Garmin Magnetometer Install - twisted pair?

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Garmin Magnetometer Install - twisted pair?

Post by Petertweed » Mon Sep 23, 2019 11:19 am

I'm soon to start installing a remote magnetometer (GMU11) to feed into a G5 to be used as an HSI (which will recieve VOR / ILS) as I want a heading rather than a GPS track based display.
The docs say it needs to be well away from electrical cables, and any that are near should be twisted pair. Obvious place to me (on our SA Bulldog) is the wingtip, but there is a nav light there. Other locations are awkward to get at. I thought about on the front of the fin but we have our VOR antenna there.

So I'm discussing with my inspector, but:
- do I need to worry about a VOR antenna affecting a magnetometer?
- are magnetometers sensitive enough to be thrown by an incandescent nav light (positive cable or route to ground) or am I being overly cautious?
- if I go wing-tip - do I need to be replacing both positive and ground (currently ground is via aircraft frame) of nav light to go through twisted pair?
- any reason why I couldn't run some standard Cat 7 network cable (which contains 4 shielded twisted pairs of 24 or 26 AWG wire) such as ... 119&sr=8-7 which could provide power & ground to nav light, power and ground to magnetometer and CAN bus to magnetometer (with a twisted pair spare) rather than using specialist aviation cable?

Peter Tweed

Matt Dovey
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Re: Garmin Magnetometer Install - twisted pair?

Post by Matt Dovey » Tue Sep 24, 2019 3:50 am

On most commercial aircraft I have dealt with the magnetometer is mounted near the wing tip but not right at the tip. One of the reasons for mounting it near the wing tip is to keep it as far away as possible from any stray RF or induced voltages. The fact that you may using shielded twisted pair may be negated if you are using an old fashioned strobe light which releases a bit burst of energy every time it flashes. I think it would also be good practice if you ran the light and magnetometer looms separately. Especially the data bus. Using the network cable may be ok for the data bus. That's if it is moister, oil and environment proof. But may not be any good for supplying power to the nav and strobe. 24 and 26 AWG sounds on the small side. What size wire does the wiring manual call up now?Also shop around for the shielded cable it may not end up costing that much in the end.
I'm not sure how much the VOR would affect it. It is just a receiver after all. But I wouldn't go to all the effort to then find out that it is. I have know static discharge from static wicks mounted on an elevator to effect a glideslope antenna.

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Re: Garmin Magnetometer Install - twisted pair?

Post by PaulSS » Tue Sep 24, 2019 12:33 pm

As Matt says, I think 24AWG would be insufficient for your incandescent nav light. An 8' length of 24AWG would give a maximum of about 4A. That might be okay but I reckon that wire would be warming up and I definitely would not want it sitting next to three others, two of which are meant to be CAN bus wires. CAT 7 wire does not have the fireproofing standards of proper aircraft-grade cable and I think you should put that far from your mind.

I've followed plenty of threads on the Kitfox Forum regarding magnetometer mounting and almost all of them have gone out to the wingtips. Likewise, they've all run close to lighting cables. Clearly the LED Nav lights don't prove any problem and I've never heard anything mentioned about LED strobes. Incandescent nav lights obviously draw a higher current but it's a 'steady' current (as opposed to the high current non-LED strobes). In other words, I think you are worrying a bit too much about the proximity of the nav light power wires. Unless the Bulldog has been re-equipped I know it won't have strobes in the wingtips, hence me not discussing that as a problem for you.

If it were me I would run two, shielded, two core, cables to the magnetometer in the wingtip. These are twisted pairs anyway and help guard against interference. I would have 22AWG for the CAN bus and 18 or 20 AWG for the power & ground. Obviously the shield would be grounded at one end only. These are fine to run alongside each other according to all the Garmin & Dynon set ups I've seen and read about.

More important is to make sure your magnetometer is away from any steel. It often felt to me that the Bulldog was carved from a solid block of the stuff, so strong as they are, but hopefully your wingtip will prove devoid of the stuff. Obviously you will separate it as much as you can from the nav light itself.

I've just had a look at part4aircraft and reckon you'd only be up for about £40 max for the cables I would use. If you've bought Garmin kit you're obviously richer than Croesus himself, so suck it up and leave the CAT 7 cable plugged into your computer :D
Paul Simmonds-Short

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Re: Garmin Magnetometer Install - twisted pair?

Post by Petertweed » Tue Oct 08, 2019 6:16 pm

Thanks very much for your replies. Despite being the original poster and setting this topic to "subscribed" I didn't get a notification of them so I apologise for not responding.

If I had old style strobes I'd be worried about the high voltage discharge causing EM interference, but as I don't, I'm not (just a rotating beacon on our Bulldog).

I plan to put the nav light power & ground into a twisted pair as as well as the magnetometer power & CAN-Bus as per the Garmin docs (which say all nearby cables should be twisted pair). I would expect a constant current to affect the magnetometer (Amp's law?), with the power drawn by the lamp higher than the magnetometer.

I'd a chat to Danny @ Mendelsohns and I was getting confused between their certified CAN bus wire ($$$) and normal 2 core screened wire which is apparently fine, so cost is no longer a factor in wire.

We've been looking at the existing cable conduits and it looks tight to re-use the nav light cable's conduit for the various options. I'll calculate minimum cable size - the bulldog runs at 24(+) Volts so less current is required for the same brightness of lamp. We measured the current as around 1 Amp but should really find the bulb spec.

I suspect the deciding factor between using CAT 7 LSZH cable (so non-harmful) will be the size of the cable bundle.

Thanks for all the advice.
Peter Tweed

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