wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
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 Posted: Wed Nov 15, 2023 2:29 pm Post subject: Power, signal, and Coax - How should they be bundled? |
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Here is my little problem. I am trying to wire my fiberglass airplane with internal antennas properly. I have little insight on electromagnetic compatibility. Please help.
I know that I am supposed to keep wiring for DC power to stuff like motors and electronics separated from data, but am struggling with knowing what is what and how much they matter.
So first issue is we are supposed to keep DC power separated from signal lines. I know which are power and which are data, and I know which are coax resonating at RF frequencies. What do I with coax? I have three antennas back there with cables going forward and another antenna going a foot or so to the ELT box.
Here is what we have in the aft fuselage:
• Position signal from my trim servos is data. The maker puts the two power wires to an electric motor and three wires for servo position on the same cable with no shielding. Five wires 24AWG, with a max 1 amp for power. Three sets of these;
• ELT, has power and GPS position data signal, all 22 AWG, 1 amp fuse, plus an RJ11 connector cable to the control box forward. No motors, just electronics;
• Three coax antenna cables, two TX/RX for COM, One RX for GS;
• LED continuous and flashing - continuous 1 amp and momentary 6 amp, and recommend shield grounded at airframe ground.
So, how do I bundle this stuff to stay quiet on signal and radios?
• Do we consider antenna coax to be power or signal or a third category?
• Does it work OK to bundle coax with power wires? Or will electric motors mess up the Voice and GS stuff?
• The makers of the servos and ELT seem to bundle up power and signal – will they be OK if I keep them that way or is it way better to separate power from signals as close to the servos as I can, run power together then bundle signals together and run them separately?
• Do I run coax cables together but away from a power bundle for ELT and LED and servos and away from signal bundles?
• I have a dipole on each fuselage wall that tends to use the whole vertical height available. I plan to run my wiring along the fuselage walls and through the center of the dipoles. That gives me a place for two bundles. Putting a third bundle at say top or bottom of the fuselage has been warned against - Jim Weir says it will mess with my COM antennas.
• I could suspend a run of wiring down the middle of the fuselage, next to the big push-pull tube for the elevators. That only gets me about 4” separation between center and sides with three bundles and would get interesting to work the support. A fiberglass wand supported at bulkheads and a some triads along the length. I do not have any desire to do that…
It all gets more interesting as we go forward, but the answers to the above questions should help with insight. The only antennas from the baggage bay through the cockpit are transponder and ADS-B on the floor with their own ground planes and two GPS antennas at the top of the roof that need no ground planes. I still only have two paths around the seats, so I suspect I must decide if my signals from the trim servos can be bundled up with the power wires or the coax. My center console is really crowded as we pass over the spar. Maybe I could run the signal bundle outside the console and put a cover over it. Ugh.
What say you guys? It sure would help if I can bundle the servo position signals with either the power wires or with the coax cables and still have it behave nicely.
Billski
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