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mhubel
Joined: 05 Sep 2009
Posts: 141
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 Posted: Tue Sep 21, 2010 8:43 am Post subject: Shielded wiring for P leads |
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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)Â 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.
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_________________
Mark Hubelbank
N708HU
CH601XL
Jabiru 3300
Rotec TBI 40-3 carb
Sensenich ground adj prop.
240 hr TAF
Pictures at photo.hubbles.com |
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BobsV35B(at)aol.com
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| Posted: Tue Sep 21, 2010 10:30 am Post subject: Shielded wiring for P leads |
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Good Afternoon Mark,
What is your opinion of the practice of using the shield on the P lead wiring as the ground path for the magneto switches and grounding the shield only on the magneto end?
Happy Skies,
Old Bob
In a message dated 9/21/2010 11:43:34 A.M. Central Daylight Time, mhubel(at)nemon.com writes:
| Quote: | 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.
|
[quote][b]
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mhubel
Joined: 05 Sep 2009
Posts: 141
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| Posted: Tue Sep 21, 2010 2:25 pm Post subject: Shielded wiring for P leads |
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Bob,
I believe this is the interesting case. Since in normal operation, the switch is open, one has a piece of unterminated transmission line running from the mag. If one assumes some VHF energy is injected into the wire at the mag end (if not none of this matters), then at some frequencies you have a transmission line which will have a high voltage on the cabin end of the shield. That is effectively an antenna. To make it worse, a lot of that antenna is in the cabin near your radios. How bad this will be depends on the wire length. I have not done formal tests on this but theory says that odd multiples of 1/4 wavelength should be the worst length. At 127 MHz 1/4 wavelength in shielded wire is about 0.39 M.
In an all metal plane, I would ground it at both ends. One can still have some RF current flowing in the shield. To minimize that, one could then terminate the line at the switch end with a series RC. This will absorbe the RF component . I would expect the effect of this terminator to be small.
Unfortunately, most engine noise is often from the high voltage side of the ignition and none of this will help much. Then only suppressor wire helps for unshielded ignition systems like the Jabiru one. It further appears that the Jabiru system has the unfortunate property that some RF energy is radiated from the mags themselves. This part may be almost impossible to completely eliminate.
Nothing I am talking about has anything to do with things like "alternator whine" which gets into earphone and microphone lines. That is a whole different story.
On 09/21/2010 2:15 PM, BobsV35B(at)aol.com (BobsV35B(at)aol.com) wrote: [quote] Good Afternoon Mark,
What is your opinion of the practice of using the shield on the P lead wiring as the ground path for the magneto switches and grounding the shield only on the magneto end?
Happy Skies,
Old Bob
In a message dated 9/21/2010 11:43:34 A.M. Central Daylight Time, mhubel(at)nemon.com (mhubel(at)nemon.com) writes:
| Quote: | 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(at)nemon.com (mhubel(at)nemon.com)
978-443-3955[b]
| | - The Matronics JabiruEngine-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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|
_________________
Mark Hubelbank
N708HU
CH601XL
Jabiru 3300
Rotec TBI 40-3 carb
Sensenich ground adj prop.
240 hr TAF
Pictures at photo.hubbles.com |
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Thu Sep 23, 2010 11:57 am Post subject: Shielded wiring for P leads |
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Mark:
The EMI given off by P leads is hardly at RF frequencies as the sound is actually generated by the mags 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 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. 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’t 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’s surprising how many times grounding comes up. Rahter than having added grounds going willy-nilly I think it’s 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’t 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(at)matronics.com [mailto:owner-jabiruengine-list-server(at)matronics.com] On Behalf Of Mark Hubelbank
Sent: September 21, 2010 10:47 AM
To: jabiruengine-list(at)matronics.com
Subject: 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.
[quote]-- Mark HubelbankNorthEast Monitoring2 Clock Tower PlaceSuite 555Maynard, MA, 01754 - USAmhubel(at)nemon.com978-443-3955[b]
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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flyadive(at)gmail.com
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| Posted: Thu Sep 23, 2010 2:01 pm Post subject: Shielded wiring for P leads |
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Noel:
Please read my responses within the body of your post.
Barry
On Thu, Sep 23, 2010 at 3:54 PM, Noel Loveys <noelloveys(at)yahoo.ca> wrote:
| Quote: | 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?
| Quote: | 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..
| Quote: | 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
[quote] 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’t 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’s surprising how many times grounding comes up. Rahter than having
added grounds going willy-nilly I think it’s 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’t 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(at)matronics.com [mailto:
owner-jabiruengine-list-server(at)matronics.com] *On Behalf Of *Mark
Hubelbank
*Sent:* September 21, 2010 10:47 AM
*To:* jabiruengine-list(at)matronics.com
*Subject:* 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
| | - The Matronics JabiruEngine-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Fri Sep 24, 2010 6:37 am Post subject: Shielded wiring for P leads |
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Ok Barry I’ll bite.
