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EMI susceptibility planning

gotyoke

Well Known Member
Hi, I'm roughing out a wiring plan for my -14 empennage and wings. I don't have the exact components figured out yet, so I don't know the pricise quantity and gages of wires, but at this stage I'm just working to validate my understanding of the concepts. I am not certain whether I'll be returning to ground tabs or grounding locally, but for now assume I'll be returning.
My particular concern is electromagnetic interference (EMI), not only with respect to audio signals, but also other potentially sensitive signals. I've been told distance seperation is the primary defense, with shielding as the next layer. With that in mind, here is my rough plan:

Empennage:
• Position light - 2 tefzel (power and return)
• ELT - 1 telephone cable (signal), 1 coax (signal)
• ADS-B - 1 coax (signal)
• Pitch AP - 2 tefzel (power and return), 1 shielded tefzel (trim signal)
• Pitch trim (controlled by AP servo) - 2 tefzel (power and return), 1 shielded tefzel (trim signal)
• COM 1 (may mount this on top well behind canopy) - 1 coax (signal)

Wings:
• Strobe (each side) - 2 tefzel (power and return)
• Nav (each side) - 2 tefzel (power and return)
• VOR/LOC/GS (right wing Archer) - 1 coax (signal)
• Landing/Taxi light (right) - 3 or 4 tefzel (power and return)
• Pitot heat (right) - 2 tefzel (power and return)
• Magnetometer (left) - 2 tefzel (power and return), 1 shielded tefzel (signal)
• Roll AP (right) - 2 tefzel (power and return), 1 shielded tefzel (signal)
• Fuel qty (each side) - ?

Which wires should be physically seperated from which others as much as possible? Which non-coax wires should be shielded as the extra layer of isolation? How would I plan to address EMI issues at the wing root connection points? Thanks.
 
EMI

Try this link to Robert L. Nuckolls book about aero electric installation.
It is free to download.

http://www.aeroelectric.com/Books/Connection/R12B_3rd-Printing_3-15.pdf

As a general guide, servo/ trim motors are known to make electric noise.
Transmitters like COM and transponders creates strong electric fields that can
disturb low voltage signals like intercom, headset and instruments.
A common negative bus is better than multiple grounds.
All cables that carry voice communications MUST have a single dedicated
ground bus. These cables MUST also be shielded.

Good luck
 
Assuming you are using LED Nav/Strobes (and if not reconsider), then for each Nav/Strobe run a single three #20 conductor shielded wire (available from Stein). The three #20 wires are:
- Nav light power
- Strobe power
- Strove sync wire

The shield provides ground for both the Nav and strobe.

Some LED Nav lights put out a lot of RFI, other don’t. The approach to wiring solves the issue.

For the ELT, in addition to the phone line for remote control and coax, run a single four conductor #22 shielded cable. The wires are:
- GPS serial output high (for position information to the ELT)
- GPS serial output low (for position information to the ELT)
- Power (you charge the ELT battery)
- Ground

For your ADS-B receiver if possible recommend mounting the receiver aft near the antenna. For my SkyView install I then run a single four #22 conductor shielded wire from the receiver to the SkyView. The wires are:
- Receiver Power
- Serial data out
- Serial data in
- Ground

For each trim servo, a single four #22 conductor cable. The wires are:
- Reference voltage
- Position signal
- Ground (cable shield)
- Trim servo motor power
- Trim servo motor power

For my SkyView autopilot servos I run a single four #22 conductor shielded wire for primary and secondary SkyView network connections, a #20 wire for power, a #20 wire for ground and a #22 wire or autopilot disconnect.

The rest are dependent on equipment used.

Carl
 
Both of your answers have led me down a path of learning that has really kickstarted my understanding of the process. Thank you.
 
A couple of things I did during my build, I think they helped -

  • Some DC-powered things like pitot heat and landing lights are locally grounded, but *everything* that consumes or produces a small signal got grounded to a forest-of-tabs device mounted to the firewall with brass bolts. The other side of the bolts, forward of the firewall, are directly grounded to the battery via braid. I know you're already thinking of doing this, here's one data point.

  • I paid extra for quality LEDs and light controllers. There's only one incandescent light in my airplane (the B&C voltage regulator Alt Fail light) and so far zero problems with noise from lights or anything else. The Baja Designs landing lights generate a tiny bit of noise that's only noticeable if com squelch is turned off and volume turned way up. I tested several cheaper LEDs and dimmer modules, they all generated enough EMI to break squelch on my com radios.
HTH

Dave
 
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