Lowering cockpit RF interference

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

Thank you Dan for your article link

John DeRosa wrote:
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.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Am I correct to assume the best grounding point would be the heaviest metal frame work closest to the instrument panel such as the control yoke or the adjustable rudder pedal track?

So in the perfect world all things metal (especially long lengths like push rods) and your negative instrument buss should be grounded to a common ground point?

On 12/20/2012 7:58 AM, wrote:
Thank you Dan for your article link

John DeRosa wrote:
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.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Am I correct to assume the best grounding point would be the heaviest
metal frame work closest to the instrument panel such as the control
yoke or the adjustable rudder pedal track?

So in the perfect world all things metal (especially long lengths
like push rods) and your negative instrument buss should be grounded
to a common ground point?

I don't know how many adhere to the "single grounding point"; however,
note that the point is for the instruments, not necessarily everything.
The other metal bits, which are not carrying signals, can be grounded
elsewhere, such as the near the battery. A powered sailplane will likely
also have an "engine" ground, so the starter currents, ignition system,
generator, etc. are kept from the instrument ground.

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

wrote:
Thank you Dan for your article link

John DeRosa wrote:
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.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Certainly not when they are in the air!

OK, that's a joke, but it does bring into sharp
relief that the "ground" concept is a fiction
(except under limited circumstances that are not
relevant here). It is also a fiction on the ground
as well, particularly where RF is concerned (see
any book on antenna design).


Am I correct to assume the best grounding point
would be the heaviest metal frame work closest
to the instrument panel such as the control yoke
or the adjustable rudder pedal track?

I doubt it, but read the service manual and contact
a competent engineer/technician. I would be
very reluctant to allow electricity to be conducted
through any part of the airframe.

On 12/20/2012 9:17 AM, Tom Gardner wrote:
wrote:
Thank you Dan for your article link

John DeRosa wrote:
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.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Certainly not when they are in the air!

OK, that's a joke, but it does bring into sharp
relief that the "ground" concept is a fiction
(except under limited circumstances that are not
relevant here). It is also a fiction on the ground
as well, particularly where RF is concerned (see
any book on antenna design).

How do you think we should be discussing the issue?

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

Eric Greenwell wrote:
On 12/20/2012 9:17 AM, Tom Gardner wrote:
wrote:
Thank you Dan for your article link

John DeRosa wrote:
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.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Certainly not when they are in the air!

OK, that's a joke, but it does bring into sharp
relief that the "ground" concept is a fiction
(except under limited circumstances that are not
relevant here). It is also a fiction on the ground
as well, particularly where RF is concerned (see
any book on antenna design).

How do you think we should be discussing the issue?

I don't really understand the question, but maybe
we shouldn't be discussing it here. Interference
arises due to many causes, is transmitted by many
means, and is received in different ways [1]. A specific
solution to one combination of source/route/destination
will not be effective for other combinations. I'm
sorry, but there aren't any generic solutions.

Probably the best general advice is to have good quality
equipment, cables and connectors, and to ensure the
connectors are properly tightened.

And even that won't prevent the "rusty bolt effect"


[1] as a single _simple_ example, given signals coupling
from one wire to another leading to interference, there
are two types of interference, near-end crosstalk and
far-end crosstalk. The causes are different and the
solutions are different.

RF introduces a whole slew of more subtle effects.
There are _many_ books on the topic, which is a good
indication that it isn't a simple problem with a simple
solution.

But of course one solution to one problem may well
be simple.

This is like posing a question to a lawyer: "It depends."

What might be helpful to non-experts is a typical step-by-step
approach to identifying the type and source of interference and then
addressing it. For you AI types (is that term still used?), an expert
system flow chart. EX: If it's a noise in the radio speaker, go to
step 2a. If it's a noise in the audio vario speaker, go to step 2b. If
it's a disturbance in the needle or indicator of an instrument, go to
step 2c. Etc. "It depends" covers too much ground.

I've chased interference before, the most maddening case being hearing
a French African channel on Voice of America non-stop thru the
aircraft radio years ago when flying at Caesar Creek, OH. It was my
brother's glider, actually, but we worked on it together. From memory:
capacitors, homemade high-pass, low-pass, and band-pass filters,
shieded cables, shielding around various components, etc. Eventually
we drove the trailer over to the VOA facility, parked on the road
outside, pulled the fuselage out, and started working. That got action
pretty fast. I didn't realize VOA had security guards. Once they
believed our story, and that the Komet trailer wasn't full of
explosives, we were actually invited in to meet the manager and get
info on freqs, antennas, power, etc. Pretty awesome numbers, as I
recall. I honestly can't remember what finally did the trick. Maybe it
was my brother moving to VA.

Chip Bearden
ASW 24 "JB"
U.S.A.