Re: NEC-LIST: 50 versus 77 (75) ohm transmission line

Dear All,

Thank you very much for all the valuable information. I
didn't expect this issue to become such a big one.

I found the discussion by Tom Bruhns and Dan Bathker very helpful.

Colleen Tallman personally sent me equations that explains the issue,
that I could find in some textbooks, and I could calculate on Excel
and verify the values.

R. P. Haviland suggested to see Henny's Radio Engineering Handbook,
(p42-12 etc. on 3rd Ed).

I could gather information to make a table as follows :

For a given outer conductor size, and when conductor loss only :

Minumum loss sqrt(epsilon)Zo = 77ohm
Max breakdown sqrt(epsilon)Zo = 60ohm
Min temp rise, inner cond. sqrt(epsilon)Zo = 36ohm
Max Power capacity sqrt(epsilon)Zo = 30ohm

The optimum inner conductor diameter occur
regardless of the dielectric material.

So, for a coax of epsilon=2.2 :

Minumum loss Zo = 52ohm
Max breakdown Zo = 40ohm
Min temp rise, inner cond. Zo = 24ohm
Max Power capacity Zo = 20ohm

And, for a foam insulated coax of velocity factor 0.8 (epsilon=1.56):

Minumum loss Zo = 62ohm
Max breakdown Zo = 48ohm
Min temp rise, inner cond. Zo = 29ohm
Max Power capacity Zo = 24ohm

I was happy to learn that we don't have to blame the HP people and
other pioneers. They were perfectly right, and we have no compromiss
with materials of epsilon around 2.2.

But with the recent foam insulated coax, we are slightly off the
optimum value (with negligibly small attenuation difference). 75ohm
TV coax may have lower loss than 50ohm, when foam insulated. (and
cheaper too).

And lastly, my apology goes to Samson Malaka, because I took up his
thread, a question on Linear Phased Antenna Array, by mistake, when I
used his posting as a template, and forgot to re-write the title.

Very sincerely

==================================================
Hideho YAMAMURA,
Senior-Engineer,
Hardware Technology Engineering Department,
Advanced Product and Technology Development Operation,
Enterprise Server Division,
Hitachi, Ltd.
==================================================
Received on Thu Mar 15 2001 - 05:10:42 EST