I think I have a ground loop problem.

FORANE

FORANE

En-Route
Joined
Mar 7, 2013
Messages
3,758
Location
TN
Display Name
Display name:
FORANE
No, I do not have a tailwheel. I am referring to wiring.

I noticed constant static in reception on my SL40 one one particular frequency. I clicked the push to talk and 2 things happened; the static went silent and the digital EGT reading on my AEROSPACE LOGIC EGT-100-4P went from the normal reading of around 1250 to a reading of about 400.
Then I clicked the push to talk again and the static in the SL40 started again and the EGT reading returned to normal. As long as the EGT readings were abnormally low, I had no static in the SL40. As long as the EGT readings were accurate, I had static in the SL40. Clicking the push to talk a couple times would cause the EGT readings to switch from normal to low and back.
Also, anytime on autopilot (old navaid ap-1) when the push to talk is pressed, the plane will roll left.

Any ideas where to start looking?
 
I'm not familiar with the hardware, but at first guess I think your problems are unrelated. The EGT hardware and autopilot should not have anything in common.

That said, the situation with the EGT hardware is interesting. Interference at one frequency when it is working properly and no interference when it is not working properly. Something in the EGT box is being toggled between working properly and not, and that is impacting the interference being generated. I can't help with what is happening, but I'd look to make sure that the EGT box is installed correctly. All required shields on cables are installed and terminated correctly. All mating surfaces on the chassis are making proper contact and are conductive. No corrosion present.

You didn't say if this was a new problem, or something that has been an issue since the hardware was installed. It might make a difference in troubleshooting.

Same approach with the AP. Is this a new problem with an old installation, or a problem with a new installation? Makes a difference, but you still need to check the same things.
 
It sounds more like an open ground somewhere rather than a "ground loop" issue per-se. In an ideal world you would have a single ground point for all of your avionics, but when you let A&Ps wire things up...

If I were to toss out a wild ass guess, you have things grounded to the panel and an open connection in the ground wire to one of the devices you mentioned and/or a poor connection between the panel and an actual ground.

But, I ain't no A&P so I don't know how electrons work in an airplane differently than they do in the real world.
 
Hey Ghery

Autopilot roll with PTT has been present for a decade (since I bought the plane).
EGT not indicating correctly (and being affected by PTT) is new.
I have contacted the manufacturer of the EGT and they said the low reading was most likely a bad ground connection. I rewired the ground to the circuit breaker panel ground with no change. Suppose next step is to run ground wire straight to motor.

Thanks for the reply.
 
Start with the radio grounds. If the power for the radio is running through the EGT instead of the radio ground wire you will create a ground offset - and given the fact that the output of thermocouples is on the order of millivolts...
 
Capt. Geoffrey Thorpe said:
Start with the radio grounds. If the power for the radio is running through the EGT instead of the radio ground wire you will create a ground offset - and given the fact that the output of thermocouples is on the order of millivolts...
Click to expand...
I will look at them. I recently had an avionics shop install a new audio panel but I believe this started before they did that work.
 
FORANE said:
No, I do not have a tailwheel. I am referring to wiring.

I noticed constant static in reception on my SL40 one one particular frequency. I clicked the push to talk and 2 things happened; the static went silent and the digital EGT reading on my AEROSPACE LOGIC EGT-100-4P went from the normal reading of around 1250 to a reading of about 400.
Then I clicked the push to talk again and the static in the SL40 started again and the EGT reading returned to normal. As long as the EGT readings were abnormally low, I had no static in the SL40. As long as the EGT readings were accurate, I had static in the SL40. Clicking the push to talk a couple times would cause the EGT readings to switch from normal to low and back.
Also, anytime on autopilot (old navaid ap-1) when the push to talk is pressed, the plane will roll left.

Any ideas where to start looking?
Click to expand...
The autopilot issue is common and the result of some RF coupling into the autopilot's analog inputs where it gets rectified. The best remedy for that is a combination of good (not 30+ year old) coax, antenna grounding, and shielding on the autopilot cables. Ferites on the radio and DG inputs to the autopilot may also help.

WRT the "static" was the squelch open without a carrier (transmission from another radio) when you heard it, or were you listening to a someone's transmission at the time? The squelch in the SL40 can be adjusted from the front panel and there are some useful diagnostic readouts available in the maintenance mode including the ambient RF noise level.

I can understand a EGT reading changing while your transmitting (same rectification issue as the autopilot) but I wouldn't expect it to last much beyond the transmission. The bistable behavior you describe is unusual but not impossible and most likely something internal to the EGT readout electronics.

