Re: OK to use TV coax for microphone?

On Sun, 04 May 2008 11:11:40 -0400, "Michael A. Terrell"
<mike.terrell@[EMAIL PROTECTED]
> wrote:

>Chris S wrote: 
>> If copper is soft then wouldn't that make it good for repeated
>> flexing in the sense that copper isn't so stiff that it wouldn't
>> split or crack?

>   Copper 'work hardens' when flexed. Over time it develops cracks, then
>breaks. Stranded wire uses smaller conductors and has less problems per
>conductor, due to a larger bending radius VS the diameter of the
>conductor.  The steel center conductor is meant for permanent
>installation, but it holds up better on the bench that solid copper
>coax.

Yep.  Way back in the 1960's, I was installing 2way radios in various
vehicles.  Someone found some RG-58/u, with a solid center conductor.
I was told to use it.  About 100 vehicles later, someone declared that
solid coax was a bad idea.  Management decided that it would replace
all the coax, but only after evidence of the first failure.  So, we
waited, and waited, and waited.  I was long gone when one of the coax
cables finally failed, about 5 years or 150,000 miles later.

Fast forward to the early 1990's and cellular telephone installs.  At
the time, the average cell phone was big, boxy, heavy, and ugly.  The
connector of fashion was the TNC.  Once again, some dealers were
installing solid center conductor coax in the vehicles.  They would
last about a month and then break.  Huh?  Solid center conductor
RG-58/u works in 100 buses for 5 years, but not in a passenger car for
a month?  Something is obviously different.

It's the connector.  In the 1960's we soldered everything.  The common
PL-259 and UG-175 adapter was as sloppy a connection that could have
been mis-designed.  30 years later, everyone was crimping the
connectors.  The breakage was always at the connector.  The crimper
was dinging the center conductor.  The sharp edge on the rear of the
center pin did the rest.  Soldering the center pin and the longer
distance between the crimp and the point at which the cable will begin
to flex (UHF versus TNC) did much to prevent breakage.

All RG-6/u "type" cables are not all solid center conductor.  Belden
Brilliance 1694F is flexible.
<http://www.belden.com/pdfs/Prodbull/NP233.pdf>
at about $0.70/ft.  However, it cannot be used with Type-F connectors.
The data sheet mumbles:
   "1694F is intended for use with connectors designed for 1694A"
which is not very helpful.

The foil outer shield usually tears in a flex test long before work
hardening does anything to the center conductor.

I've watched cable flex testers locally:
<http://www.ideinc.com/plan_cycle.html>
<http://www.ideinc.com/plan_tester.html>
(Bring hearing protection).  These simulate normal cable bending on
telephone headsets.  The flexing is far more radical than anything
that would be encountered with RF coax cable.  I asked if they had
ever tried flexing RF type coax cable.  As I vaguely recall, neither
solid or stranded center conductor coax cables survived for very long,
but I don't recall the numbers.  I'll ask.

Incidentally, there's also the tiny triboelectric noise created by the
mylar coated insulated shield, rubbing against the dielectric:
<http://en.wikipedia.org/wiki/Triboelectric_effect>
Good microphone cable should be "bonded" (i.e. glued) to prevent this
admittedly tiny noise problem.  You're not going to find that in
commodity RF coax cable.

-- 
Jeff Liebermann     jeffl@[EMAIL PROTECTED]
 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558