Hopefully A Simple One...

A friend/coworker of mine has a glider that he likes to fly, and recently rewired his entire instrumentation panel. Since then, his radio has a ton of static, and squelches quite often.

He replaced all of the instruments, kept the same radio, and also added a computerized mapping gadget. The wiring has a "star" ground to a bus bar (connected to the battery), and a fuse block provides individual, fused circuits to each instrument.

The static is greatly reduced if he has the mapping computer on, but is present other times as well.

My guess is to put an LC filter near the power entry of both the radio and the mapping computer. Is that a good start?

Second, can you suggest appropriate filters? I know that Rat Shack has filters for car audio, but I imagine that something more suitable is out there.

i would start by looking closely at how the physical layout of the re-wiring is .

an LC filter should not be difficult to build ...
i don't think those "brute force" trash shack filter kits are designed for much current .

See if any of these help:

http://cor.com/

We used their AC filters when I was in manufacturing. Don't have any first hand experience with the DC line.

That was also my first guess, as one wire routed thru/near the wrong place can cause havoc. If he can see any correlation between appearance of the static and instrument variations, it could help pinpoint the problem area(s).

Some added (tinfoil?) shielding might also be an option if he has any idea where to put it.

fold foil ... place on head ?


some ferrite ring chokes slipped over the equipment power lines might help .
keep the rf from being radiated back down the line and keeps it out of the equipment .

it sounds like he may be having problems with rf being radiated from a device that is close to the radio/coax/antenna .
some of those displays are real garbage generators and radiate directly from the display itself .

Yer killin' me here...

Alright. I'll tell him to take a look and make sure that power and antenna lines are nicely separated, and look into some LC filters.

The Cor units are pretty slick, but at $70 each... wow.

As for ferrite beads... if I take the toroids from an SMPS and have him wrap the wires through a few times, will they work well?

I'm waiting to hear back from him on power draw and the freqs that the radio uses.

depends on which toroids you use.. of course the only other toroid they put in smps is powdered iron and those are easy to differentiate.

anyhow, a common mode choke on the radio supply leads should do it.

often the radio and the power supply form part of the antenna network unintentionally. on an airplane where there is no "ground" you might need to reconfigure the antenna topology.

How about a little troubleshooting before the hacking begins?

This is a big clue. I would normally expect the interference to go up when the device is on since that's when the aggressor currents would be greatest. My blind guess here would be to look at the switching regulator in the mapping device.

Was the cable bundling and routing changed significantly? If so how and where, the answers to those two questions are also good clues.

Question 1: What frequency does the radio operate at? If it supports multiple bands is the static present on all the bands?

Question 2: Is the coupling between aggressor( presumably the mapping device) and victim( the radio) conducted or radiated?

Try running the mapping device on a separate battery. Disconnect it's supply and ground leads and connect to its own battery. Notice any effect on the static? If there is no effect the coupling is probably radiated. If the static goes away the coupling is conducted.

Off the wall questions: Does the mapping device support bluetooth? Where is its GPS antenna located?

There's nothing like a good mystery, good luck to you.

So, I spoke with him again, and all if the cables are quite close, his antenna wire is just a couple of inches from all if the power wires. And in an attempt of being neat, he cut them all to exactly the right length to just reach the instruments, so he doesn't have length even to wrap around ferrite toroids, let alone reroute them.

I also meant to say that the static is WORSE when the mapping computer is on, so I suspect it to be a main culprit.

I am going to give him some clamp on ferrite beads, and tell him that if they don't work, he needs to start cutting wires, rerouting, and filtering the radio and computer more aggressively.

Running a separate battery isn't an option, he is limited by the size and location of battery compartment in the cockpit.

He also runs an HD video recorder, but that uses the linear regulator that I built him specifically because the switchers he tried caused radio interference.

Another big clue. It's a safe bet that the mapping computer has at least one switcher inside of it.

The purpose of the battery is as a test to determine if the noise coupling is conducted, not to be a fix for the problem. Putting ferrites on cables won't help if the coupling is radiation from the mapping box directly to the antenna. The general idea is to use the troubleshooting time and effort as well as possible, don't just try something and see if it works.

Cheers,

Cool, I appreciate the explanation. I will pass it along.