Today's Message Index:
----------------------
1. 07:29 AM - Re: Re: a bit O/T ...AEA Responds (Robert L. Nuckolls, III)
2. 07:32 AM - Re: Antenna static discharge protection (Robert L. Nuckolls, III)
3. 09:48 AM - Ground loops (John Grosse)
4. 11:08 AM - Re: Ground loops (Robert L. Nuckolls, III)
5. 11:14 AM - Re: Ground loops (Robert L. Nuckolls, III)
6. 11:15 AM - Re: Ground loops (Charlie England)
7. 01:17 PM - Re: Ground loops (John Grosse)
8. 01:38 PM - Re: power line interferances (James Robinson)
9. 02:34 PM - Re: power line interferances (Robert L. Nuckolls, III)
10. 03:35 PM - Re: Ground loops (Robert L. Nuckolls, III)
11. 05:59 PM - WxWorx REWX9ID USB Weather Receiver For Sale... (Matt Dralle)
Message 1
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| Subject: | Re: a bit O/T ...AEA Responds |
>Perhaps we can get together some time & pursue it, either in person
>or on Bob's potential new politics list.
I'm working with Matt to fix some bugs in the
list-server engines I mentioned. When Matt
has some spare time, he'll get them squared
away.
At that time, I'll invite anyone who desires
to join a discussion group for what I'll call
Simple-Ideas. An exploration of fundamental
truths that transcends politics (but is often
stirred in with politics).
Subscription to these lists is by request/invitation.
I maintain the registered address lists manually
for now . . . as opposed to the user management
system that supports this list.
Will let you all know when it's up and running.
Bob . . .
Message 2
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| Subject: | Re: Antenna static discharge protection |
At 10:40 PM 7/1/2011, you wrote:
>Ah! I hadn't considered a shunt inductor!
>
>Thanks for the info! If anyone else knows where to find data on in
>flight ESD I'd still love to see it.
We had a big shake-up at Beech about 25 years
ago to deal with some static discharge wick issues
on the fleet. There are pretty simple bench tests
to test the relative effectiveness of a static
wick. I'm aware of no flight test data . . . you
might poke around in the patents (freepatentsonline.com)
and NASA documents.
Bob . . .
Message 3
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I think I understand about avoiding ground loops by providing only a
single path to ground, but I'm confused by the fact that the avionics
manufacturers (Garmin and PS Engineering in particular) tell you to
ground both ends of a shielded cable. Specifically, when going from a
mic or phone input to the audio panel or going from a com radio to the
audio panel all the wiring diagrams show the shield grounded at both
ends. Doesn't this create a ground loop?
Can someone please enlighten me.
John Grosse
Message 4
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| Subject: | Re: Ground loops |
At 11:43 AM 7/2/2011, you wrote:
>
>I think I understand about avoiding ground loops by providing only a
>single path to ground, but I'm confused by the fact that the
>avionics manufacturers (Garmin and PS Engineering in particular)
>tell you to ground both ends of a shielded cable. Specifically, when
>going from a mic or phone input to the audio panel or going from a
>com radio to the audio panel all the wiring diagrams show the shield
>grounded at both ends. Doesn't this create a ground loop?
If they are grounded to different locations . . . absolutely.
I am increasingly mystified by the instructions
from folks who should know better are mis-using shields over
wires . . . and especially 'grounding both ends' when the
shield is NOT being used as a signal path.
Here's an example of an audio system using NO shielded
wires. It also takes potential for ground loops into
account with note 2.
http://www.aeroelectric.com/PPS/Audio/2000014C.pdf
Here's a drawing I produced where shields ARE part
of the signal pathways. Note specific treatment
callouts for shields at both ends.
http://www.aeroelectric.com/PPS/Audio/DualCommAudio.pdf
Same thing here . . .
http://www.aeroelectric.com/PPS/Audio/hv1_760vhf.pdf
>Can someone please enlighten me.
If the shield over a wire is ALSO a signal path,
i.e. the 'ground' connection for a remotely mounted
mic or headset jack, then that shield goes to ground
at ONE END ONLY which will be the "lo" side of that
feature AT THE INTERCOM or RADIO's ground . . . which
in turn is taken to the instrument panel ground for
extension to aircraft ground as illustrated in Z-15.