I’m 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’s first trans Atlantic broadcast.... it was only a resounding letter “S” 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 ¼ 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(at)matronics.com [mailto:owner-jabiruengine-list-server(at)matronics.com] On Behalf Of FLYaDIVE
Sent: September 23, 2010 7:25 PM
To: jabiruengine-list(at)matronics.com
Subject: Re: 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(at)yahoo.ca (noelloveys(at)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?
| Quote: |
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..
| Quote: |
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.
| Quote: |
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’t 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’s surprising how many times grounding comes up. Rahter than having added grounds going willy-nilly I think it’s 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’t 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(at)matronics.com (owner-jabiruengine-list-server(at)matronics.com) [mailto:owner-jabiruengine-list-server(at)matronics.com (owner-jabiruengine-list-server(at)matronics.com)] On Behalf Of Mark Hubelbank
Sent: September 21, 2010 10:47 AM
To: jabiruengine-list(at)matronics.com (jabiruengine-list(at)matronics.com)
Subject: 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. | Quote: | | -- Mark HubelbankNorthEast Monitoring2 Clock Tower PlaceSuite 555Maynard, MA, 01754 - USAmhubel(at)nemon.com978-443-3955 Mark Hubelbank |
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[quote][b]
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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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dave.go
Joined: 20 Jul 2009
Posts: 26
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| Posted: Fri Sep 24, 2010 7:09 am Post subject: Shielded wiring for P leads |
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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.
[quote][b]
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don.honabach(at)pcperfect
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| Posted: Fri Sep 24, 2010 10:01 am Post subject: Shielded wiring for P leads |
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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 conclusions:
<![if !supportLists]>1. <![endif]>Keep it simple **** – for me that is a single point ground connected right at the battery. This keeps troubleshooting straight forward and I don’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 least is the safest.
<![if !supportLists]>2. <![endif]>When available – follow the advice from the gadget’s manual I’m installing for shielding at one end or both ends.
<![if !supportLists]>3. <![endif]>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 than a science”. As such, expect to tweak and EXPERIMENT during your first 40 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 that is the biggest complaint someone can find about my plane, I’m not going to lose any sleep.
Thanks,
Don Honabach
Zodiac 601HDS (N601HDS)
Jabirua 3300A – 150 Hours
From: owner-jabiruengine-list-server(at)matronics.com [mailto:owner-jabiruengine-list-server(at)matronics.com] On Behalf Of Dave
Sent: Friday, September 24, 2010 8:07 AM
To: jabiruengine-list(at)matronics.com
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.
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Sat Sep 25, 2010 10:38 am Post subject: Shielded wiring for P leads |
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Definitely me! But geek??
lol
Noel
From: owner-jabiruengine-list-server(at)matronics.com [mailto:owner-jabiruengine-list-server(at)matronics.com] On Behalf Of Dave
Sent: September 24, 2010 12:37 PM
To: jabiruengine-list(at)matronics.com
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.
| Quote: | | http://www.matronics.com/Navigator?JabiruEngine-List |
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Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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flyadive(at)gmail.com
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| Posted: Sun Sep 26, 2010 7:02 am Post subject: Shielded wiring for P leads |
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Nole:
You must have the name spelled wrong because I was not able
to research anything on that name (Reg Fressenden). Marconi was given much
of the credit for the discovery of radio transmission ONLY because
he had money backing him and had a stage prescense.
Hertz, did a lot of teaching and lectures but his ideas were not widely
accepted, only because he did not produce as many or as elaborate a device.
Many years later they finally gave him the reconion he deserved and changed
the basic unit of frequency from the Cycle to HERTZ.
As I mentioned and you reiterated the first transmitters did use Spark Gaps
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Float Flyr
Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland
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| Posted: Sun Sep 26, 2010 5:18 pm Post subject: Shielded wiring for P leads |
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Bonne Vol
Noel
From: owner-jabiruengine-list-server(at)matronics.com [mailto:owner-jabiruengine-list-server(at)matronics.com] On Behalf Of FLYaDIVE
Sent: September 26, 2010 12:27 PM
To: jabiruengine-list(at)matronics.com
Subject: Re: Shielded wiring for P leads
Nole:
You must have the name spelled wrong because I was not able to research anything on that name (Reg Fressenden). Marconi was given much of the credit for the discovery of radio transmission ONLY because he had money backing him and had a stage prescense.