The one thing all this may have in common is RF leakage from the antenna cable and/or a corroded Comm antenna ground connection.
 
gismo said:
The autopilot issue is common and the result of some RF coupling into the autopilot's analog inputs where it gets rectified. The best remedy for that is a combination of good (not 30+ year old) coax, antenna grounding, and shielding on the autopilot cables. Ferites on the radio and DG inputs to the autopilot may also help.

WRT the "static" was the squelch open without a carrier (transmission from another radio) when you heard it, or were you listening to a someone's transmission at the time? The squelch in the SL40 can be adjusted from the front panel and there are some useful diagnostic readouts available in the maintenance mode including the ambient RF noise level.
The static was exactly like the squelch was open. It was constant yet today being Christmas, few people were on the radios. I know how to adjust the squelch on the SL40 and have done so but that was not the problem. A change in frequency and the static was gone until I went back to the problem frequency. I am not familiar (did not notice) with the ambient Rf noise though in the SL40.
I can understand a EGT reading changing while your transmitting (same rectification issue as the autopilot) but I wouldn't expect it to last much beyond the transmission. The bistable behavior you describe is unusual but not impossible and most likely something internal to the EGT readout electronics.
Yes, after a click on the PTT or two, the EGT would change to or from the normal reading and once changed it would stay in the changed state.

The one thing all this may have in common is RF leakage from the antenna cable and/or a corroded Comm antenna ground connection.
Click to expand...
We did have this problem with an RG400 cable which had vibrated enough to damage the cable due to lack of cable fastening at the antenna to cable connection. We ran new cable. Bear in mind though this is a Lancair and as a composite aircraft, grounding of an antenna is just a ground plane.
 
Henning said:
Do you have a ground plane for the antenna?
Click to expand...
There is a whip antenna embedded in the vertical empennage with a sheet of aluminum connected to the base which serves as a ground plane.
The other antenna linked in my last post does not have a ground plane and is not designed for one.
The SL40 I think is connected to the whip.
 
FORANE said:
There is a whip antenna embedded in the vertical empennage with a sheet of aluminum connected to the base which serves as a ground plane.
The other antenna linked in my last post does not have a ground plane and is not designed for one.
The SL40 I think is connected to the whip.
Click to expand...
How well is the whip's ground plane connected to other conductive parts of the airplane and is the connection between the antenna lead shield and the ground plane tight and corrosion free? I don't have much experience with grounding in a composite airframe but I suspect that leaky coax and poor connections are an even bigger problem there than in an aluminum can.
 
I would suspect bad grounds behind that panel first. Then the antenna coax grounding at both ends. Leaking RF can also trip alternators offline if the regulator/overvolt detectors are electronic. They pick up stray RF and interpret it as an overvolt condition. Common in aluminum airplanes in the cabin ceiling where condensation corrodes the coax grounding at the antenna.

Dan
 
FORANE said:
Bear in mind though this is a Lancair and as a composite aircraft, grounding of an antenna is just a ground plane.
Click to expand...


Is that so? You mean after all I taught Lance he used ground planes in the airplane instead of dipoles? You had best check that for sure.

Jim

,
 
First off, thanks guys for trying to help me sort this out.

gismo said:
How well is the whip's ground plane connected to other conductive parts of the airplane and is the connection between the antenna lead shield and the ground plane tight and corrosion free? I don't have much experience with grounding in a composite airframe but I suspect that leaky coax and poor connections are an even bigger problem there than in an aluminum can.
Click to expand...
What I refer to as a ground plane is not electrically connected to anything but the antenna base and RG400's BNC connector. It is not connected to any other conductive parts of the plane. We recently replaced this coax with new RG400. It is inside the aircraft empennage not exposed to any elements; I checked the connection with a meter and did not have any resistance.

Dan Thomas said:
I would suspect bad grounds behind that panel first. Then the antenna coax grounding at both ends. Leaking RF can also trip alternators offline if the regulator/overvolt detectors are electronic. They pick up stray RF and interpret it as an overvolt condition. Common in aluminum airplanes in the cabin ceiling where condensation corrodes the coax grounding at the antenna.

Dan
Click to expand...
Hey Dan
A bad ground behind the panel was my first thought. I intend to run a new ground for the EGT display. The coax are grounded at the radio ends but as Jim points out below, I guess they are really dipoles at the antenna end and as such are not grounded per se - just connected to the RG400 BNC connector.



weirdjim said:
Is that so? You mean after all I taught Lance he used ground planes in the airplane instead of dipoles? You had best check that for sure.

Jim

,
Click to expand...
Jim, please forgive me if use incorrect nomenclature as I have not had the benefit of formal training in such issues.
 