You can have LOTS of grounds if they are strung out
in a planned, linear fashion. The "loop" happens when
a conductor operating at ground is tied to the airframe
in more than one place . . . when that structure
carries potentially antagonistic 'noise currents'.
Suggest you review
http://www.aeroelectric.com/articles/richter/response_1.pdf
http://www.aeroelectric.com/articles/richter/response_2.pdf
by searching on the word 'shield' in these two documents
and then review the context in each case.
The 'Richter Affair' was some years back but it seems
that many individuals have the same mis-conceptions
in common about shields. It's a sad day when the installers
instructions are not well thought out.
Bob . . .
Bob . . .
Message 5
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| Subject: | Re: Ground loops |
At 11:43 AM 7/2/2011, you wrote:
>
>I think I understand about avoiding ground loops by providing only a
>single path to ground, but I'm confused by the fact that the
>avionics manufacturers (Garmin and PS Engineering in particular)
>tell you to ground both ends of a shielded cable.
Can you point me to wiring diagrams on the 'net that
speak to this?
Bob . . .
Message 6
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| Subject: | Re: Ground loops |
On 07/02/2011 11:43 AM, John Grosse wrote:
> <grosseair@comcast.net>
>
> I think I understand about avoiding ground loops by providing only a
> single path to ground, but I'm confused by the fact that the avionics
> manufacturers (Garmin and PS Engineering in particular) tell you to
> ground both ends of a shielded cable. Specifically, when going from a
> mic or phone input to the audio panel or going from a com radio to the
> audio panel all the wiring diagrams show the shield grounded at both
> ends. Doesn't this create a ground loop?
>
> Can someone please enlighten me.
>
> John Grosse
Hi John,
One thing to remember is that for signals in an unbalanced circuit,
'ground' is usually referring to the return path to complete a circuit.
The return is usually at 'ground' potential, but in some circuits, it
might not be.
A ground 'loop' is actually having 2 different paths for the ground, as
you say. In most audio circuits, the shield is used for the return path.
With the mic circuit, the mic jack is almost always specified as
isolated from the airframe to avoid that 2nd return path.
The radio to audio panel issue is a bit harder to reconcile with the
'rule', but signal levels are higher there & the shield path is
typically so short that secondary return paths don't seem to be as much
of an issue. If you allow one end of the shield to float, and let the
return find its own path through the airframe, I'd bet that you'd be
creating the kind of problems that the 'rule' is trying to prevent.
That's not a very satisfying answer, I know. Maybe someone else can give
a more technically specific answer.
Charlie
Message 7
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| Subject: | Re: Ground loops |
Thanks for the responses.
I misspoke about PS Engineering. They ground the shield at one end, but
here's the diagram for a Garmin SL40: You'll notice that they
specifically say to ground both ends of the shield to the case.
John
> ------------------------------------------------------------------------
>
> Robert L. Nuckolls, III <mailto:nuckolls.bob@aeroelectric.com>
> July 2, 2011 1:11 PM
>
>
> <nuckolls.bob@aeroelectric.com>
>
>
> Can you point me to wiring diagrams on the 'net that
> speak to this?
>
>
Message 8
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| Subject: | Re: power line interferances |
Hi Bob
I have talked to you before about my needing to move the batteries in my Glasair
111 from the firewall to behind the baggage compartment. My latest problem with
this conversion is the routing of the 2 #2s and 2 #4s. To run down the center
console there are a lot of things running in that area, but there may be enough
room. The other option is to go under the wing, but that has it's problems as
well. There is minimum room between the bottom of the wing and the belly panel,
and a VOR antenna is in the way. My main question is: Are there potential
problems I need to factor into the equation as to where these big wires run in
relationship to the other wires and antennas. Any suggestions or ideas.
Jim
James Robinson
Glasair lll N79R
Spanish Fork UT U77
Message 9
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| Subject: | Re: power line interferances |
At 03:35 PM 7/2/2011, you wrote:
>Hi Bob
>I have talked to you before about my needing to move the batteries
>in my Glasair 111 from the firewall to behind the baggage
>compartment. My latest problem with this conversion is the routing
>of the 2 #2s and 2 #4s.
Why two #4. The two batteries can share a single
#2 ground.
