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olechap
Joined: 08 Jul 2007
Posts: 3
Location: Ellington, CT
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 Posted: Sun Jul 22, 2007 3:46 pm Post subject: Double-shielded coax |
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Can someone explain the benefits of double-shielded coax cable?
A friend of mine mentioned it's either a requirement or strongly suggested
when doing the GNS430 WAAS upgrade - so what happens, does it help reduce
line loss and/or does it really reduce EMI?
Also, we were discussing whether it'd be beneficial to use double shielded
from the mag p-lead -> key switch -> mag ground and then subsequently
grounding the outer-most shield to the airframe. I wasn't sure where to
stand with this concept as I thought the idea of using a single shielded
wire isolated from the A/C ground enabled current pulses going opposite ways
to cancel most noise.
Any thoughts will be appreciated!
-Andy
RV-7 FWF
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nuckollsr(at)cox.net
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| Posted: Tue Jul 24, 2007 1:46 pm Post subject: Double-shielded coax |
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At 07:43 PM 7/22/2007 -0400, you wrote:
| Quote: |
Can someone explain the benefits of double-shielded coax cable?
A friend of mine mentioned it's either a requirement or strongly suggested
when doing the GNS430 WAAS upgrade - so what happens, does it help reduce
line loss and/or does it really reduce EMI?
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Double shielding has some exceedingly small benefits to propagating
radio frequency energy from point A to point B through exceedingly
RF noise environments. Since GPS transmitted signals are below ambient
noise, it's a good idea not to "waste" your system's inherent signal
to noise ratio with a potentially degraded feedline installation. Now,
having said that, it's unlikely that anyone crafting an OBAM aircraft
will realized benefit from the double shielding. The major jump in
coax quality for RG400 vs. RG58 is modern, higher temperature,
environmentally robust insulations. PVC and Polyethylene were whippy-do
materials in WWII, but cousins to teflon are the materials of choice
today.
The choice is not so much one of performance as-installed but one of
longevity in service combined with doing the best we know how to do.
It's similar to the evolution in finishes. Nobody would consider painting
their airplane with cellulose based dope (al la Hindenberg) no matter how
good it looks when compared to an equally fresh coat of Imron.
| Quote: | Also, we were discussing whether it'd be beneficial to use double shielded
from the mag p-lead -> key switch -> mag ground and then subsequently
grounding the outer-most shield to the airframe. I wasn't sure where to
stand with this concept as I thought the idea of using a single shielded
wire isolated from the A/C ground enabled current pulses going opposite ways
to cancel most noise.
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No. Shielding in these applications are for mitigation of ELECTROSTATIC
coupled noises and there is no benefit to using double shielded coax
for p-leads. Ground as shown in the z-figures at the engine end only.
Bob . . .
----------------------------------------
( "Physics is like sex: sure, it may )
( give some practical results, but )
( that's not why we do it." )
( )
( Richard P. Feynman )
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