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Signal issues with Garmin GI106A (CDI)

Hello,

I have an RV6 with appx 900 engine hours, based out of Colorado Air and Spaceport KCFO. My father built it in 2003 and I have been flying it regularly since 2014. In Fall 2020 I began my IFR training and began to rely on the Garmin GI106A VOR LOC GS GPS-CDI for approaches including VOR, ILS (Localizer and Glideslope), and GPS (input from GNS430). During my training I noticed the following two issues which I am trying to understand and fix.

First, when performing a VOR test at the ground facility at Centennial Airport, after tuning the VOR and centering the needle at 180, I expected to see the From flag and when centering the needle at 360 I expected to see the To flag. Strangely the flag did not give any indication, neither to or from flag.

When performing a VOR check from the air (at a different VOR) the CDI has given the expected to/from flag when tuned into the VOR.

The other issue I’ve experienced is when doing an ILS approach (specifically at KCFO ILS RWY 26). I have found the ILS signal to be sporadic based on the following observations. The Lateral flag appears while the needle reflects deviations as expected, although I do not know how accurate. Also, the
Vertical flag appears while the needle reflects what I believe are accurate glide slope deviations. It is my understanding that the appearance of these flags (a two headed arrow) indicates that no signal is being received.

As a side note, the GI106A behaves accurately when the CDI soft key is set as GPS. It is only when it is set at VOR when I’ve noticed these issues.

My CFII suggested that I inspect the connections between the GI106A and the antennae. I have called Garmin but have had a hard time reaching a knowledgeable resource via their phone support although I am planning to call again and see what help I can get.

However, I would like to know if any of you folks at Vans Airforce have run into this issue and have any ideas for me. Is there a connection issue, a mechanical issue with the instrument, or could it be user error?

Thank you
Bryan
 
More info please

What antennas do you have and where are they mounted?
It could be separate GS and VOR/LOC antennas, or they might be using a diplexer.
Are the antennas outside or somewhere else like in the wingtip, inside the canopy, inside the cowl?
 
Antennaes

There are two antennae mounted under the empennage aft of the rudder. The two antennae are both attached to a single black circular piece that is attached to the bottom of empennage.
 
Sounds like the antenna you've got on the tail is like this (or similar) https://www.aircraftspruce.com/catalog/avpages/av532.php

that is ONE antenna, despite having two rods. One coax will go to it. This is your VOR and Localizer antenna (for sure). What's less certain is whether it also is driving the glide slope... or not. For that you need to trace your coax cables under the panel. The Nav radio should have 2 coax jacks on the back. If there is a diplexer, then I'd expect the two cables to come together somewhere under the panel into one of these: https://www.aircraftspruce.com/catalog/avpages/coupler507.php
 
Since your NAV antenna is on the bottom of the plane (aft of the rudder??? how's that?)

there may be some issues with the plane blocking the signal when tuning the VOT on the ground. Try again, with the plane oriented N, then W, then S, then E... if the results change we know the plane is likely blocking the antenna from getting a good signal on the ground.

All in all it seems like the signal received (by the NAV radio, nothing to do with the CDI) is weak. That can be:
  • Bad Antenna
  • Bad Coax
  • Bad Coax connector(s)
  • NAV radio problem

And, if the installation uses a diplexer, that will reduce the signal strength somewhat.
 
@blaplante

I meant that the antenna is located forward of the rudder not aft, my mistake.
The antennae you pictured is a near match to what I have.
I will retry the VOT test as you mentioned, this makes sense.
I checked the electrical schematic drawings that my dad documented when he built it and it shows that there is one antenna for both VOR/GLIDE SLOPE. I am assuming (and will double check with him tomorrow) that he means VOR/LOCALIZER/GLIDE SLOPE.

I will check the antennae and coax and connector- I’m not sure the best way to check the connector: if I can see it through the exterior inspection plate, if I need to crawl back into the empennage, or if I can detach the antennae from the bottom. Any thoughts there?

As a side note, I have noticed that one of the two stainless steel elements that make up the antenna is “tweaked” for lack of better term. In other words, rather than the two elements being “on the same plane” one seems to be tilted as if it is rotated in the socket such that it points at a different angle. A picture would do a better job describing it. I wonder if it got knocked off kilter when I landed on a bumpy grass field or something and maybe that caused that issue. Perhaps that is part of this issue. I wonder how easy it would be to repair that, or at least inspect it more closely? I recently was cleaning the bottom of the body of oil/dirt and cleaned the antennae (very dirty). I gently tried to adjust that one element but it didn’t seem to “want” to movement so I didn’t force it.

Thank you,
Bryan
 
Last edited:
Brian,

My indicator behavior is similar to your, in that the G/S off flag stays in view even though the gs was valid. It was the instrument's error. I now have a placard that the gs off flag is inop. It is on my #2 radio. I always fly ILS from the this radio, and have the #1 which has the 430w and separate display programed to an LPV if available, otherwise it to has the ILS in it. I like to have a backup approach and/or a verification. Many times the ILS and LPV slopes will not be perfectly coincident. I always error to the higher side.
The instrument bench check literally took minutes and no guessing.

