N

N62XS

Well Known Member
DG on the GRT EFIS Magnetometer heading swings about 50 degrees when SL-30 transmits. The AHRS unit is mounted about 4" from the back of the SL-30, in the forward baggage compartment, with the wiring from the magnetometer running near the rear of the SL-30. The obvious solution is to move the AHRS or the wiring, but I am hoping someone has an idea on how to shield the wires and prevent the relocation of the wiring or rewiring of this installation.
 
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Magnetometers are very sensitive to magnetic fields caused by electrical current. When you transmit, the radio is drawing a large (several amp) current, creating this field.

Shielding will not correct this problem. The solution is provide both your power and ground from a location that is as far away from the magnetometer as possible, and to twist the power and ground leads together.

The twisted wires help to cancel the magnetic interference by producing identical, yet opposite magnetic fields in close proximity.

This is probably too difficult to make practical in an aircraft, where ground buses are rarely near the power bus. In many cases, the airframe also carry current, such as for landing lights in the wing.

I think you will need to bite the bullet and relocate your AHRS to a position that is far away from any other electrical devices, such as the aft fuse or wing.


Vern Little.
 
A Couple Questions

Robby,

Two questions so I can ponder.....

1) Where is your magnetometer itself,and does the wiring for that pass very close to the SL-30?

2) What kind of antenna are you using for the SL-30, and where is it located?

I first used a cheap wire whip antenna under the seat in my -8 when I was building, and when I transmitted, all the LED indicators in the panel went nuts - I was radiating as much energy UP into the cockpit as I was down out of the aircraft. A good antenna with shielded connections fixed that. The lesson being that you need the RF line to the antenna well shielded and isolated from any of the delicate stuff.

Paul
 
SL-30 problems

My problems with xmitting on SL-30 are:
1. spikes in EGT with false cooling warnings
2. causes TrueTrack autopilot to pitch up.
3. weak receive and transmit (20 miles is good). It varies from hour to hour. Sometimes aircraft orientation to controller matters, sometimes not.

Since I know the radio has a problem, I am now waiting for the loaner/replacement. However, TrueTrack advised me to separate the antenna from the pitch servo wires. They are probably right, but I'm wating until I get the second radio.

My antenna is Archer in rt. wing. Avionics shop said "very little reflected energy" when they checked it out.

So - maybe the SL-30 is being bad and maybe we have to move wires anyhow. My AHRS is in fwd left corner of cabin under deck and SL-30 is just right of center in panel of -7A.

Just data - hope it helps. How's RD?
 
Answers

Paul:

Magno is in the right wing tip and the wiring runs right behind the SL-30 to the AHRS. I'm using two belly mount Comant comm antennas, the expensive ones. GRT thinks it is the proximity of the magno wires to the rear of the SL-30 and I agree, but moving anything will be a real bear, as the wiring harness is cut to fit and any move will require either lengthing the harness or splices in the pitot/static tubes.
 
Ground testing?

Interesting topic (we have TT and SL-30 too). Is this something that can be demonstrated / proved / reproduced on the ground - or is it airbourne only? - ie. Can others test before buttoning up the forward skin?

Carl
 
zkvii said:
Interesting topic (we have TT and SL-30 too). Is this something that can be demonstrated / proved / reproduced on the ground - or is it airbourne only? - ie. Can others test before buttoning up the forward skin?

Carl
Click to expand...


Carl - my problems showed up early in ground testing - just key the mike in the workshop (make sure the antenna is installed), and watch for effects in other systems.

Robby - sound slike you already know what you have to do....I keep a little list of "things to do the next time the panel is out" on the wall in the hangar....

Paul
 
On the Ground

Carl:

in the AHRS maintenance page, the Magnetometer Heading reading changes when I key the mike. It also happens when I key the SL-60, but only about 10 degrees. The display swings as well, but not instantaneously and does not return to normal for eight to ten minutes.
 
On first flight, my SL-30 received fine. However when we taxied out as a flight of two, my plane (N410S) couldn't be heard by ground or tower. We could *barely* hear it in the chase plane about 10-15 ft away. It sounded very rough - very difficult to describe - and I thought something was "spiking" at certain intervals.

Anyway we took off using the chase plane's radio. Mike could talk to us seemingly fine air-to-air.

Further ground testing revealed that on battery power other planes taxiing on the ground could hear me just fine (even inside my hangar). Alan A. and I also did some testing with a scope in my hangar (thanks Alan!). It's not the coax or antenna. I hope to take the plane back up today and see what's up. I wonder if it's only a problem with the alternator on.

