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Garmin GA-35 Antenna Hard Failure In-Flight

krw5927

Well Known Member
I know there have been reports of this previously, but yesterday returning from Kansas City and about 20 miles from home, my Garmin GNS480 with GA-35 antenna suddenly lost GPS navigation integrity. The signal status page showed zero signal on any satellite, which has never happened before.

At the same time, I noticed the G5, Dynon Skyview, and Ipad had all lost their position locks as well. The Skyview also displayed a message of degraded attitude condition or something of that nature.

Having read of this GA-35 antenna failure before, I tried turning off the GNS480, and all the other devices resumed GPS position lock. As an experiment, I turned the 480 back on, and watched everything else lose lock immediately again. It was a very repeatable failure.

Because the GNS480 feeds ADSB out, I filed an ASRS report even though I was only in Class E and G airspace for the rest of the flight. Had I been in IMC, this failure would've been quite exciting!

I know others have had some success working with Garmin to replace the faulty antenna. Mine is several years old so probably outside of any warranty period. Any advice on working with Garmin would be appreciated - I don't exactly have a "dealer" to work with. Perhaps I just bite the bullet and buy a new GA-35?
 
Contact Trek Lawler at Garmin. He will most likely have you work thru a distributor (Stein) no matter what he comes up with.
 
Kurt - I'm sorry you experienced this failure. With this failure mode the GA35 antenna turns into a GPS jammer, normally taking out all the other GPS receivers in the airplane.

While Trek Lawler is a great guy who does his best to help, the corporate line from Garmin is the source of the fault is excessive torque of the mounting fasteners. Of course that also means any warranty claim will be denied.

Dare I be so bold as to suggest that antennas should be designed so that most reasonable fastener torques don't cause the antenna to fail in such a manner as to interfere with every other receiver in the aircraft? Dare I suggest this should be documented in a Service Bulletin distributed to all owners, and that Garmin should have some skin in the game when it comes to replacing antennas which clearly have a fatal design flaw?

Nah, might as well save my breath to cool my porridge since nobody in the FAA seems to think that wiping out all the GPS navigators on an aircraft is a problem! There's a law against using cell phones on airliners, but no law against manufacturing avionics antennas that become jammers - that would be too logical.

Ok, time to hop off the soap box...
 
Steve, see this thread.

http://www.vansairforce.com/community/showthread.php?t=139576

Post 6 has a lot of good info that might address your questions.

Ah, yes, I remember now. What I stated above is not true. (post deleted) The base plate is not threaded. The screws pass through from above and get nuts on the underside. So yes, I can see that over-tightening could crush or crack the plastic cap. Especially if the antenna is attached to a curved skin. I made aluminum wedges to put between the curved skin and the base of the antenna, so it clamps down flat on the wedges.

gosh, it was only 3 weeks ago and I have already forgotten.

According to Brian, it looks like they have improved them to reduce the chance of cracking.
 
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There are other options, but you may not like the price :eek:

GPS/VHF Antenna TNC/BNC Comant CI-2580-200
GPS/VHF Antenna TNC/BNC Comant CI-2728-200
GPS/XM/VHF Antenna TNC/BNC Comant CI-2580-410
GPS/XM/VHF Antenna TNC/TNC/BNC Comant CI-2728-410
GPS Antenna TNC Comant CI-428-200
GPS/XM Antenna TNC/TNC Comant CI-428-410
 
Hello Kurt,
thanks for the email, appreciate the heads up on this and will do what I can to help. but basically after about 5 years of troubleshooting this issue which started in 2010 we ultimately found that it was in fact an over tightening of the screws on the antenna. As much as I'd hope that an antenna could be made that would not allow this to happen doing so would most likely be such a heavy unit that you would not be able to carry any passengers, even over torquing a cylinder head will cause it to crack so not sure there is anything that could prevent an issue like this.
During this timeframe when this issue was first noted and we did not know what was causing it we were replacing them at no charge, keep in mind the GA 35 is not something we build in house, but rather have a vendor build and provide them for our kits. so it was quite a bit of money out of pocket over a 5 year period. we then found the problem with overtightening them as we could never reproduce it with the returned antennas as they were no longer under any stress during our testing. in short what was happening is the ground plane was coming loose from the internal circuit board in the unit and it was turning from a receiver to a transmitter oscillating at its tuned frequency of 1.575 GHz which as you now know is the frequency of the GPS data. we went through a couple of different shell changes with the vendor and did change to a different material that is intended to be more robust and less susceptible to the cracking if the screws are overtightened, plus there is also a big notice in the packaging about the max tightening of the fasteners in the package that has to be removed before you can install the new antenna.