Hertz, did a lot of teaching and lectures but his ideas were not widely accepted, only because he did not produce as many or as elaborate a device. Many years later they finally gave him the reconion he deserved and changed the basic unit of frequency from the Cycle to HERTZ.
As I mentioned and you reiterated the first transmitters did use Spark Gaps. But what you are referring to with the generator (not alternator) came years later. It was know as a Rotary Spark Gap. The thought --- THOUGHT --- Was that the speed of the Rotary Spark Gap would control the frequency - Well, ANYTHING that turns on and off at a regular rate does have a frequency but was NOT the controlling factor. Marconi made the same mistake. That is where Hertz came in. Marconi wanted to make transatlantic communication an EVERYDAY event. Everyday in the ability to send a radio wave any time he wished. So he made bigger and bigger - Higher and Higher antennas. The bigger he went the less his chances of transatlantic communications became. Do you see where I am going?
Hertz said make them smaller... The LENGTH of the antenna was and still is the controlling factor - FREQUENCY.
As a HAM you should remember the formula:
L (in meters) = 300,000,000 / Freq (in MHz). That is a basic PHYSICS formula. So:
Freq = 300,000,000 / L (aka 299,792,458 meters per second = speed of light)
So, what did the Rotary Spark Gap do?
It created a longer lasting spark - Almost a continuous spark. Because the spark had to travel down the Antenna (wire) and then Radiate Out on a Frequency determined by the Length of the Antenna (Aerial) Oh! They did not have Antennas in those days.
Of Course - The frequency was no where accurate. It was a Very, Very Wide Band.
Now, how do I know all this trivia? I made a 300,0000 Volt Spark Gap Transmitter using a series of three neon sign AUTOTRANSFORMER'S. Found one alone worked better than all three.
Back to Ground Planes - As I said in my original post we are discussing VHF frequencies. Sure at the high end of VHF and of course in UHF and (SHF) Microwave Shields can radiate. But we are not talking those frequencies. And if we were other procedures would be used. Not for this discussion.
So, here is the other formula you should be acquainted with:
L (in feet) = 468/Freq (in MHz) this of course is for a dipole but divide that by 2 and you have a vertical. Verticals are what 98.76% of our planes use. <--- percentage data PFMA.
This will show that the length required for your 1/4wave... BUT! Wait a second. RF is Grounded at BOTH ends. AF is grounded at the source. So where is this magical RF coming down a AF line AND if it did, because of poor installation then it would have to be inductively coupled (Capacitive - No Way at our frequencies) AND there are so many reasons why that would not happen either. Power being #1.
And finally to respond to our not so geek minded friend. The reason why I perused this long (I'm sorry) discussion is to KILL the misconceptions and Old Wives Tails that are so easily propagated over the internet. ALL sorts of oral defecation propagate faster than the TV - Radio and especially the New Paper. While Noel is well meaning in his post the information is not correct, close but no cigar. And as YOU mentioned we are trying to HELP the fellow asking the question. Incorrect information does not help.
I'm off. Must go flying now - In a static free plane.
Barry
On Fri, Sep 24, 2010 at 10:25 AM, Noel Loveys <noelloveys(at)yahoo.ca (noelloveys(at)yahoo.ca)> wrote:
Ok Barry I’ll bite.
I’m 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’s first trans Atlantic broadcast.... it was only a resounding letter “S” 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 ¼ 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(at)matronics.com (owner-jabiruengine-list-server(at)matronics.com) [mailto:owner-jabiruengine-list-server(at)matronics.com (owner-jabiruengine-list-server(at)matronics.com)] On Behalf Of FLYaDIVE
Sent: September 23, 2010 7:25 PM
To: jabiruengine-list(at)matronics.com (jabiruengine-list(at)matronics.com)
Subject: Re: 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(at)yahoo.ca (noelloveys(at)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?
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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
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[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..
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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.
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[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.
| Quote: |
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’t 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’s surprising how many times grounding comes up. Rahter than having added grounds going willy-nilly I think it’s 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’t 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(at)matronics.com (owner-jabiruengine-list-server(at)matronics.com) [mailto:owner-jabiruengine-list-server(at)matronics.com (owner-jabiruengine-list-server(at)matronics.com)] On Behalf Of Mark Hubelbank
Sent: September 21, 2010 10:47 AM
To: jabiruengine-list(at)matronics.com (jabiruengine-list(at)matronics.com)
Subject: 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. | Quote: | | -- Mark HubelbankNorthEast Monitoring2 Clock Tower PlaceSuite 555Maynard, MA, 01754 - USAmhubel(at)nemon.com978-443-3955 Mark Hubelbank |
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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats |
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