Today's Message Index:
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1. 07:37 AM - Re: Shielded wiring for P leads (Noel Loveys)
2. 08:09 AM - Re: Shielded wiring for P leads (Dave)
3. 11:01 AM - Re: Shielded wiring for P leads (Don Honabach)
Message 1
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| Subject: | Shielded wiring for P leads |
Ok Barry I=92ll bite.
I=92m not sure about this one...
For phone, in the earliest part of the twentieth century Reg Fressenden,
a
Canadian had a transmitter set up in Connecticut. He used a generator
(Anderson alternator) with many poles and running at extremely, for the
day,
high speeds to develop the frequency he wanted for the first broadcast
radio. If memory serves me right on his first test the darn thing blew
up
on the first trial but he had a back up for his first transmission
connected
to his antenna. That makes him the inventor of radio as we know it
today.
The earliest Morse code transmitters used spark gaps. The system was a
muther of a coil attached to as much dc voltage as they could muster and
a
key wired in series. The secondary coil as connected to what is
essentially
a spark gap and a tank circuit to amplify only one frequency. Problems
abounded. As the spark electrodes heated up the width of the spark
changed,
the batteries ran down and one tank is not enough filtration/tuning so
spark
transmitters are noisy all over the bands. A long time before I became
involved in Amateur Radio spark transmitters became illegal but there
are
always a few around who will try them. Then again there are lots of
unlicensed operators on the bands.
BTW I have been to but not operated the radio station at the receive
site of
the world=92s first trans Atlantic broadcast.... it was only a
resounding
letter =93S=94 picked up by an antenna actually flown aloft by a kite.
My two
most favourite things, flying and Radio.
Ground planes not just happening you are right they are constructed but
not
always intentionally. You are also right that the shield is not a
ground
plane. However ground planes can occur when grounded shields are
attached
to metallic bodies at the end remote from the source.. especially if one
of
those bodies happens to be =BC wavelength the com frequency as can
easily
happen in the panel of one of our small planes.
As for reinventing the wheel... Why not? Here it is complete with bent
spokes ;-) http://www.gizmag.com/go/3603/
Noel
From: owner-jabiruengine-list-server@matronics.com
[mailto:owner-jabiruengine-list-server@matronics.com] On Behalf Of
FLYaDIVE
Sent: September 23, 2010 7:25 PM
Subject: Re: JabiruEngine-List: Shielded wiring for P leads
Noel:
Please read my responses within the body of your post.
Barry
On Thu, Sep 23, 2010 at 3:54 PM, Noel Loveys <noelloveys@yahoo.ca>
wrote:
Mark:
The EMI given off by P leads is hardly at RF frequencies as the sound is
actually generated by the mags
[Barry] - There SURE IS RF given off by a Mag. How? Well when the
points
OPEN they create a SPARK... The Spacing of the Points is know as a Spark
Gap... A.K.A. Spark Gap Transmitter... It is the exact same thing Hertz
and
Marconi experimented with way back in the days of "The Eiather" . So
YES,
Mark and Noel you sure do want to Shield and Ground Both Ends.
LOADED QUESTION Noel: Back in the day what did they use to control the
Frequency?
or alternator both of which are well into audio frequency. Even to
shield a
wire from RFI grounding at one end prevents the accidental construction
of a
ground plane
[Barry] - Ground planes don't just happen ... Especially like in the
above
explanation
A ground plane is Frequency, Length, Area and Distance dependent. A
shield
is NOT a ground plane..
at the end furthest from the source. Even when wiring very complex
panels
in large aircraft shielded wires are generally shielded only at the
source
end.
[Barry] - Gaggle - Please go back and read my post on the RULES of
Grounding
and Shielding.
THEY WORK!
Shielding is broken down into AF & RF. That is all I'm going to say for
now. Please Read My Post.
Let's not re-invent the wheel. Especially with bend spokes.
Barry
E.E. & M.E.
For example; The mic wire which comes from the transceiver to the audio
panel should only have the shield grounded at the Transceiver end. The
mic
wire that goes from the audio panel to the pilot/co-pilot jacks (they
are
different) will have the shield grounded at the audio panel. The mic
cord
itself is grounded through the shield attached to the base ring of the
jack.
All that takes care of stray RF getting into the Mic system (hopefully
;-D)
but then there are the EMI sounds that can get into the DC source to the
radios. This EMI is things like alternator noise and ignition noise.
Again
to prevent shielded wire from becoming a capacitive bridge into the DC
power
wires for the radios it is normally only shield grounded at the source
(close to the generator) end.
With some of these systems you may actually come across a situation
where an
extra ground will protect a system... just don=92t ask anyone to
explain why.
Some sources of EMI and there are a lot more than I can list here
Fouled spark plugs, Worn crystallized plug wires, worn capacitors in the
alternator, Poor grounding on an ECU, Poor grounding of the engine case
to
the electrical circuit, Poorly grounded strobe system, Poor grounding of
the
radio stack, poor grounding of the braid at the antenna end of feed
coax.
