Lowering cockpit RF interference

Hello all EEs.

What is standard operating procedure for when and where to use shielded wire in cockpit wiring?

What is standard operating procedure for when and where to use ferrite sleeves?

Do both the positive and negative lead need to pass through the ferrite sleeve?

Thanks for the education.

wrote:
Hello all EEs.

What is standard operating procedure for when and where to use shielded wire in cockpit wiring?

What is standard operating procedure for when and where to use ferrite sleeves?

Do both the positive and negative lead need to pass through the ferrite sleeve?

Thanks for the education.

It can all be a bit trial and error, but
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
gives some of the theory and practical aspects of it.

Key points:
- mount "close" to interference source
(close = less than 1/20th of the wavelength)
- mount "close" to whatever is being affected by
the interference
- multiple loops are better, usually
- resonance can cause surprises; be prepared to experiment
- any piece of wire can be an antenna, including the
cable shields

On Friday, December 14, 2012 11:04:39 AM UTC-8, wrote:
Hello all EEs. What is standard operating procedure for when and where to use shielded wire in cockpit wiring? What is standard operating procedure for when and where to use ferrite sleeves? Do both the positive and negative lead need to pass through the ferrite sleeve? Thanks for the education.

What equipment do you have in the cockpit,specify all equipment, Make and Model?

What kind on interference are you getting?

Richard
www.craggyaero.com

http://www.zug.com/img/ThreadImg/A/8...?ts=1355523304

On 12/14/2012 11:04 AM, wrote:
Hello all EEs.

What is standard operating procedure for when and where to use shielded wire in cockpit wiring?

What is standard operating procedure for when and where to use ferrite sleeves?

Do both the positive and negative lead need to pass through the ferrite sleeve?

Thanks for the education.

The simple answer: whenever the equipment installation manual tells you
to use it.

Typically use shielded wire for antenna cables (ie, coax) and microphone
cables.

Newer equipment usually comes with sleeves on the cables they supply.

If the equipment installations done properly, nothing additional is
needed in an unpowered glider. Powered gliders can have interference
generated by the engine running, but again, if the engine electrics are
installed properly and in good condition, there usually isn't a problem.

Generally, I'm amazed at what people can get away with doing!

Most electrical noise problems come from specific problems in the
installation, and can be tough to track down. Ferrites can be an easy
fix sometimes; shielded wiring isn't so easy to install, and it's often
hard for even an experienced EE or technician to know exactly which
cable(s) is the problem and should be replaced.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On Fri, 14 Dec 2012 14:24:28 -0800, Eric Greenwell wrote:

Most electrical noise problems come from specific problems in the
installation, and can be tough to track down. Ferrites can be an easy
fix sometimes; shielded wiring isn't so easy to install, and it's often
hard for even an experienced EE or technician to know exactly which
cable(s) is the problem and should be replaced.

The only electrical noise problem I've had was when I installed a 28v T&B
which was powered of the glider's 12v supply with a solid state 12v-28v
DC-DC converter. The solution was to put an electrolytic capacitor across
the 12V supply to the converter. This was a specialized noise cancelling
capacitor and IIRC it is 150 uF with a 1.5A transient current capacity.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

I had the same problem, though with a 12v T&B, and used a ferrit in the
power lead to solve it.


The only electrical noise problem I've had was when I installed a 28v T&B
which was powered of the glider's 12v supply with a solid state 12v-28v
DC-DC converter. The solution was to put an electrolytic capacitor across
the 12V supply to the converter. This was a specialized noise cancelling
capacitor and IIRC it is 150 uF with a 1.5A transient current capacity.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Sat, 15 Dec 2012 11:49:06 -0700, Dan Marotta wrote:

I had the same problem, though with a 12v T&B, and used a ferrit in the
power lead to solve it.

I think I tried a ferrite core on the 12v side of the voltage converter
without success and then remembered that Maplins (the UK equivalent of
RadioShack) had these capacitors at a reasonable price. Better yet it was
easy to fit: I just soldered it across the T&B power switch.


