>> Everything is pretty straight forward in the
>> basic VFR panel, but I keep running into second guessing what to do with
>> wire shields.
>
>EVERY WIRE IS AN ANTENNA. Whether it be a transmitting ( makes emissions)
or a
>recieving antenna ( picks up emissions).
>
>Shielding is used to reduce emissions from the transmitting wire and to
provide
>blocking from outside sources for the recieving wire. ( Could actually be the
>same wire but hang in there.
>
>Example - Remember the old twin lead TV antenna wire ( I know, it's still
being
>used). Remember how it would pick up the ignition noise from a passing car or
>how you could tune out unwanted signals and ghosts with aluminum foil?
Thats why
>they went to coaxial ( shielded ) cable, now you don't have problems with
>outside noise sources.
Sorry, but not true. The old twin-lead feedline was ballance TRANSMISSION
LINE which was quite free of coupling to the outside world both as
a radiator and receiver of energy. Ignition noise from passing vehicles
was brought in through the ANTENNA which was designed to pick up stuff
from the air. Twinlead was more difficult to work with than Coax but it
had lower losses.
>Every wire that passes current is a radiator of magnetic energy. The speed
and
>amplitude of the current change is what dictates the amount of magnetic
energy
>released ( broadcast ), so to keep this energy from getting into your
soup, put
>a grounded barrier around it, then it's basically tin-canned.
NOT if it's a TRANSMISSION LINE . . . meaning that for every electron that
ventures one way on the feedline, the companion electron is headed the
other way and tightly coupled to it . . . as in televions twinlead, coax
cable, twisted pair cables, etc. Shielding does NOT mitigate magnetic
coupleing, only electro-static coupling. I designed a prop synchronizer
some years ago that detected engine phase angle by measuring the MAGNETIC
field around a SHIELDED spark plug wire . . . the field was quite strong
and easily detected outside the shielding.
>There is nothing wrong with grounding both ends of a shield but what you
created
>by doing so is actually a ground wire that may or may not be cause for
problems
>later on if problems pop up with the normal grounding system, this means
that a
>lot more current flow could be returning via the shield than it can handle
and
>"POOF"! You may get to see some smoke!
POOF is pretty common in my experience . . . it happend to shielded
p-lead wires on two airplanes during my 6-month tenure as an airport
owner about 10 years back. Shielding should be used only if the
manufacturer of the equipment you are installing recommends it. Their
installation instrucitons should be quite specific as to how the shields
are treated. There is NO HARD FAST RULE . . . it depends on what noise/
propogation mode/victim combination is being addressed. The biggest
chapter in my book is on electrical noise . . . dealing with electrical
interference between systems on an airframe have been some of the most
vexing problems of my career.
A guy called me about ten years ago and described a very agressive
shielding and filtering activity on his airplane that took lots of
hours, dollars and added pounds to his airplane. After reading the
list list to me I asked, "Gee, I can't think of anything you NEED,
do you have a noise problem?" "No," sez he, "I haven't flown the
airplane yet."
The moral is, start with good practice for architecture. Single
point ground blocks, avoiding the use of airframe and/or engine
mounts to carry battery, alternator or starter currents. Shield
the p-leads by grounding shields to magneto housing at engine end
and using shield to provide a ground at the switch end - but don't
ground switch to anything behind the panel. Shield strobe light
wires per manufacturer's instructions. Shield any other wires per
manufacturer's recommendations. Mount microphone and headset jacks
so that they are INSULATED from a metalic airframe. Interestingly
enough, I've wired lots of headset and microphone jacks with twisted
pair/trio wire and NO shielding with no problems.
Observation of some simple rules for design and initial installtion
will make the chances of future problems very remote. Don't add
filters and/or sheilding, "'cause it sounds like a good idea."
If a noise problem arises in the future, you'll need to identify
source, propogation mode, and victim before deciding how to
attack the problem. The most common addition of useless shielding
was on alternators for the purposes of reducing noise in the
ADF. The guys at Cessna threw on some shielded wire, added some
filters, flew the airplane and pronounced it good. When I asked
if the shielding helpped, they said "you bet!" Actually, nobody
tried it without the sheilding and the filter was REALLY doing
all the work. None the less, tons of shielding material has been
added to tens of thousands of airplanes since.
Before you throw the book at a noise problem, drop us a note.
Let's size the task and pick the right tools.
Bob . . .
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