Gee it=92s surprising how many times grounding comes up. Rahter than
having
added grounds going willy-nilly I think it=92s better to make sure the
ones
installed and which worked for sometimes years are all clean. To clean
these grounds you must disconnect them, clean the contact areas and
reattach
them. Looking at a connection doesn=92t count.
While you are at it why not make a diagram of the wiring of your plane.
Just a simple block diagram will do. Show the colours of the wires, the
connection plugs and where all the grounds are. Such a diagram will
make
future troubleshooting a whole lot easier both for yourself and future
owners of your plane. Put all your diagrams in your aircraft (log) book
so
you can find it easily.
Noel
From: owner-jabiruengine-list-server@matronics.com
[mailto:owner-jabiruengine-list-server@matronics.com] On Behalf Of Mark
Hubelbank
Sent: September 21, 2010 10:47 AM
Subject: JabiruEngine-List: Shielded wiring for P leads
To all working with noise in aircraft wiring. I have been following the
listings on the subject of shielding and as a design engineer, I have
one
thing to add (without the intent of generating any controversy).
This may help in cases where noise is comming from the ignition "P"
leads
which run from the mags to the ignition key switch. There is a lot of
talk
on grounding one end vers both ends of a shield. When one is working
with a
RF situation, one needs to ground both ends otherwise the shield will
simple
pick up the signal from the center conductor and act as an antenna
itself.
After all an antenna at VHF frequencies is simply a short wire connected
at
one end. In the case of the "P" leads, you may be radiating RF energy
picked
up from the mags into the instrument panel area, exactly where you
really
don't want it.
Thus when trying to prevent RF from "leaking" out, one needs to ground
the
shield at both ends. This is more problematic in a composite aircraft
and
other measures may be needed at the instrument panel end if there is
nothing
to ground to. In those cases, some other RF tricks may be required. I
have
not actually had to "de-noise" such an installation but I would first
try a
combination of terminations (50 ohm carbon resistor in series with 0.001
uf
ceramic cap - very short leads from shield to center conductor) and
ferrite
suppressors on the wire. Also note that by using wire that has high loss
at
VHF some of the conducted interference is attenuated. Typically non-coax
shielded wire (MIL-C-27500) is in this class.
Note that the connect shield at one end logic was originated to deal
with
low frequency issues like ground loops. An even better solution in some
of
those cases is to float the device at one end. For example for a
microphone
lead, one can use insulating washers to separate the jack from the
airframe
and then use the shield to provide the return path. This is the best way
to
go when possible. All this is mentioned to various degrees in the
literature
but I thought it does not hurt to give the example.
--
Mark Hubelbank
NorthEast Monitoring
2 Clock Tower Place
Suite 555
Maynard, MA, 01754 - USA
mhubel@nemon.com
978-443-3955
ist"
target="_blank">http://www.matronics.com/Navigator?JabiruEngine-List
tp://forums.matronics.com
_blank">http://www.matronics.com/contribution
Message 2
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| Subject: | Re: Shielded wiring for P leads |
Yawnn... Have we figured out which geek is the most stubborn yet?
I think trying to help anybody got trampled into in the dirt a long time
ago.
Message 3
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| Subject: | Shielded wiring for P leads |
Just my two cents...
I've gone done this road and after talking to a few EEs and one that spent
his life designing radar systems for the military, I came the following con
clusions:
1. Keep it simple **** - for me that is a single point ground connecte
d right at the battery. This keeps troubleshooting straight forward and I d
on't have to worry about inspecting multiple ground points, and yadda yadda
. At the end of the day, a simple, easy to understand system FOR ME at leas
t is the safest.
2. When available - follow the advice from the gadget's manual I'm ins
talling for shielding at one end or both ends.
3. Make sure I have access panels/access to any area where I'm worried
about noise (i.e. headset/radio) as the after listening to the experts; I
believe this statement that I heard, "RF/noise engineering is more an art t
han a science". As such, expect to tweak and EXPERIMENT during your first 4
0 hours.
This method worked great for me and I have absolutely no noise issues. The
worse thing anyone can say about my system is that I added 1 or 2 pounds of
cabling because I ran some ground wires instead of using the frame. If tha
t is the biggest complaint someone can find about my plane, I'm not going t
o lose any sleep.
Thanks,
Don Honabach
Zodiac 601HDS (N601HDS)
Jabirua 3300A - 150 Hours
From: owner-jabiruengine-list-server@matronics.com [mailto:owner-jabiruengi
ne-list-server@matronics.com] On Behalf Of Dave
Sent: Friday, September 24, 2010 8:07 AM
Subject: Re: JabiruEngine-List: Shielded wiring for P leads
Yawnn... Have we figured out which geek is the most stubborn yet?
I think trying to help anybody got trampled into in the dirt a long time ag
o.
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