Please report back your findings.
 
the 430 has 4 coax jacks on the back. Looking from the rear side, the g/s is the lower left coax. The NAV is on the bottom to the right of that. Given 1 antenna I expect these two connect in a diplexer as noted above. So there are 5 coax connections for you to inspect right there.

You are looking for any corrosion or loose crimps. Disconnect and look inside each one. Look at how the coax is connected. If you flex it does the coax move in the connector? If so, that's no good.

Do the same back at the antenna.

If the antenna got tweaked, it may have broken a wire inside the little round thingy, and you are only using a half of an antenna.
 
Will report back

Thank you for the feedback, I really appreciate it. I will report back after I inspect connections. Have a great week. - Bryan
 
Results

I removed the empennage fairing and inspected the antennae connections. All connections were secure and looked ok. Beneath the empennage, I inspected the puck and antennae elements and noted that one is bent. I crawled under the instrument panel and inspected the duplexer and connections to the GNS 430. Connections to the GNS 430 looked fine, but I found that the Glideslope cable from the GNS 430 was connected to the Nav connection on the duplexer. And the VOR/Localizer cable from the GNS 430 was connected to the Glideslope connection on the duplexer.

I contacted RAMI, the manufacturer of the VOR/LOC/GS Receive Only Antenna - AV-532L. Ben, the sales engineer indicated that having those connections reversed in the duplexer will cause problems because the Glideslope operates on a different set of frequencies than the VOR/Localizer. He said that the bent element isn't an issue, that the antennae is very simple and there are no additional parts within the puck that would be jarred or broken. So, I went ahead and moved the Glideslope cable to the GS connector and the VOR/Localizer cable to the Nav connector on the duplexer. I flew the airplane with foggles while my father was safety pilot in right seat.

First I intercepted the DEN VOR and flew it for about 15 miles, noting that the Glideslope flag was on (and unfortunately I cannot remember for sure if the VOR flag was on, but I think that it was). Then, I practiced an ILS approach on RWY 35 at KCFO. I found that the Glideslope flag disappeared and showed an acceptable /accurate glide slope depiction. The Localizer/VOR flag continued to appear although on the approach the needle was alive. My father felt that the needle was showing accurate deflection but I thought that it was not. I saw the needle to the right of center, flipped my foggles, and found that the airplane was right of the runway centerline (and not on the back course)! However, I was a bit fatigued at the time and might not have seen it correctly. Also, I could be covering up for bad flying. ;)

Ben from RAMI also stated that the antenna is mounted in a location which is not acceptable for ideal performance (too much adjacent metal to the antenna whips as they exit the insulator puck) and the transmission line (coax) does not incorporate the required balun transformer.

For the mounting, he recommended that I can either fabricate an insulated spacer (1.5 to 2" long) to bring the antenna puck further away from the adjacent metal on the belly or relocate the antenna further back on the airframe, like just in front of the tailwheel. I prefer the option of inserting a spacer over relocating the antennae in front of the tailwheel, because it seems easier to add the spacer.

For the transmission line, he indicated that I can fabricate a coaxial balun transformer into the existing transmission line or purchase their transmission line only (with integral balun transformer) from Aircraft Spruce.

I want to re-test the Localizer/VOR flag next time I go up, to verify whether the flag ever disappears or is always on. I am inclined to follow RAMI's recommendations on inserting the spacer and incorporating the balun transformer. However, I am unclear about this step - do I need to replace the coaxial cable from antennae all the way to duplexer with RAMI's transmission line with integral balun transformer? In speaking with my dad, he cannot remember why he used COAX rather than the transmission line with integral balun transformer. He also didn't recall considering the spacing between the empennage and the antennae puck.

Please let me know of your experience, advice, or any information that you might have.

Thank you
Bryan
 
Well,….
If the diplexer was ideal, you would have gotten no signals with the gs and vor connections reversed. I’m surprised putting it right didn’t fix the issues. Triple check the the antenna, vor, and gs connections are now correct. Test the diplexer by connecting the antenna directly to the nav input on the 430. The gs won’t work, but does that improve the vor or localizer performance? If so the diplexer may be kaput.
I am really not a fan of the dipole down so low. A significant eye hazard anytime you’re down there, as well as problems on a tail dragger being so close to the ground. Personally I like the Archer wingtip design, but at least think about putting it up high.
Putting in a matching network will help a bit but that’s not your real problem.
 
Well, no diplexer is ideal. But I agree, I'm surprised that there wasn't a substantial improvement. Lack of the Balun could reduce the signal by maybe 50% at worst I think.

With your 430 and CDI, you can cross check the VOR against the GPS. Fly somewhere say 40 miles away from a VOR. Tune in the VOR and set the CDI to display the VOR. Do Direct To on the GPS to the VOR. What does the GPS say the heading is to the station? What does the CDI say is the radial TO when you center the needle. Do they match? How close do you need to get until they do? How does the VOR signal sound when you Ident the VOR? Faint? Scratchy? Fades? Hearing buzzing or other interference noise?

Do you have any USB adapters in the plane? Try unplugging those. Try turning off landing lights, beacon, and strobes. ... while listening to the VOR audio.
 
Oh, and I'm not a fan of diplexers in general. It will always reduce the signal somewhat. A separate glideslope antenna eliminates that loss.
 
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