My antennas don't really run with the autopilot. All my antennas are bundled together until they go thru the spar. They are in close proximity to the EIS 4000 wire bundle but not touching it.

Sorry if this post is all jumbled in thought... just trying to explain my symptoms. Only thing I can come up with is swap radios and see if my radio is bad in another person's plane or try a known 'good' one in my plane.

Howard and I happened to have bought our brand new SL-30's from the same source, though!
 
Thanks Guys for the info, I have some seperatation of the aerials, SL-30 and the AHRS, Magnatrometer cables but it is difficult with only 4' x 2' panel space :)

Keep the rambling / thoughts coming - good education for the rest of us.
 
I had a similar problem on my BMA, although the interference was coming from the WigWag system on my landing lights. Every time the WigWag flashed, the compass swung about 5 degrees. The magnetometer was mounted in the wingtip, about 2 feet from the landing light, but the wires all ran in the same conduit and passed within a couple inches of the magnetometer. Shielding the landing light cable didn't work and I couldn't get enough separation to make everything work, in the wing tip, although distance from the light wire did help. (I did not try twisting the power and ground wires to the landing light, since I was grounding to the wing.)
I finally relocated the magnetometer behind the rear baggage compartment, on my RV8. Just built a small shelf on two angles spanning the two upper longerons.
I don't know how your magnetometer is set up, but if it has a connector, just make a new connecting cable and run it to a new location, away from your interference sources. (Such as the radios, stobe power sources, etc.)
You could make up a long test connector cable and try out possible locations until you get a good one and then mount it permanently. The interfence is registered by the magnetometer, not by the wires connecting it to the EFIS, so you should be able to run them just about anywhere, but using a shielded cable probably wouldn't hurt.
Anyway, that's what worked for me.
 
RD is doing fine. He just turned two last month!

GRT is sure the problem is with the Magnetometer wires being bundled with the Comm antenna wires, behind the radios. I plan to seperate them this PM. Without the radios on, I have no problem. For those going to LOE, I hope to demonstrate the problem over an adult beverage.

IronFlight, what time are you launching out. Our planned fuel stops are at Yazoo City, MS(Just gotta get some of that Jerry Clower Memorial Fuel) and deep in the heart of TX. We plan to launch between 6:00AM and 7:00AM EDT, from JYL. Want to meet along the way?
 
On the Ground

My symptoms can be demonstrated on the ground, even with an alternate (Comant) antenna. However, the ground test does not eliminate the proximity of the antenna wire to the TT AP pitch bundle. My loaner/replacment radio is in the UPS system as we speak.
 
Magno

I had the same kind of problem. In my testing I wraped strips of alu. foil around the magno wires(only is the area of the radio), then I wraped the foil with 22ga bare wire(about 3/4-1" apart), then grounded this wire. This helped some. Then I wraped the ant. wire(only in the area of the magno wires) and the problem want away. I have not done this yet, but this fall I plan on replacing my ant. wire(RG-142) with Belden 9222(it has an extra shield).
Check out the Nov. issue of Kitplanes, it has a very good article on making coax cable.

Frank Thorman
N821BF
240+hrs flying
 
Hard Knox said:
IronFlight, what time are you launching out. Our planned fuel stops are at Yazoo City, MS(Just gotta get some of that Jerry Clower Memorial Fuel) and deep in the heart of TX. We plan to launch between 6:00AM and 7:00AM EDT, from JYL. Want to meet along the way?
Click to expand...


Robby - Not sure yet when I'll be airborne - my schedule is frequently not my own to set...my goal is to leave early Friday, but I might be delayed - I'm sure I'll hear folks along the way!

Paul
 
After flying mine for about 1.5 hrs today, one possible problem I had was it was not firmly seated in the tray. I fiddled with it some on the ground and it seemed to help - I could hear tower and they could hear me throughout the whole flight. It was in the tray correctly before and I couldn't pull it out (without unscrewing the retaining screw) but perhaps another 'jam' in there solved it?

HOWEVER, I still have what seems like wind noise from a mic. It's not the squelch, headset or audio panel. Tried everything in the air to eliminate it. It goes away on the ground and when I'm in the pattern. But up at altitude and cruising around the problem arises. Noooo... it's not real wind noise as I can adjust it with the volume on the audio panel.

Still not sure if I want to go too far with my current radio situation.
 
To help eliminate the wind noise get an avionics shop to turn down the internal mic gain in the radio. This I have had to do on dozens and dozens of radios.
 
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