> To run down the center console there are a lot of things running
> in that area, but there may be enough room. The other option is to
> go under the wing, but that has it's problems as well. There is
> minimum room between the bottom of the wing and the belly panel,
> and a VOR antenna is in the way. My main question is: Are there
> potential problems I need to factor into the equation as to where
> these big wires run in relationship to the other wires and
> antennas. Any suggestions or ideas.
Run them all together. There are no proximity issues
with respect to other wires/accessories.
Bob . . .
Message 10
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| Subject: | Re: Ground loops |
At 03:13 PM 7/2/2011, you wrote:
>Thanks for the responses.
>I misspoke about PS Engineering. They ground the shield at one end,
>but here's the diagram for a Garmin SL40: You'll notice that they
>specifically say to ground both ends of the shield to the case.
>
>John
Okay, I'll bet you a dollar to a donut that
the "audio grounds" connected with wires are internally
grounded to chassis on both devices. Adding a shield
over a twisted pair adds no benefit for two devices
located next to each other. In this case, adding shields
and grounding at both ends is only redundant to the
ground-to-ground wire . . . hence, no risk.
You could just as easily wire these boxes up with twisted
pairs and no shields.
<ceengland@bellsouth.net>
The radio to audio panel issue is a bit harder to reconcile with the
'rule', but signal levels are higher there & the shield path is
typically so short that secondary return paths don't seem to be as
much of an issue. If you allow one end of the shield to float, and
let the return find its own path through the airframe, I'd bet that
you'd be creating the kind of problems that the 'rule' is trying to prevent.
I think the potential is already there with the 'audio
grounds' If the radio were, say, in a rear cockpit and
the audio panel in front . . . and the battery grounds were
in the tail, then there is potential for ground loop coupling
whether or not a shield is present.
That's not a very satisfying answer, I know. Maybe someone else can
give a more technically specific answer.
Most general aviation systems make prolific use
of chassis ground for both power and signals within
any given device. This generally not a problem for
small aircraft . . . especially when we go to some
pains to avoid noisy circuits circulating on the airframe.
Military stuff uses chassis ground only for shielding
where LOCAL bonding to airframe is desirable. RFI filters
grounded to chassis bring signal and power wires
in to dedicated, floating grounds within the device.
This makes is MUCH easier to manage ground systems.
So I guess I'd have to continue to say there are no
hard 'rules' for handling shields because we seldom
are privy to wiring details inside the black boxes.
If the schematic John provided us was for some really
'advanced' equipment, those paired audio lines would
be true balanced pairs and only one "low side" would
be grounded internally to inside the source box. The load
end would be totally floating with a ton of common mode
rejection. Shielding would still be of no particular benefit.
All we can do is the best guess and then go after a
noise problem should it happen to pop up. Our best guess
is adequate probably 99% of the time.
What we can say for sure is that all the schematics
showing shields that are not part of a return path
could easily be replaced with a wire. Shielding has
essentially zero probability of added value in our
working environment.
Bob . . .
Bob . . .
Message 11
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| Subject: | WxWorx REWX9ID USB Weather Receiver For Sale... |
I have a 1-year old WxWorx REWX9ID USB XM Aircraft Weather and XM Audio receiver
for sale. It was just removed from my RV-8 and is in perfect working order.
Includes Antenna, Mount, Receiver Unit, and USB Interface.
It is compatible with the Grand Rapids EFISs and a number of others. Gives text
and graphical representations of weather including Rain, Wind, Temp, etc. Requires
separate monthly XM subscription which is not included in this offering.
Attached are some pictures of the actual unit for sale.
Works great and I wouldn't fly without a similar product. I'm replacing the functionality
with an ADS-B receiver.
Additional information on the unit can be found here:
https://www.wxworx.com/portable-receiver-overview
https://www.wxworx.com/interface-modules
Aircraft Spruce has the unit for $595:
http://www.aircraftspruce.com/catalog/avpages/fl190True.php
I will take $400 firm for the unit as shown plus $10 for shipping. First come,
first served. Email me with questions.
I can accept PayPal http://www.paypal.com using "dralle@matronics.com" or I can accept Visa/MC directly. I will take a 2.5% discount for a check, but it will have to clear before I ship.
Best regards,
Matt Dralle
-
Matt Dralle
RV-8 #82880 N998RV "Ruby Vixen"
http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log
http://www.mattsrv8.com/Rebuild - Post Landing Mishap Rebuild Log
http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel
Status: 170+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...
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