with that being said, I will be happy to do what I can to help if you can get with your local Garmin dealer with the serial number of the antenna and have them contact me
 
Hello Kurt,
thanks for the email, appreciate the heads up on this and will do what I can to help. but basically after about 5 years of troubleshooting this issue which started in 2010 we ultimately found that it was in fact an over tightening of the screws on the antenna. As much as I'd hope that an antenna could be made that would not allow this to happen doing so would most likely be such a heavy unit that you would not be able to carry any passengers, even over torquing a cylinder head will cause it to crack so not sure there is anything that could prevent an issue like this.
During this timeframe when this issue was first noted and we did not know what was causing it we were replacing them at no charge, keep in mind the GA 35 is not something we build in house, but rather have a vendor build and provide them for our kits. so it was quite a bit of money out of pocket over a 5 year period. we then found the problem with overtightening them as we could never reproduce it with the returned antennas as they were no longer under any stress during our testing. in short what was happening is the ground plane was coming loose from the internal circuit board in the unit and it was turning from a receiver to a transmitter oscillating at its tuned frequency of 1.575 GHz which as you now know is the frequency of the GPS data. we went through a couple of different shell changes with the vendor and did change to a different material that is intended to be more robust and less susceptible to the cracking if the screws are overtightened, plus there is also a big notice in the packaging about the max tightening of the fasteners in the package that has to be removed before you can install the new antenna.

with that being said, I will be happy to do what I can to help if you can get with your local Garmin dealer with the serial number of the antenna and have them contact me

Trek, I (and I'm sure others) really appreciate you coming on the board to reply! I will contact the Garmin avionics dealer in my area to see if they can help run some communication.

Also, in hopes that I don't create a new problem for myself, and for future posterity for others on the board, is the recommended torque for the GA35 antenna fasteners still 12-15 in-lbs as depicted in the installation manual linked here? http://static.garmin.com/pumac/GA35GPS_WAAS-antenna_AntennaInstallationInstructions.pdf

Thanks again for your time!
 
Another GA35 Issue

Hi Trek,
I've also had some reception issues, I left you a PM with more info.
 
Minimum ground plane diameter?

Contact Trek Lawler at Garmin.
How would one do that, please? I have a question on how to install the GA-35 in a fiberglass airplane. I'm planning on installing a GDL-82 in my Lancair, but the instructions tell me I need a 20" dia ground plane under the GA-35 antenna. Certainly that can't be right because I know Lancair pilots which only have 6" dia. ground planes and those guys report no problems. My tail cone doesn't have room for a 20" ground plane so I really need to find out what the minimum diameter is.
 
How would one do that, please? I have a question on how to install the GA-35 in a fiberglass airplane. I'm planning on installing a GDL-82 in my Lancair, but the instructions tell me I need a 20" dia ground plane under the GA-35 antenna. Certainly that can't be right because I know Lancair pilots which only have 6" dia. ground planes and those guys report no problems. My tail cone doesn't have room for a 20" ground plane so I really need to find out what the minimum diameter is.

I pulled the below ref. from the GTN install manual mainly because it included a blurb about non metalic aircraft.
Both the GDL and GTN ref the same 7.5" radius ground plane (not 20").

"the GPS/SBAS antenna requires a minimum ground plane radius of 7.5" inches around the perimeter of the antenna. Refer to figure 2-2. For metal aircraft, the surrounding metal skin on which the antenna is mounted supplies the ground plane. For nonmetallic aircraft, the ground plane can be composed of heavy duty aluminum foil tape, such as 3M P/N 438, 3M P/N 436, or other adhesive backed dead soft foil with aluminum 7.2 mils or greater."
 
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I think you guys are mixing Radius and Diameter. 7.5" Radius = 15" Diameter.
Figure 6-4 in the GDL-82 installation manual shows the ground plane extending a minimum of 7.5" from the perimeter of the antenna. Since the longest dimension of the GA-35 antenna is 4.68", I took that to mean 7.5+7.5+4.68" = 19.68". I just rounded off to 20". FWIW, I know of a Lancair that only has a 6"x6" ground plane and the pilot reports that it works fine "most of the time". Upon doing some research this morning, it seems like the gain of a WAAS GPS antenna is much better with the larger ground plane. I think Garmin came up with the 7.5" number because that works out to a full wavelength at GPS frequencies. It seems like overkill to me, but WTFDIK?
 
