On Sat, 02 Jul 2005 05:31:02 GMT, Owen wrote:

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...

I wasn't the only one?


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.

0.0002 is from the ITT Handbook, fourth and fifth editions.

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%.

Of course. But I said that I didn't wade throught the numbers [g].


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.

I admit I discount the last digit of preceison in the values. As Dan
says so well in a note on his web site:

"Caution: The computed values for K1 and K2, like all computed results
in both the XLZIZL package and the TLDetails program, are shown with a
precision of a few digits beyond what is reasonable in normal
engineering practice. This is done to allow you to spot trends and do
theoretical studies. Don't allow yourself to become overly concerned
with the exact values for K1 and K2. The loss characteristics for any
transmission line will vary with manufacturing tolerances, age,
bending, exposure to heat and sunlight, and even changes in the
ambient temperature. The values used here, and indeed in any modeling
package, must be considered as "best guess" estimates of the actual
attenuation for any given line."


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.

Another cautionary note:

While Dan was working on his programs we corresponded a lot via email.
In one instance, he said "X" and I said "Y" about Heliax.

After some fussing around, we learned that my paper Andrew catalog no.
35 has different loss figures for the LDF series than does the "new"
online catalog no. 38. As we would say in America, like trying to
shoot at a moving target.

Also, from my catalog 35, Vp is given as:

LDF3-50 88%
LDF4-50 88%
LDF5-50 89%
LDF6-50 89%
LDF7-50 88%

So even that varies in a random fashion.

Regards,

Wes