Last edited:
OK, a ground plane is a large sheet of metal (thickness not important). In order to be an effective ground plane, it needs to be somewhere close to 24" in radius with the antenna connector at the center of that circle or rectangle of metal.

It can be an irregular shape (i.e. not a circle, or a square, but it needs to be SOMEWHAT the same size in most or all directions from the connector.

The radiating rod needs to be approximately 22" (more or less, according to the thickness of the radiator) sticking vertically up OR down from the center of that ground plane.

Exactly what does that ground plane/radiating rod look like in your aircraft?

If that is in fact what is in there? Or is it a true dipole with two radiating rods in something approximating a straight line?

Jim
 
Pics show more:
5961838901723947698

5961839195057531394

20131226_173338.jpg



20131226_173001.jpg


I am not good with pics so let me know if you cannot see these.

To give an idea of size, the top of the tail is less than 6 feet off the ground. The ground plane runs side to side in the rear empennage toward the top which may be seen in the outside of fuselage pic is very narrow. The ground plane is at most 1 foot wide and maybe 2-3 feet long. The whip extends up into the vertical empennage. The closer RG400 cable goes to the Garmin 430 GPS antenna on a separate shelf. Out of sight just forward of the Garmin 430 GPS antenna is the other com antenna which is one of these:
http://www.aircraftspruce.com/catalog/avpages/antennasystems.php
 
While checking for good grounds be sure to check the engine/engine mount to fuselage ground. A poor or intermittant ground to the engine can cause all sorts of interesting whines, pops, crackles and instrument fluctuations.

Just something else you might check...

Chris
 
Hiperbiper said:
While checking for good grounds be sure to check the engine/engine mount to fuselage ground. A poor or intermittant ground to the engine can cause all sorts of interesting whines, pops, crackles and instrument fluctuations.

Just something else you might check...

Chris
Click to expand...

Bad engine-to-airframe grounds can do some amazing stuff. The alternator charging current will look for alternate paths and sometimes find them through CHT or oil temperature probes and through the instrument and cause high indications and a lot of concern for the pilot when there's really nothing wrong other than a bad ground. Starting the airplane with a really bad engine ground can heat up the throttle/prop/mixture/carb heat control cables and ruin them real quick. Electrons aren't too fussy about where they flow as long as it's the easiest path.

Dan
 
OP here.
I am reading all posts. Plan to delve into this issue Saturday. Between a busy work schedule and wrapping up installation of a Trio Pro Pilot autopilot in the RV, I just have not had the time to look into it.
Thanks for the ideas.
 
FYI, the installation instructions for the EDM700 state that it must be grounded at the engine. This is to eliminate erratic readings. Maybe your installation requires the same. Worth checking.
 
Pegasus said:
FYI, the installation instructions for the EDM700 state that it must be grounded at the engine. This is to eliminate erratic readings. Maybe your installation requires the same. Worth checking.
Click to expand...

Thanks for the tip. Yes, I read that also. Strange that it worked fine for years as the builder had wired it and now is causing issues. I replaced the ground wire and changed its ground point with no effect (though did not wire to engine yet). I replaced the power wire and changed its point on the circuit breaker assembly with no effect.

I looked into it yesterday.
Rerouted RG400 away from the EGT power wire: helped some.
Swapped RG400 SL40 & G430: significantly worsened the condition.
Turned off SL40: EGT-100 gauge functions perfectly.
Disconnected power cable to EGT-100: SL40 functions perfectly.
Pulled circuit breaker to EGT-100: SL40 squelch break persists and will return or resolve as before with a few taps on the push to talk.

The squelch break only occurs on the SL40, not the G430. The interference with the EGT-100 readings only occurs with relation to the SL40.
 
I believe I have found the problem...

The SL40 tray has a couple feet long RG400 cable running from it. There was a dead short between the center pin and the outside of the BNC connector.
Interesting, if I transmit with the antenna connected, then disconnect the antenna RG400 from the SL40's tray RG400 pigtail, the antenna RG400 will have a dead short between center pin and exterior of BNC connector for a few minutes. After a few minutes the antenna RG400 will no longer have any continuity between center pin and exterior shielding.

I checked the SL40 and it has a short between the center post and the ground of its antenna out. There is some resistance but there is a short.


Thanks to those who have offered up suggestions in this thread.
 
Last edited:
Bent RG-400 can allow the center conductor to "migrate" to the outside of the radius and touch the braid, internally. It's really bad with cheap coax that has foam insulation, RG-400 is a little better, but the RF world moved on from RG-400 in vibration-prone environments years ago. Why aviation still uses the crap, is beyond me.
 
You must log in or register to reply here.
Back
Top