For nonmetallic aircraft, the ground plane can be composed of heavy duty aluminum foil tape, such as 3M P/N 438, 3M P/N 436, or other adhesive backed dead soft foil with aluminum 7.2 mils or greater.
Thanks, Walt. If I really need the 20" dia. ground plane, that's what I'm going to have to do. I'll apply the foil to the underside of the fuselage skin above the baggage compartment and mount the GPS antenna out in the wind. I really didn't want to mount the antenna outside the aircraft but I don't have enough room for that large of a ground plane inside. I question why a full-wave ground plane is required, though. With a comm whip antenna, only 1/4 wave is required. I think the 20" is overkill.
 
John - a couple of thoughts for you... (Coming from another "plastic airplane" guy...)

1) if you wish to have your GA35 mounted internally, build an aluminum shelf for it. Make it so the shelf holds the antenna up close against the inside of the fuselage. The outboard edges of the shelf would then bend downwards to follow the profile of the fuselage. At the point of contact between the shelf and fuselage (which would be at or below the base of the GA35 antenna so as to not shade the antenna from a clear "view" of the horizon) start your foil running further down the curvature of the fuselage. You won't be able to achieve a good ground plane in the fore and aft directions, but will get good coverage in the lateral axis.

2) many folks get away with much smaller ground planes for GPS. Garmin's guidance is based on their need to provide guidance to installers to ensure best probability of GPS system performance meeting or exceeding TSO minimums. This means they have to take all factors to their worst-case condition when they do these engineering calculations. If, for instance, you have much less antenna cable loss than the maximum allowable, you will have bought some extra performance margin that would not have been in Garmin's "worst case" engineering analysis. These factors, when considered as a whole, mean that some installations work well with less antenna ground plane.

3) keep in mind the conductivity characteristics of your fuselage. If you have any conductive materials embedded in the composite matrix in order to improve lighting or P-static protection, you will want to mount all your antennas on the exterior of the fuselage.
 
1) if you wish to have your GA35 mounted internally, build an aluminum shelf for it.
That was my original plan, but I ran up against a couple problems. First, the mounting screws for the GA-35 are on top and I would have no screwdriver access with the antenna mounted right up against the "ceiling". That would mean I would have to make the shelf removable, adding complexity back in a hard to reach area. Also my VOR antenna is already mounted in that area, meaning I would have to move it elsewhere. Here's the view of the area I have to work with (All that junk hanging in front is where my Garmin Virb camera mounts. The V-shaped foil VOR antenna and coax are on the "ceiling". That long, red stripe is the Marker Beacon antenna, which I will probably never use.):

l6nX5s.jpg


Here's a screen grab of the Garmin GDL-82 instructions:

ukwDvi.jpg


So now I'm thinking of mounting the antenna on an aluminum shelf maybe 12"x12" somewhere back in there. No metal embedded in the fiberglass.

Edit: SBAS is "Space Based Augmentation System" AKA "WAAS". I had to look it up.
 
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If Mount my ga35 inside the engine compartment on the motor mount with axel clamps does all the metal around counts as ground plane?
 
If Mount my ga35 inside the engine compartment on the motor mount with axel clamps does all the metal around counts as ground plane?
No. Bad idea. The base of the GA-35 has to be physically and electrically in contact with the ground plane. Also, the heat and vibration in there would kill the GA-35.
 
Several folks have mounted their GPS antennas under the engine cowl. Some have had good luck with this installation. Others... not so much. The reality is that heat is the enemy of reliability when it comes to electronics. Heat is also the enemy of good radio frequency amplifiers (such as those found in the GA35 antenna). On principle, mounting the antenna in a cooler area should ultimately be better than mounting an antenna in an area as harsh as an engine compartment.
 
I see what you guys are saying but so many people mount their antenna in the engine compartment! I wonder if it works for most people or that will go bad quick! Is the GA35 the only the needs a ground Plane or they all call for that? Thanks
 
...with that being said, I will be happy to do what I can to help if you can get with your local Garmin dealer with the serial number of the antenna and have them contact me

Trek, thanks again for your offer to help. Unfortunately it seems I've run into some resistance with my local Garmin dealer. They were all too eager to "sell me a new GA35 antenna", however they've been rather unresponsive to the request to simply relay the serial number of my antenna to you as you requested.

This is one time the dealer network is a hindrance to real customer service.

I'll likely be buying a new GA-35 antenna. But most definitely NOT from my local dealer.
 
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