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  #1  
Old 10-17-2010, 09:15 PM
marchudson's Avatar
marchudson marchudson is offline
 
Join Date: Feb 2006
Location: East Texas
Posts: 187
Default Running out of ideas

On how to fix the problem with RFI/EMI.

Components that I have:

GNS430W (Comm 1)
SL-40 (Comm 2)
RG-400 coax cable
2 - RAMI bent whip antennas mounted on belly
Dynon Skyview with EMS module
Single point ground for everything.

Problem I'm having:

When I transmit on comm 1 AND comm 2, the EMS readings go nuts. CHT/EGT moves anywhere from 10 deg to 100 deg depending on the frequency that I'm transmitting. If transmitting on 118.0 I'll get just a little change, on 136.0, I get a huge change. When transmitting on 124.0, the ammeter goes from +8 amps (normal) to -35 amps. Talked to the Dynon folks and they said that I'm the only one with this problem?????

Solutions I've tried:

1. Replacing both cables with store bought cables and routed them in random locations to see if this makes a difference. (no difference noted)

2. Power the Dynon with just the standby battery (no difference). Note Dynon system is still tied to aircraft ground system.

3. Disconnect the sensor wires from the EMS. result - system behaves normally but I don't have any engine monitor readings either. (Maybe noise is not entering EMS module on power supply lines)

4. Place a 12" X 12" piece of aluminum foil on the antenna, touching nothing else but the antenna. (normal indications - I deduce by this that the coax is probably not leaking due to this, don't know)

5. Transmit on Comm 1 and compared results to transmitting on Comm 2. Both radios produce the same adverse results on the EMS.

My next step is to place ferrite chokes on the 3 EMS cables. But is this just putting a bandaid on the problem? I would rather fix the root problem.

OK... I'm open for suggestions. Do I need to purchase a SWR meter or purchase a different brand antenna?

Thanks for your time
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  #2  
Old 10-17-2010, 09:37 PM
Norman CYYJ Norman CYYJ is offline
 
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Location: Victoria B.C.
Posts: 1,230
Default

Make sure that the antennae are properly grounded to the airframe. The base of the antennae need to be bonded where they are mounted. It does sound as though you have extremely high VSWR and a poorly grounded antenna can do this. Good luck.
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  #3  
Old 10-17-2010, 11:32 PM
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AltonD AltonD is offline
 
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Location: Dothan, Alabama
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Default

Grounds. Make sure you have a very good ground from all components, including the engine. Did you use a ground block or are your components grounded via the frame?
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  #4  
Old 10-17-2010, 11:47 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Default antenna ground

I agree, its the first thing I thought of. The antenna is not grounded at its mount, and/or the shield braid of the RG-400 is not grounded to the connector(s).

Try leaving the engine thermocouples connected to the Dynon, but not plugged into their spots on the engine. So the probes are just hanging in the air. That way, the probes can not pick up any ground noise. If they still act up, the noise is coming in through the power and ground leads to the Dynon. If they are quiet, you know the noise is coming into the probes. Either way, you are putting a lot of energy into the ground somewhere.
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  #5  
Old 10-18-2010, 06:05 AM
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marchudson marchudson is offline
 
Join Date: Feb 2006
Location: East Texas
Posts: 187
Default Grounds

Thanks for the quick replies guys. My initial thought was also the ground. My first action was to check the continuity of both antenna bases to the airframe and I am getting zero resistance. The engine is grounded to the airframe via a #2 welding cable and I checked the resistance from several locations on the engine to the grounding block, zero ohms. As far as the coax shield grounding is concerned, I checked both with the meter and then removed them from the equation when I temporarily replaced them with store bought RG-58 cables.

Steve - I actually tried your idea of unplugging a few of the CHT probes from the engine. When they are left near the engine but not touching anything I get the same results. If I let the probes dangle below and very near the main gear, the problem gets much worse. I figured that they are in the direct path of the radiated energy when dangling.

Alton - yes I have a single point ground for all equipment in the aircraft and everything is running to it.

Sounds like guys think I might need to get an SWR meter to determine the health of the antenna / cable combination.
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  #6  
Old 10-18-2010, 07:21 AM
R.P.Ping R.P.Ping is offline
 
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Location: Peoria, AZ
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Default Bounce back

I had some initial problems when I was checking systems out in my garage. Just couldn’t get the led lights not to glow when I transmitted. Checked grounds, moved antenna wires, checked antenna connections. Ended up it was the signal bouncing around inside the garage. All my work trouble shooting was for nothing. If you can move your project outside it will probably work just fine.
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  #7  
Old 10-18-2010, 07:48 AM
rich rich is offline
 
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Location: Fort Worth, Tx
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Default RF/EMI

I had the same problem with my SL-40 and RMI eng. monitor.
When I would transmit the RMI CHT/EGT readings would go nuts and set off alarms.
Take a hand held com and while transmiting run it up and down your wire bundles and observe your EMS. When the numbers start to change you found a "LEAK" where RF can enter. I found my worst leak where the CHT/EGT'S connect to a rotary switch . I wrapped some braided shielding around the area so at least the alarms don't go off now.
I too have everything grounded to a grounding block on the firewall.
If you find the "FIX" let me know.
Good luck,
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  #8  
Old 10-18-2010, 04:31 PM
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KRviator KRviator is offline
 
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Location: Sydney, Aust.
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marchudson, you've said when you disconnect the sensor inputs to the EMS, the problem goes away.

It's likely to be a PITA, but have you considered disconnecting them all individually, then reconnecting them one at a time to isolate which one(s) give you the strange readings? At least then you can concentrate on CHT4 or your Tach input or whatever it is that is causing the problem directly.

Another suggestion would be to verify zero resistance between all 6 ground pins on the DB37 EMS Module plug and the ground point. Something I've also noticed in the SkyView installation documentation, Table 15 specifies Pin30 as a ground, but the tables for various engine examples show it as "not connected". No idea if this could cause your issues but it might be something to raise when you're on the blower with Dynon.
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  #9  
Old 10-18-2010, 07:36 PM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Default Dynon thermocouple bundle connector?

So, now I'm wondering if it is in the connector on the back of the EMS where all the thermocouples plug in. On my Dynon, EMS-D120, that cable was made by Stein, I think, and it works fine.

Your hand-held radio test, your probes dangling test, and your SL-40 off the airplane test, all suggest that RFI is getting into the thermocouple signals, not antenna or airframe grounds. Since we all use thermocouple wire that is unshielded and we don't have problems, its not the wires. So I'm wondering if there is something in the connector - maybe bad grounds, or maybe the thermocouple leads are grounded wrong at the "cold junction" inside the box or at the connector, or something like that.

Based on everything you've tested, I would conclude that it is a Dynon problem, and ask them to check it out.

Tell us what the antenna signal testor says.....but that seems like low probabily given that it is a production antenna.

This is a strange one, to be sure.
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