The only electrical noise problem I've had was when I installed a 28v
T&B which was powered of the glider's 12v supply with a solid state
12v-28v DC-DC converter. The solution was to put an electrolytic
capacitor across the 12V supply to the converter. This was a
specialized noise cancelling capacitor and IIRC it is 150 uF with a
1.5A transient current capacity.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Saturday, December 15, 2012 2:32:26 PM UTC-8, Martin Gregorie wrote:
On Sat, 15 Dec 2012 11:49:06 -0700, Dan Marotta wrote:

I think I tried a ferrite core on the 12v side of the voltage converter

without success and then remembered that Maplins (the UK equivalent of

RadioShack) had these capacitors at a reasonable price. Better yet it was

easy to fit: I just soldered it across the T&B power switch.

martin@ | Martin Gregorie

gregorie. | Essex, UK

org


A couple of thoughts here . . .

When you say you soldered it "across" the power switch, that might lead one to assume you soldered it to the two terminals of your switch, i.e. in parallel with the switch contacts). Normally a filter capacitor would be connected across the power leads, i.e. from + to -. While a filter inductor (such as a ferrite) is connected in series.

When trouble shooting RFI, it helps to be able to quantify results. If it's interferance on comm freqencies, a hand held radio can be useful - squelch off, and distance away from suspected RFI source as appropriate one can use the handheld as a field strength meter for radiated interferance.

This is not so much use when dealing with closely spaced instruments on the panel where one is causing problems with another (LNAV did that to Comm on one of previous glider). There, just slapping in clamp-ferrites did the job.

If you have enough slack in wiring leads, and the size of the ferrite allows, you get almost double the iductance for loopiing the wire/s through the ferrite twice, (or even more for 3 times etc). Sometimes adding a capacitor in concert with the ferrite helps even more, especially when noise is coupling via power leads (ferrite or inductor in series and cap across the wires in parallel to form a classic pi filter).

Ferrites are not all the same, they have a frequency band at which they will be most effective. Same thing with the size of capacitors used in filtering apps. (do a search on inductive and capacitive reactance and resonant frequecy). In the U.S., Digi-Key is one good source for ferrites etc.

bumper
zz
Minden, NV

bumper

Your question is less about "I have a problem and want to fix it." versus, "I don't want any problems in the first place. What are the best practices to follow?"

Literally the only interference problem I have every had was a vario that deflected when I keyed the transceiver. Solution: replace the vario. I must live a blessed life. This has also been in pure gliders only so I don't have to worry about ignition noise which is by far the worse culprit for interference. However, new electronics run at some high frequencies (1Mhz processors are sloooow these days), so interference can happen.

So back to what I do to prevent problems in the first place.

1) Start with QUALITY - Components, wiring, terminals, switches, etc. Buy from an aviation source like Wicks or Aircraft Spruce. Radio Shack and ACE Hardware DO NOT have aviation aisles. Modern avionics seldom seem to create issues as they have to pass some pretty stringent testing to be certified. Step away from anything still using crystals (are they still legal?).
2) Coax for microphones and antennas. But every microphone and antenna already comes with coax. The more important bit is making sure that the shield has a good ground at the avionics end. A good BNC connection for the antenna (have someone knowledgeable help). Many transceivers have a separate ground pin for the microphone - use it!
3) Ferrite beads - While my EE specialty is not in the RF spectrum (get it?), putting these on doesn't hurt and might help ... so I use them. As someone in this thread said, run both the +/- wires through the bead twice if you have the slack. Non-split beads are generally better than split beads. The trick is to prevent high frequency noise from one device getting into another device through your power bus.
4) Air gaps - Wires that might have data flowing through them, like a GPS to a PDA, can induce noise into another wire running right along side of it in a bundle. So separate any suspect wires from one another.
5) Grounding - Ground all your devices to a single common location as close to the electronics as possible, including the negative side of the battery.. Even ground the metal stick, push tubes, etc, if possible.

Good luck, John