It certainly speaks a lot of Tom's consistency. <grin>
Keith
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Lux
Sent: Sunday, January 16, 2005 10:31 PM
To: Tom Rauch; towertalk@contesting.com
Subject: NASA and braid was Re: [TowerTalk] There's 'ground',and then
there's 'ground'
>
> Many construction standards prohibit braided straps in RF or lightning
> paths unless the connection absolutely must have braiding in order to
> withstand flexing, and then the braid is often substantially oversized
> to make up for its reduced current capacity. It's my understanding
> NASA restricts use of braiding, and I can cite many cases where
> braiding will either cause excessive loss or actually fail in high
> current RF systems.
I'm going to guess that NASA doesn't like braid because of the possibility
of small metallic fibers coming out and winding up in the wrong place in
flight hardware, not because of any RF properties.
I'll also have to say that what NASA requires or advocates may or may not be
relevant to amateur radio (or any other industry) practice. Without knowing
the details of the ostensible NASA proscription on braid, I can't really say
why they do it, but it might be because of some reason totally unrelated to
the RF properties.
Tom, you've written several times about NASA not liking braid, but I can't
find any definitive reference in any NASA documentation that says this.
Back in 2001, you wrote (in part):
NASA and others prohibit the use of braiding in lightning grounds, and
you'll never see it in BC stations where the ground is involved in lightning
or RF applications unless it is way overkill size and very short.
and in 1997: NASA prohibits use of braiding for lightning protection and RF
grounds unless the connection absolutely has to be flexed.
I found these when hunting for NASA requirements for something regarding
the use of braided conductors.
I did find the following NASA specification (June 2004 date)
http://www.ccb.org/docs/NASAASC/NS16065.pdf
which specifically allows braid for bonding conductors.
And then there is the following NASA document:
http://klabs.org/DEI/References/design_guidelines/design_series/1231.pdf
"Design considerations for lightning strike survivability"
which makes no mention of braid, pro or con, except in the context of
requiring braid shields to be "grounded"
Then, there's JPL D-8208 (rev I) "Electronic Packaging and Cabling", with
which I have entirely too much familiarity...
http://standards.jpl.nasa.gov/contractor/docs/d8208i.pdf
One might want to look on Page 292 of the spec, which says, in part:
" The S/C chassis provides very low impedance between all subsystems. When
the RFS (Radio Frequency Subsystem) is powered from a source other than the
S/C, a low impedance path between the RFS chassis and its power source
chassis must be provided. Heavy *braided* wires will provide an acceptable
impedance. The power source shoudl also have a heavy *braided" wire
connecting it to solid earth ground."
This isn't lightning protection related, of course.
There's a whole raft of stuff about using braid as a thermal path, as well
(since it has many parallel wires, and is flexible under vibration and
thermal distortion)
I also note that we use braid as a bonding conductor at JPL a lot
particularly in ESD protection schemes (not that it might not be prohibited
by some spec, or even a bad idea, but I would think that some auditor would
have raised the flag by now, and we would have had a lab-wide purge of braid
(like we have had for "pink poly" or for "disposable" wrist straps, and
such)).
I grant that using small braid in a lab for a ESD ground isn't lightning
protection.
But the summary is that I can't find any reference in NASA procedures or
documentation about prohibiting its use. If you could give me some pointers
to where it might be, I'll hunt them down.
Jim, W6RMK
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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