave Measurements" by General
Radio gives it as 0.0003.

Ok, as I posted in another msg, my figure came from the ITT Ref Hbk,
and even at 2e-4, it comes short of being the entire explanation of G
derived from published loss figures. I accept that the ITT book is
much lower than others.

Just Googling, I see Reg's site shows 2e-5,

2. Again, without having followed the derivation, I find the k2
values to be different from those given by the handiwork of Dan,
AC6LA, in his XLZIZL.xls workbook or his TLdetails program. Dan used
published attenuation values and Excel regression analysis to
determine the values of k1 and k2. See:


Dans k2 figures are based on units of MHz and feet, mine are Hz and
metres, and when you allow for the units base, they reconcile to
within 1%.

http://www.qsl.net/ac6la/bestfit.html

3. Also, General Radio says, "alpha(diel) does not depend at all on
the dimensions of the line..." This suggests that there should be no
difference in k2 between LDF4-50 and LDF6-50.

I believe that is true, my derivation is that k2=9.09e-8 * D /vf (for
units of Hz and metres). So, the "leakage" loss depends on D and 1/vf
(or permittivity**0.5), and dimensions don't enter the equation.

What sent me down this track is trying to reconcile this with the
published specs which claim more loss than is explained by the
dielectric.

Dan's numbers show that
to be the case.

Dan's figure (in my ZLZIZL) is, like mine, a little lower (25%) for
the larger line. It is the observation that it varies that suggests
there is more to it than D alone.


4. Any loss that doesn't follow the sqrt(f) rule (radiation,
wire-to-wire resistance of braid?, etc) as you suggest falls into the
k2 term.

5. High-quality, high-frequency (microwave) flex cables do away with
braid, or at least solder fill it, and use tape-wound shields.

Yes, see my other post regarding the LMR cables which, like the LDF
series, show much less variation in k2 with cable size than moving
from RG58C/U to RG213.

Thanks for the thinking Wes, Owen

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